OCS Study 

MMS 2005-008 

 

Coastal Marine Institute 

Visibility and Atmospheric Dispersion 

Capability over the Northern Gulf of Mexico 

Estimations and Observations 

of Boundary Layer Parameters 

Cooperative Agreement Coastal Marine Institute 

Louisiana State University

U.S. Department of the Interior 

Minerals Management Service 

Gulf of Mexico OCS Region 
OCS Study 

 MMS 2005-008 

 

Coastal Marine Institute 

Visibility and Atmospheric Dispersion 

Capability over the Northern Gulf of Mexico 

Estimations and Observations 

of Boundary Layer Parameters 

Authors 

S.A. Hsu 

B.W. Blanchard 

March 2005 

Prepared under MMS Contract 

1435-01-99-CA-30951-17810 

by 

Coastal Marine Institute 

Louisiana State University 

Baton Rouge, Louisiana 70803 

Published by 

U.S. Department of the Interior Cooperative Agreement Minerals Management Service Coastal Marine Institute 

Gulf of Mexico OCS Region Louisiana State University
DISCLAIMER 

This report was prepared under contract between the Minerals Management Service (MMS) and 

the Coastal Studies Institute, Louisiana State University. This report has been technically 

reviewed by the MMS, and it has been approved for publication. Approval does not signify that 

the contents necessarily reflect the views and policies of the MMS, nor does mention of trade 

names or commercial products constitute endorsement or recommendation for use. It is, 

however, exempt from review and compliance with the MMS editorial standards. 

REPORT AVAILABILITY 

Extra copies of this report may be obtained from the Public Information Office (Mail Stop 5034) 

at the following address: 

 U.S. Department of the Interior 

 Minerals Management Service 

 Gulf of Mexioc OCS Region 

 Public Information Office (MS 5034) 

 1201 Elmwood Park Boulevard 

 New Orleans, Louisiana 70123-2394 

 Telephone: (504)736-2519 or 

 1-800-200-GULF 

CITATION 

Suggested citation: 

Hsu, S. A. and B. W. Blanchard. 2005. Visibility and atmospheric dispersion capability over the 

northern Gulf of Mexico: Estimations and observations of boundary layer parameters: 

Final Report. U.S. Dept. of the Interior, Minerals Management Service, Gulf of Mexico 

OCS Region, New Orleans, LA. OCS Study MMS 2005-008. 184 pp. 

ABOUT THE COVER 

NOAA-12 satellite imagery for 4 March 2002 1637 CST reveals the locations of coastal marsh 

fires in southwest Louisiana and the resulting plumes extending southward over the shelf waters 

(courtesy of the Earth Scan Lab, LSU). 

iii 
ABSTRACT 

Two visibility and meteorological stations were made operational in the Louisiana coastal waters 

beginning in November 2001. An hourly data record is being archived, and near-real time 

information is made available over the internet. Simplified formulas are derived for the offshore 

atmospheric boundary layer which require only easily obtainable input parameters. With these 

formulations, monthly mean variations of stability, mixing height, and ventilation factor (an 

indicator of dispersion capability) are provided for the northern Gulf of Mexico. A method for 

determining whether fog or haze conditions are observed at a monitoring station is given. Using 

this algorithm, the effects of fog and haze at three coastal stations are discussed. Several periods 

of reduced visibility at our primary monitoring station are examined using pertinent weather 

maps and satellite observations. It is shown that naturally occurring fog conditions are by far the 

most common cause of reduced visibility along the Louisiana coast. Haze can be observed each 

month; however, the total duration is small and the extent to which it reduces visibility is usually 

much less than that due to fog. Plumes from inland (near-coast) areas can affect the OCS region. 

v 
TABLE OF CONTENTS 

Page

List of Figures ................................................................................................ ix 

List of Tables ................................................................................................. xv 

I. Executive Summary ................................................................................... 1 

II. Field Program ........................................................................................... 3 

III. Scientific Methodology ........................................................................... 63 

 A. Overwater Stability Determination ..................................................... 63 

 B. Bowen Ratio Variations Over the Gulf of Mexico .............................. 67 

 C. Overwater Mixing Heights and Ventilation Factor ............................. 72 

IV. Applications............................................................................................. 75 

V. Some Characteristics of Fog and Haze Along the Louisiana Coast ......... 107 

VI. Applications of Satellite Imagery During Periods of Reduced Visibility 

Over the Northern Gulf of Mexico.......................................................... 129 

 A. November 25, 2001.............................................................................. 129 

 B. 4 - 5 March, 2002................................................................................. 133 

 C. 27 - 28 March 2002.............................................................................. 137 

 D. 10 April 2002....................................................................................... 145 

 E. 30 August 2002.................................................................................... 151 

 F. 14 - 15 September 2002 ....................................................................... 157 

 G. 14 - 15 December 2002........................................................................ 164 

 H. 26 April 2003....................................................................................... 172 

 I. Satellite Detection of Plume Geometry ................................................ 178 

VII. Summary................................................................................................. 181 

VIII. References............................................................................................. 183 

vii 
LIST OF FIGURES 

Figure Page 

1 Locations of the NDBC and WAVCIS instrumented platforms in the northern 

Gulf of Mexico....................................................................................................... 2 

2 WAVCIS platforms CSI-3 (top) and CSI-6 (bottom) (courtesy of the WAVCIS 

program)................................................................................................................. 5

3 November 2001 hourly meteorological record from station CSI-3....................... 7 

4 November 2001 hourly visibility and temperature records for station CSI-3........ 8 

5 December 2001 hourly meteorological record for station CSI-3........................... 9 

6 December 2001 hourly visibility and temperature records for station CSI-3........ 10 

7 January 2002 hourly meteorological records for station CSI-3............................. 11 

8 January 2002 hourly visibility and temperature records for station CSI-3............ 12 

9 February 2002 hourly meteorological records for station CSI-3........................... 13 

10 February 2002 hourly visibility and temperature records for station CSI-3.......... 14 

11 March 2002 hourly meteorological records for station CSI-3............................... 15 

12 March 2002 hourly visibility and temperature records for station CSI-3.............. 16 

13 April 2002 hourly meteorological records for station CSI-3................................. 17 

14 April 2002 hourly visibility and temperature records for station CSI-3................ 18 

15 May 2002 hourly meteorological records for station CSI-3.................................. 19 

16 May 2002 hourly visibility and temperature records for station CSI-3................. 20 

17 June 2002 hourly meteorological records for station CSI-3.................................. 21 

18 June 2002 hourly visibility and temperature records for station CSI-3................. 22 

19 July 2002 hourly meteorological records for station CSI-3................................... 23 

20 July 2002 hourly visibility and temperature records for station CSI-3.................. 24 

21 August 2002 hourly meteorological records for station CSI-3.............................. 25 

22 August 2002 hourly visibility and temperature records for station CSI-3............. 26 

23 September 2002 hourly meteorological records for station CSI-3......................... 27 

24 September 2002 hourly visibility and temperature records for station CSI-3........ 28 

25 October 2002 hourly meteorological records for station CSI-3............................. 29 

26 October 2002 hourly visibility and temperature records for station CSI-3............ 30 

27 November 2002 hourly meteorological records for station CSI-3......................... 31 

28 November 2002 hourly visibility and temperature records for station CSI-3........ 32 

29 December 2002 hourly meteorological records for station CSI-3......................... 33 

30 December 2002 hourly visibility and temperature records for station CSI-3........ 34 

31 January 2003 hourly meteorological records for station CSI-3............................. 35 

32 January 2003 hourly visibility and temperature records for station CSI-3............ 36 

33 February 2003 hourly meteorological records for station CSI-3........................... 37 

34 February 2003 hourly visibility and temperature records for station CSI-3.......... 38 

35 March 2003 hourly meteorological records for station CSI-3............................... 39 

36 March 2003 hourly visibility and temperature records for station CSI-3.............. 40 

37 April 2003 hourly meteorological records for station CSI-3................................. 41 

38 April 2003 hourly visibility and temperature records for station CSI-3................ 42 

39 May 2003 hourly meteorological records for station CSI-3.................................. 43 

40 May 2003 hourly visibility and temperature records for station CSI-3................. 44 

ix
Figure Page 

41 June 2003 hourly meteorological records for station CSI-3.................................. 45 

42 June 2003 hourly visibility and temperature records for station CSI-3................. 46 

43 July 2003 hourly meteorological records for station CSI-3................................... 47 

44 July 2003 hourly visibility and temperature records for station CSI-3.................. 48 

45 August 2003 hourly meteorological records for station CSI-3.............................. 49 

46 August 2003 hourly visibility and temperature records for station CSI-3............. 50 

47 September 2003 hourly meteorological records for station CSI-3......................... 51 

48 September 2003 hourly visibility and temperature records for station CSI-3........ 52 

49 October 2003 hourly meteorological records for station CSI-3............................. 53 

50 October 2003 hourly visibility and temperature records for station CSI-3............ 54 

51 July 2003 hourly meteorological records for station CSI-6................................... 55 

52 July 2003 hourly temperature records for station CSI-6........................................ 56 

53 August 2003 hourly meteorological records for station CSI-6.............................. 57 

54 August 2003 hourly visibility and temperature records for station CSI-6............. 58 

55 September 2003 hourly meteorological records for station CSI-6......................... 59 

56 September 2003 hourly visibility and temperature records for station CSI-6........ 60 

57 October 2003 hourly meteorological records for station CSI-6............................. 61 

58 October 2003 hourly visibility and temperature records for station CSI-6............ 62 

59 An overwater relationship between z/L and Rb (see text for details)..................... 66 

60 Seasonal distribution of Bowen ratio vs. temperature difference for buoy 42035 

in the coastal waters............................................................................................... 69 

61 Seasonal distribution of Bowen ratio vs. temperature difference for buoy 42002 

in the deep Gulf waters........................................................................................... 69 

62 Mean and standard deviation of Bowen ratio vs. temperature difference for 

classified data from buoy 42035 in the coastal waters........................................... 70 

63 Mean and standard deviation of Bowen ratio vs. temperature difference for 

classified data from buoy 42002 in the deep Gulf waters...................................... 71 

64 Derived relationships between the Bowen ratio and sea-air temperature 

difference from the coast to the deep Gulf waters. ............................................... 71 

65 Approximate locations of National Data Buoy Center Coastal-Marine 

Automated Network and offshore buoys along with WAVCIS platforms CSI-3 

and CSI-6 superimposed on a MODIS satellite image.......................................... 77 

66 January monthly mean values of air temperature, sea temperature, and stability 

(z/L)........................................................................................................................ 78 

67 January monthly mean mixed heights (m) and ventilation factor index................ 79 

68 February monthly mean air temperature, sea temperature, and stability (z/L)...... 80 

69 February monthly mean mixed heights (m) and ventilation factor index.............. 81 

70 March monthly mean air temperature, sea temperature, and stability (z/L).......... 82 

71 March monthly mean mixed height (m) and ventilation factor index.................... 83 

72 April monthly mean air temperature, sea temperature, and stability (z/L)............ 84 

73 April monthly mean mixed height (m) and ventilation factor index...................... 85 

74 May monthly mean air temperature, sea temperature, and stability (z/L)............. 86 

75 May monthly mean mixed height (m) and ventilation factor index....................... 87 

76 June monthly mean air temperature, sea temperature, and stability (z/L)............. 88 

77 June monthly mean mixed height (m) and ventilation factor index....................... 89 

x 
Figure Page 

78 July monthly mean air temperature, sea temperature, and stability (z/L).............. 90 

79 July monthly mean mixed height (m) and ventilation factor index....................... 91 

80 August monthly mean air temperature, sea temperature, and stability (z/L)......... 92 

81 August monthly mean mixed height (m) and ventilation factor index.................. 93 

82 September monthly mean air temperature, sea temperature, and stability (z/L).... 94 

83 September monthly mean mixed height (m) and ventilation factor index............. 95 

84 October monthly mean air temperature, sea temperature, and stability (z/L)........ 96 

85 October monthly mean mixed height (m) and ventilation factor index................. 97 

86 November monthly mean air temperature, sea temperature, and stability (z/L).... 98 

87 November monthly mean mixed height (m) and ventilation factor index............. 99 

88 December monthly mean air temperature, sea temperature, and stability (z/L).... 100 

89 December monthly mean mixed height (m) and ventilation factor index.............. 101 

90 August to October 2003 computed mean mixed heights from the Louisiana 

coast to deep Gulf................................................................................................... 102 

91 Annual distribution of computed mixed heights from the Louisiana coast to 

deep Gulf................................................................................................................ 105 

92 Frequency of occurrence of fog and haze as defined by the ASOS at CSI-3. 

Note that records for May and October 2002 are less than 75% complete............ 109 

93 Frequency of occurrence of restricted visibility at CSI-3...................................... 110 

94 Frequency of occurrence of fog and haze as defined by the ASOS at station 

CSI-6...................................................................................................................... 111

95 Frequency of occurrence of fog and haze as defined by the ASOS at the 

Boothville station................................................................................................... 112 

96 Frequency of restricted visibility at the Boothville station.................................... 113 

97 Frequency of occurrence of fog and haze as defined by the ASOS at NDBC 

GDIL1.................................................................................................................... 114

98 Frequency of occurrence of restricted visibility at NDBC GDIL1........................ 115 

99 January wind rose for CSI-3, and visibility 'roses' for the CSI-3 and Boothville 

stations.................................................................................................................... 116 

100 February wind rose for CSI-3, and visibility 'roses' for the CSI-3 and Boothville 

stations.................................................................................................................... 117 

101 March wind rose for CSI-3, and visibility 'roses' for the CSI-3 and Boothville 

stations.................................................................................................................... 118 

102 April wind rose for CSI-3, and visibility 'roses' for the CSI-3 and Boothville 

stations.................................................................................................................... 119 

103 May wind rose for CSI-3, and visibility 'roses' for the CSI-3 and Boothville 

stations.................................................................................................................... 120 

104 June wind rose for CSI-3, and visibility 'roses' for the CSI-3 and Boothville 

stations.................................................................................................................... 121 

105 July wind rose for CSI-3, and visibility 'roses' for the CSI-3 and Boothville 

stations.................................................................................................................... 122 

106 August wind rose for CSI-3, and visibility 'roses' for the CSI-3 and Boothville 

stations.................................................................................................................... 123 

107 September wind rose for CSI-3, and visibility 'roses' for the CSI-3 and 

Boothville station................................................................................................... 124 

xi
Figure Page 

108 October wind rose for CSI-3, and visibility 'roses' for the CSI-3 and Boothville 

station..................................................................................................................... 125 

109 November wind rose for CSI-3, and visibility 'roses' for the CSI-3 and 

Boothville station................................................................................................... 126 

110 December wind rose for CSI-3, and visibility 'roses' for the CSI-3 and 

Boothville stations.................................................................................................. 127 

111 Time series (CST) of observed parameters on 25 November 2001....................... 130 

112 NWS surface weather charts for 0000Z and 0300Z 26 November 2001............... 131 

113 0400Z 26 November 2001 NWS Weather Depiction chart................................... 132 

114 1910 CST 25 November 2001 GOES-8 infrared image........................................ 132 

115 Time series (CST) of observed parameters on 4 - 5 March 2002.......................... 133 

116 NWS surface weather charts for 0000, 0300, and 0600 Z 5 March 2002.............. 134 

117 NWS Radar Summary chart for 0314 Z 5 March 2002......................................... 135 

118 1400 CST 4 March 2002 NOAA-16 visible image................................................ 135 

119 1637 CST 4 March 2002 NOAA-12 visible image and near-infrared image........ 136 

120 1813 CST 4 March 2002 NOAA-15 near-infrared image...................................... 137 

121 Time series (CST) of observed parameters on 27 - 28 March, 2002..................... 138 

122a,b NWS Surface Weather Charts for 0300 and 0600 Z 28 March 2002.................... 139 

122c,d NWS Surface Weather Charts for 0900 and 1200Z 28 March 2002..................... 140 

123 NWS Weather Depiction charts for 07, 10, and 13 Z 28 March 2002, top to 

bottom, respectively............................................................................................... 141 

124 1917 CST 27 March 2002 NOAA-15 infrared image............................................ 142 

125 2039 CST 27 March 2002 GOES-8 infrared image............................................... 142 

126 0136 CST 28 March 2002 NOAA-16 infrared image............................................ 143 

127 0609 CST 28 March 2002 GOES-8 infrared image............................................... 143 

128 0739 CST 28 March 2002 NOAA-15 visible image and infrared 

image...................................................................................................................... 144

129 Time series (CST) of observed parameters on 10 April 2002................................ 145 

130 NWS Surface Weather charts for 21 Z 10 April and 00 Z 11 April 

2002........................................................................................................................ 146 

131 NWS Weather Depiction chart for 19 Z 10 April 2002......................................... 147 

132 1221 CDT 10 April 2002 MODIS true color image.............................................. 147 

133 1456 CDT 10 April 2002 NOAA-16 visible image and infrared image................ 148 

134 1751 CDT 10 April 2002 NOAA-16 visible image and infrared image................ 149 

135 1939 CDT 10 April 2002 GOES-8 infrared image................................................ 150 

136 2039 CDT 10 April 2002 GOES-8 infrared image................................................ 150 

137 Time series (CST) of observed parameters on 30 August 2002............................ 151 

138 NWS Surface Weather chart for 1500 Z 30 August 2002...................................... 152 

139 NWS Radar Summary for 1515 Z 30 August 2002............................................... 152 

140 NWS Weather Depiction charts for 13 Z and 16 Z 30 August 2002..................... 153 

141 0945 CDT 30 August 2002 GOES-8 visible image and infrared image................ 154 

142 1015 CDT 30 August 2002 GOES-8 visible image and infrared image................ 155 

143 1146 CDT 30 August 2002 NOAA-17 visible image and infrared image............. 156 

xii 
Figure Page 

144 1234 CDT 30 August 2002 MODIS true color image........................................... 157 

145 Time series (CST) of observed parameters on 14 - 15 September 2002............... 158 

146 NWS Surface Weather Charts for 0600, 1200, and 1500 Z 15 September 2002... 159 

147 NWS Weather Depiction charts for 07Z and 13Z 15 September 2002.................. 160 

148 NWS Radar Summary chart for 1415Z 15 September 2002.................................. 160 

149 0145 CDT 15 September 2002 GOES-8 infrared image........................................ 161 

150 0309 CDT 15 September 2002 NOAA-16 infrared image.................................... 161 

151 0915 CDT 15 September 2002 GOES-8 visible image and infrared image.......... 162 

152 1049 CDT 15 September 2002 NOAA-16 visible image and infrared image....... 163 

153 Time series (CST) of observed parameters on 14 - 15 December 2002................ 164 

154 NWS Surface Weather charts for 0000 Z and 0600 Z 15 December 2002............ 165 

155 NWS Surface Weather charts for 1200 Z and 18 Z 5 December 2002.................. 166 

156 NWS Weather Depiction charts for 07 Z and 10 Z 15 December 2002................ 167 

157 NWS Weather Depiction charts for 13 Z and 16 Z 15 December 2002................ 168 

158 1918 CST 14 December 2002 NOAA-15 infrared image...................................... 169 

159 2015 CST 14 December 2002 GOES-8 infrared image......................................... 169 

160 0615 CST 15 December 2002 GOES-8 infrared image......................................... 170 

161 0740 CST 15 December 2002 NOAA-15 visible image and infrared image......... 171 

162 Time series (CST) of observed parameters on 26 April 2003................................ 172 

163 NWS Surface Weather Chart for 2100 Z 26 April 2003........................................ 173 

164 NWS Surface Weather charts for 0000 Z and 0300 Z 27 April 2003.................... 173 

165 1149 CDT 26 April 2003 MODIS true color image; close-up identifies fires 

and smoke plume originating on the central Louisiana coast................................ 174 

166 1459 CDT 26 April 2003 NOAA-16 visible (top) and infrared (middle) images; 

bottom panel is close-up of middle IR image........................................................ 175 

167 1545 CDT 26 April 2003 GOES-12 visible image and infrared image................. 176 

168 1958 CDT 26 April 2003 NOAA-16 infrared image............................................. 177 

169 2015 CDT 26 April 2003 GOES-12 infrared image.............................................. 177 

170 2134 Z 6 December 1999 NOAA-14 near-infrared image showing a smoke 

plume extending from the central Louisiana coast out over the Gulf of Mexico 

shelf waters............................................................................................................. 179 

171 NWS Surface Weather chart for 2100 Z 6 December 1999................................... 180 

172 NWS Surface Weather chart for 0000 Z 7 December 1999................................... 180 

xiii 
LIST OF TABLES 

Table Page 

1 Data Return Rate (%) for Station WAVCIS CSI-3................................................. 6 

2 Data Return Rate (%) for Station WAVCIS CSI-6................................................. 6 

3 Pollution dispersion forecast categories related to atmospheric ventilation 

(product of wind speed and mixing depth) (after Eagleman, 1996, based on the 

air pollution dispersal index used by the State of Colorado Department of Health 

in Denver)................................................................................................................ 73 

4 Locations and pertinent periods of record for coastal and offshore stations used 

in this report............................................................................................................. 76 

5 Frequency of occurrence (%) of stability classes, August to October 2003............ 103 

6 Monthly frequency of occurrence (%) of stability class at CSI-3........................... 103 

7 Annual variation of average computed mixed height extending from shoreline to 

deep Gulf during the approximate period of November 2001 through October 

2003.......................................................................................................................... 104 

xv 
I. EXECUTIVE SUMMARY 

In 1999, the Environmental Protection Agency (EPA) promulgated new regulations for 

protecting and improving visibility in the national parks and wilderness areas (the Regional Haze 

regulations). EPA regulations regarding the viewing of scenic vistas in locations such as the 

Breton Island National Wilderness Area have focused new attention on visibility impacts of

pollutant emissions. The Minerals Management Service (MMS), which oversees offshore 

activity including air quality aspects, has funded several studies associated with these issues in 

the Gulf of Mexico region. This Final Report presents the activities and findings of one such 

project designed to investigate visibility and mixing height over the Gulf of Mexico. 

In the marine environment, visibility can be affected by sea spray, hence wind and wave 

measurements are needed. Visibility sensors were thus deployed on two instrumented platforms 

in the Louisiana shelf waters (see Fig. 1) and an hourly archive of pertinent meteorological and 

oceanographic data was accumulated. The hourly reports were also made available in near-real 

time over the internet. 

Visibility is directly related to the distribution of aerosols, particulates, and pollutants, 

which is largely determined by the mixed layer properties. Routinely available measurements 

were employed to develop air-sea interaction formulas describing stability characteristics and 

mixing height over the Gulf. These parameters are essential inputs for air quality modeling 

efforts. Monthly average data from National Data Buoy Center (NDBC) offshore buoys and 

Coastal-Marine Automated Network (C-MAN) stations is used to illustrate spatial variations in 

the Gulf region. 

Definitions of haze and fog are applied to the data from several stations to determine the 

potential frequency of occurrence of these phenomena. Seasonal variations are presented, along 

with the distribution of severely restricted visibility conditions. 

Several low visibility episodes have been identified during our measurement period. 

Meteorological conditions, and satellite imagery when available, associated with these events are 

discussed. It will be shown that significant reductions in Louisiana coastal and offshore 

visibility are almost entirely due to transient natural conditions (fog). Episodes of haze are 

generally short-lived and affect visibility much less. Offshore haze can result from plume drift 

generated by coastal sources. Neutral and unstable conditions dominate over the Gulf. Spatial 

and temporal variations of the mixed layer height result, with an average computed height of 

only 445 m at our measurement station. When combined with low average wind speeds 

(ventilation factor), areas of potentially low atmospheric dispersion capabilities occur, 

particularly in late summer. 

1 
Figure 1. Locations of the NDBC and WAVCIS instrumented platforms in the northern Gulf of 

Mexico. For this study, platforms CSI-3 and CSI-6 were equipped with visibility 

sensors (map courtesy of WAVCIS webpage). 

2 
3 

II. FIELD PROGRAM 

In order to study characteristics of the atmospheric boundary layer and visibility over the 

Louisiana coastal waters, we proposed to deploy two surface stations; one near shore and the 

other closer to the shelf break. Under a separate program (WAVCIS, WAVe Current 

Information System), the Coastal Studies Institute, Louisiana State University, is constructing a 

network of instrumented platforms in the OCS region (see Fig. 1). Both oceanographic and 

meteorological parameters are recorded hourly at these stations and transmitted to LSU for 

quality control and archiving. The data are then made available in near-real time via the web at 

www.wavcis.lsu.edu (detailed information about this program can be found at the website). Two 

WAVCIS stations were eventually utilized for our visibility measurements. 

The first was near-shore at 92°03.68' 29°26.47' (south of Marsh Island, WAVCIS CSI-3) 

in late October 2001. Unfortunately, several logistic and contractual obstacles delayed the 

construction of the deeper water platform for some time. In late June 2003, the second visibility 

station came on line at WAVCIS CSI-6, located at 90°29' 28°52' (south of Terrebonne Bay). For 

this study, we integrated a Belfort Model 6100 visibility sensor and a Rotronics Model MP101A 

relative humidity/air temperature probe into the existing WAVCIS sensor suite. 

The Belfort Model 6100 is designed to measure visibility conditions over a range of 0 - 

10 miles. Visibility is detected using widely accepted principles of forward scattering. A highoutput infrared LED transmitter projects light into a sample volume, and light scattered in a 

forward direction is collected by the receiver. The light source is modulated to provide excellent 

rejection of background noise and natural variations in background light intensity. The sensor 

outputs an analog signal which is proportional to visibility. Accuracy is +/- 10%. If measured 

visibility falls below 100', then the accuracy is fixed at 10'. The Rotronics MP101A is 

commonly used in remote weather stations and ocean buoys. Sensor accuracy at 25°C is +/- 

1.5% and +/- 0.2°C. 

WAVCIS platforms CSI-3 and CSI-6 are shown in Fig. 2. For CSI-3, the anemometer, 

air temperature/humidity sensor, visibility sensor, and pressure sensor are mounted at 23.4 m, 23 

m, 21.9 m, and 13.3 m asl, respectively. They are similarly mounted on CSI-6 at heights 37.4 m,

37 m, 35.9 m, and 16.3 m, respectively. The sensors are sampled at 1 Hz for a 10-minute period 

at the beginning of each hour. As can be seen from Fig. 2, open space on production platforms is 

usually very limited. Best efforts were made to mount the sensors in a location providing clear 

air from all directions. 

For each hourly record, standard meteorological formulas (see, e.g., Hsu, 1988) were 

applied to derive a dew-point temperature as follows: 

The saturation vapor pressure, es, was obtained from the air temperature by 

e e s

Tair Tair = ∗ ∗ ∗ + 61078 2 302 7 5 237 3 . ( . . /( . )) (1) 
4 

By definition, relative humidity is the ratio of actual vapor pressure to saturation vapor pressure, 

or 

es

RH

e = (2) 

Therefore, vapor pressure e is 

RH

e es = ∗

100 (3)

A simple program was written to estimate a wet-bulb temperature from the measured air 

temperature and relative humidity by referencing tables from Weast et al. (1964). If the 

measured air temperature or the estimated wet-bulb temperature were below zero, then the dewpoint temperature was given by 

T

e

e dew =

 

− 



 

 9 321  61078 . log .



  2614  61078 . log . (4)

otherwise 

T

e

e dew =

 

− 



 

 7 5  61078 . log .



  237 3  61078 . log . (5) 

Tables 1 and 2 list the data return rates for CSI-3 and CSI-6, respectively. With the 

exception of May and October 2002, monthly records were generally better than 95% complete. 

Missing data was mostly due to station maintenance or sensor failure. Note that these 

measurements are continuing as of this publication, and can still be accessed through the 

WAVCIS website. 

Monthly time series of measured parameters for the period of November 2001 through 

October 2003 are presented in Figs. 3 to 58. 
Figure 2. WAVCIS platforms CSI-3 (top) and CSI-6 (bottom) (courtesy of the WAVCIS 

program). 

5 
Table 1. 

Data Return Rate (%) for Station WAVCIS CSI-3 (Shaded Months < 75% Complete) 

Month Visibility Tair Relative Humidity

2001

November 99 99 99

December 99 100 99

2002

January 97 97 97

February 79 79 79

March 100 100 100

April 100 100 100

May 70 70 70 

June 98 98 98

July 97 97 97

August 98 98 98

September 95 95 95

October 97 97 71 

November 99 99 78

December 99 99 99

2003

January 97 97 97

February 98 98 98

March 93 93 93

April 80 99 99

May 100 100 100

June 99 99 99

July 100 100 100

August 92 99 99

September 99 99 99

October 100 100 100

Table 2. 

Data Return Rate (%) for Station WAVCIS CSI-6 (Shaded Months < 75% Complete) 

Month Visibility Tair Relative Humidity

2003

July 0 99 0 

August 100 100 99

September 99 99 99

October 100 100 95

6 
Figure 3. November 2001 hourly meteorological record from station CSI-3. 

7 
Figure 4. November 2001 hourly visibility and temperature records for station CSI-

8 
Figure 5. December 2001 hourly meteorological record for station CSI-3. 

9 
10 

Figure 6. December 2001 hourly visibility and temperature records for station C
Figure 7. January 2002 hourly meteorological records for station CSI-3. 

11 
12 

Figure 8. January 2002 hourly visibility and temperature records for station CSI-3
Figure 9. February 2002 hourly meteorological records for station CSI-3. 

13 
Figure 10. February 2002 hourly visibility and temperature records for station CSI-

14 
Figure 11. March 2002 hourly meteorological records for station CSI-3. 

15 
Figure 12. March 2002 hourly visibility and temperature records for station CSI-3. 

16 
Figure 13. April 2002 hourly meteorological records for station CSI-3. 

17 
Figure 14. April 2002 hourly visibility and temperature records for station CSI-

18 
Figure 15. May 2002 hourly meteorological records for station CSI-3. 

19 
Figure 16. May 2002 hourly visibility and temperature records for station CSI-3

20 
Figure 17. June 2002 hourly meteorological records for station CSI-3. 

21 
Figure 18. June 2002 hourly visibility and temperature records for station CSI-3. 

22 
Figure 19. July 2002 hourly meteorological records for station CSI-3. 

23 
Figure 20. July 2002 hourly visibility and temperature records for station CSI-3. 

24 
Figure 21. August 2002 hourly meteorological records for station CSI-3. 

25 
Figure 22. August 2002 hourly visibility and temperature records for station CSI-

26 
Figure 23. September 2002 hourly meteorological records for station CSI-3. 

27 
Figure 24. September 2002 hourly visibility and temperature records for station CSI-

28 
Figure 25. October 2002 hourly meteorological records for station CSI-3. 

29 
Figure 26. October 2002 hourly visibility and temperature records for station CSI-3

30 
Figure 27. November 2002 hourly meteorological records for station CSI-3. 

31 
Figure 28. November 2002 hourly visibility and temperature records for station C

32 
Figure 29. December 2002 hourly meteorological records for station CSI-3. 

33 
Figure 30. December 2002 hourly visibility and temperature records for station 

34 
Figure 31. January 2003 hourly meteorological records for station CSI-3. 

35 
Figure 32. January 2003 hourly visibility and temperature records for station CSI-3

36 
Figure 33. February 2003 hourly meteorological records for station CSI-3. 

37 
Figure 34. February 2003 hourly visibility and temperature records for station C

38 
Figure 35. March 2003 hourly meteorological records for station CSI-3. 

39 
Figure 36. March 2003 hourly visibility and temperature records for station CSI-3

40 
Figure 37. April 2003 hourly meteorological records for station CSI-3. 

41 
Figure 38. April 2003 hourly visibility and temperature records for station CS

42 
Figure 39. May 2003 hourly meteorological records for station CSI-3. 

43 
Figure 40. May 2003 hourly visibility and temperature records for station CSI-3. 

44 
Figure 41. June 2003 hourly meteorological records for station CSI-3. 

45 
Figure 42. June 2003 hourly visibility and temperature records for station CSI-3

46 
Figure 43. July 2003 hourly meteorological records for station CSI-3. 

47 
Figure 44. July 2003 hourly visibility and temperature records for station CSI-3

48 
Figure 45. August 2003 hourly meteorological records for station CSI-3. 

49 
Figure 46. August 2003 hourly visibility and temperature records for station CS

50 
Figure 47. September 2003 hourly meteorological records for station CSI-3. 

51 
Figure 48. September 2003 hourly visibility and temperature records for station 

52 
Figure 49. October 2003 hourly meteorological records for station CSI-3. 

53 
Figure 50. October 2003 hourly visibility and temperature records for station 

54 
Figure 51. July 2003 hourly meteorological records for station CSI-6. 

55 
Figure 52. July 2003 hourly temperature records for station CSI-6. 

56 
Figure 53. August 2003 hourly meteorological records for station CSI-6. 

57 
Figure 54. August 2003 hourly visibility and temperature records for station C

58 
Figure 55. September 2003 hourly meteorological records for station CSI-6. 

59 
Figure 56. September 2003 hourly visibility and temperature records for station C

60 
Figure 57. October 2003 hourly meteorological records for station CSI-6. 

61 
Figure 58. October 2003 hourly visibility and temperature records for station CSI

62 

64 

where u* is the friction velocity, ρ is the air density, Cp is the specific heat capacity at constant 

air pressure, T is the absolute air temperature at the reference height, κ is the von Karman 

constant, g is the gravitational acceleration, H is the vertical heat flux, and B is the Bowen ratio 

of sensible to latent heat flux. The parameters u*, B, and also the cloudiness are not normally 

measured in the marine environment. According to Arya (1999), the primary advantages of 

Pasquill's stability classification scheme as used in the OCD model are its simplicity and its 

requirement of only routinely available information from surface meteorological stations, such as 

mean surface wind speed, solar radiation, and cloudiness. Therefore, our purpose is to provide a 

practical way to estimate L using routinely recorded overwater measurements. 

At the air-sea interface, according to Hsu (1988), we have 

τ ρ = = u C * ρ d U 2

10

2 (7) 

and 

H C = − ρ p T C ( ) Tsea Tair U10 (8)

where τ is the shearing stress, Cd is the drag coefficient, CT is the sensible heat flux coefficient, 

Tsea and Tair (in °C) are sea and air temperatures, respectively; and U10 (in m s

-1) is the wind 

speed at 10 m above the surface. 

Substituting Eqs. (7) and (8) into (6), we obtain 

( ) z g zC T T

B T sea air

air z d

− +





 

 κ 1  0 0. 7

L ( ) T U C = − + 27316 2 3/2 . (9) 

where z is a reference height normally set to 10 m.

According to Hsu (1999) 

B T( ) sea Tair = − 0146

0 49

. . (10)

Data from the geographical regions of the Equatorial Atlantic and East China Sea were 

used to derive Eq. (10). Because the resulting correlation coefficient was 0.94, it is employed for 

this study as a first approximation. In a following section, we will show that slightly different 

relationships are found in the Gulf region. 

Now, if we group the measured parameters from routine ship or buoy observations, i.e., 

Tsea, Tair, and Uz, we get 
65 

z −

L

C CT d Rb A Rb = = κ 3/2

 (11)

where A needs to be determined from pertinent field measurements, and Rb is the bulk 

Richardson number. 

According to Smith (1980), CT = 1.10 * 10-3 under unstable (or Tsea > Tair) conditions and 

0.83 * 10-3 under stable (or Tair > Tsea) conditions; hence A is expected to be different for 

unstable vs. stable conditions. Also, under unstable conditions 

( )

( ) R

T U b

air z

= −

 

+

0 07

27316 2

.

.

g z T T

B sea air − + 1  (12)  



and under stable 

g z( ) T T

T U air z

−

( ) 273.16

Rb

sea air = + 2 (13)

Note that when Tsea < Tair, evaporation is suppressed and assumed here to be less active so that 

the water vapor correction term (1 + 0.07/B) is dropped. 

To identify the coefficients for Eq. (11), pertinent data sets provided by Hwang and 

Shemdin (1988) and Donelan et al. (1997) with Tair, Tsea, U10, and z/L measurements were 

incorporated in the analysis. Our results are shown in Fig. 59. If one accepts these high R2

values, Fig. 59 should be useful operationally. Now, substituting the proper value of "A" for 

unstable and stable conditions respectively, we have 

z

L

Rb = 10.2 (14) 

with R2

 = 0.87 for unstable condition, and 

L

z

Rb = 6 3. (15)

with R2

 = 0.97 for stable condition. Furthermore, by substituting Eq. (12) into (14) and 

rearranging, one gets 

( ) L

T U air z = − + 2732 2 . (16)

( ) T T

B sea air − +





 

 100 1  0 0. 7

for unstable conditions. Similarly, substituting Eq. (13) into (15), we have 
( ) T U

T T

air z

air sea

+

−

2732

62

2 .

( ) L = (17)

for stable conditions. 

Eqs. (16) and (17) are our proposed method to determine overwater L using routine 

measurements of wind and air and sea temperatures. 

Figure 59. An overwater relationship between z/L and Rb (see text for details). 

66 
67 

B. Bowen Ratio Variations Over the Gulf of Mexico 

The ratio of the sensible heat flux to the latent heat flux is commonly referred to as the 

Bowen ratio (see, e.g., Roll, 1965) and takes the form

( )

Hl L CT E ( ) qsea qair − B

H C C T T s p T sea air = = − (18)

where the specific humidity, qsea, is related to the saturation vapor pressure, esea, through (Hsu, 

1988) 

P sea   q

esea = 

  0 6. 2  (19) 

and 

[ ] ( ) esea

Tsea Tsea = ∗ + 61078 10 7 5 237 3 . . / . (20)

For the specific humidity and saturation vapor pressure of the air, the dew point 

temperature (Tdew) replaces Tsea in Eqs. (19) and (20). 

The Bowen ratio has been found to vary widely in the marine environment. Using 

surface data recorded during a strong cold air outbreak over the Gulf of Mexico in 1996, Hsu 

(1998) demonstrated that a compelling linear correlation between the vapor pressure difference 

and sea-air temperature difference does exist. With this result, and from Eq. (18), it was also 

shown that B can be related to the sea-air temperature difference alone as 

B a ( ) Tsea Tair

b = − (21)

Hourly data from several NDBC stations were used to derive values of a and b over the 

Gulf of Mexico, which varied from 0.077 to 0.078 and 0.67 to 0.71, respectively. Hsu (1999) 

went on to further verify Eq. (21) using data from a tropical ocean location and a cold air 

outbreak region, ultimately deriving Eq. (10). This equation has since been advocated for use by 

Hsu and Blanchard (2003) in marine atmospheric boundary layer applications. 

While very high correlations were established, relatively small data sets were used in 

formulating Eq. (10). Furthermore, Hsu (1998) showed several variations for Eq. (21) with a 

distinct difference between coastal and deep Gulf stations. The purpose of this analysis task is to 

investigate the spatial and temporal variations of Eq. (21) over the Gulf using long term records 

from available buoy, Coastal Marine Automated Network (C-MAN) stations, and our WAVCIS 

platform. 

Records from two C-MAN stations (GDIL1 and DPIA1), WAVCIS platform CSI-3, and 

11 moored buoys (42001, 42002, 42003, 42041, 42019,42020, 42036, 42039, 42040, 42007, and 

42035), ranging from the deep Gulf to near shore, were used (see Figs. 1 and 65 and Table 4 for 

locations). The period of record was 1999 through 2002, and was selected primarily for two 
reasons. First, hourly dew point temperatures generally became routine at most stations during 

1999 with the following exceptions: 42003, year 2000; 42019, year 2000; 42020, year 2000; 

42041, year 2002; and DPIA1, 2002. The complete year of 2002 from WAVCIS CSI-3 was 

used. 

Second, by using a multi-year record, any bias introduced into the data due to ENSO 

effects should be minimized. ENSO (El Nino / Southern Oscillation) is the term for the coupled 

ocean-atmosphere interactions in the tropical Pacific characterized by episodes of anomously 

high sea surface temperatures in the equatorial and tropical eastern Pacific; associated with largescale swings in surface air pressure between the western and eastern Pacific. ENSO is the most 

prominent source of interannual variability in weather and climate around the world (Geer, 

1996). El Nino and La Nina are the extremes of the ENSO cycle, and can produce large shifts in 

the usual weather patterns. For the southeastern U.S. and the Gulf of Mexico region, this can 

mean cooler, wetter winters (El Nino), or warmer, drier winters (La Nina). According to the 

NOAA Climate Diagnostics Center (2004) and the Climatic Prediction Center, 1999 experienced 

weak to moderate La Nina conditions which continued through most of 2000 (the third quarter 

being Neutral) until the second quarter of 2001, after which more Neutral conditions prevailed. 

Weak El Nino began in the second quarter of 2002 and strengthened slightly through the years 

end. 

The only data editing performed was to remove every hourly record which was missing 

any of the pertinent parameters: wind speed, pressure, or sea- air- or dew-point temperatures. 

Finally, since we were only considering unstable conditions, a subset was created containing 

only records in which Tsea > Tair. For each hour, the Bowen ratio was computed using Eqs. (18) 

through (20) with CT = 1.13 * 10-3 (Large and Pond, 1982); Cp = 1004 J kg-1; CE = 1.12 * 10-3

(Smith, 1988); and LT = 2.5 * 106

 J kg-1 (Hsu, 1988). Note that the wind speed parameter 

cancels out in Eq. (18); hence sensor height corrections for the various platforms was not 

necessary. 

The resulting unstable data sets were analyzed in two ways. A relationship between the 

Bowen ratio and sea-air temperature difference was derived for each station for the entire period 

of record. Since this usually included over 20,000 data points, the data was reduced into about 

20 classes with a range of 0.5 to 0.8°C. For each Class, a mean and standard deviation of the 

Bowen ratio was computed, and then the power curve (Eq. 21) fit to the data. The unstable data 

sets were also further separated into monthly sets, and Eq. (21) fit to the smaller record. 

Monthly analysis at each station yielded a family of curves similar to those shown in 

Figs. 60 and 61. This was not an unanticipated result, particularly for the coastal and near shore 

stations. Fall and winter frontal passages over this region bring colder and much drier 

continental air over the shelf waters. The large sea-air temperature differences, as well as those 

between qsea and qair (due to low dew point temperatures), produce a wide range of sensible and 

latent heat fluxes. This effect diminishes later in the season as the shelf waters cool. In summer, 

much smaller gradients are observed as conditions become more homogeneous. Nevertheless, 

the seasonal variations of the mean Bowen ratio are significant, as much as 2 to 3 times in 

magnitude at the near shore stations. This distribution extends even over the deep Gulf stations, 

although the variations are considerably smaller. 

68 
Figure 60. Seasonal distribution of Bowen ratio vs. temperature difference for buoy 42035 in the 

coastal waters. 

Figure 61. Seasonal distribution of Bowen ratio vs. temperature difference for buoy 42002 in the 

deep Gulf waters. 

69 
Extremely high correlation coefficients were obtained for the relationships derived from

most of the classified data sets. Somewhat poorer values were produced at DPIA1 and the 

WAVCIS station, however only one year of data was available at these locations. Figs. 62 and 

63 show examples of the near coast and deep Gulf, respectively. Clearly, much higher Bowen 

ratios and greater temperature differences generally occur near shore than over the deep Gulf, 

and these characteristics are thus described by different relationships. 

The various stations were then grouped into four categories - deep Gulf (buoys 42001, 

42002, 42003, and 42041); shelf break region (buoys 42019, 42020, 42036, 42039, and 42040); 

shelf waters (buoys 42007 and 42035, WAVCIS CSI-3); and coastal (C-MAN GDIL1 and 

DPIA1). Very similar relationships were found amongst the stations in each group. Little 

difference was noted between the coastal stations and those in the adjacent shelf waters, hence 

these regions were combined. 

Our final result is shown in Fig. 64. A mean relationship was derived from the stations 

within each region of coastal waters, shelf break, and deep Gulf. The relationship provided by 

Hsu (1999) and Hsu and Blanchard (2003) is also depicted. Each region is found to exhibit a 

unique relationship, with much higher ratios observed in the coastal region decreasing as one 

moves offshore. The relationship proposed by Hsu and Blanchard (2003) appears best suited to 

the coastal region, completely overestimating conditions over the deep Gulf and shelf waters 

over most of the sea-air temperature difference range. However, since it has been verified in 

such diverse geographic locations, it remains the recommended formula for operational use. 

Figure 62. Mean and standard deviation of Bowen ratio vs. temperature difference for classified 

data from buoy 42035 in the Gulf coastal waters. 

70 
71

Figure 63. Mean and standard deviation of Bowen ratio vs. temperature difference for classified 

data from buoy 42002 in the deep Gulf waters. 

Figure 64. Derived relationships between the Bowen ratio and sea-air temperature difference 

from the coast to the deep Gulf waters. Dotted line is Eq. (10) as proposed by Hsu 

(1999) and Hsu and Blanchard (2003). 
72 

C. Overwater Mixing Heights and Ventilation Factor 

Due to potential evaporation, the air over the water is usually moister than that over land, 

and the top of the marine boundary layer is often times capped by clouds (see, e.g., Garratt, 

1992). On the basis of analysis of vertical soundings by research aircraft, rawinsondings, and 

radar wind profilers and Radio Acoustic Sounding Systems (RASS), it has been shown by 

Garratt (1992) that the mixing height h = LCL, the lifting condensation level, under cumulus 

cloud conditions (where LCL = cloud base). The height of the LCL may be estimated by (see 

Hsu, 1998) 

H T LCL ( ) air Tdew = − 125 (22)

where HLCL is in meters and the dewpoint depression at the sea surface in degrees Celcius. 

 If Tdew is not available, it may be estimated by (Hsu, 1988, p. 21) 

e

e dew

air

air



 

− 



 

 7 5  61078 10 . log .

T =   237 3 61078 10 . log . (23)

From Hsu (1998) 

1

e Pq air air = 0 6. 2 (24)

and for operational applications (Hsu, 1998, Fig. 3) 

( ) q q (T T ) sea air sea air − = 56. . 8 + 0 37 − (25)

where qsea and esea are given by Eqs. (19) and (20), respectively. 

Since both Tsea and Tair can be obtained routinely by buoys, ships, and satellites, Eq. (22) 

can be used to estimate the mixing height if fair weather cumulus clouds are present. 

On the eastern and Gulf coasts of the U.S., as well as over the East China Sea, cold air 

outbreaks are common in the winter season. Under these conditions, according to Hsu (1997), 

the mixing height, Zi, is convectively unstable that 

0

Z w i v = + 369 6004( ) ' ' θ (26)

where Zi is in meters and ( w ) is the buoyancy flux in meters per second Kelvin. v ' ' θ

0
For operational applications, the buoyancy flux at the sea surface is found to be 

0 B ( ) w C v T U ( ) Tsea Tair ' . ' θ 10 1

0 07 = − +





 

  (27)

where CT = 1.1 * 10-3 under unstable conditions (Smith, 1980) and B is the Bowen ratio provided 

in Eq. (10). 

During certain periods, the sea-surface temperature is less than the air temperature. 

Therefore, the boundary layer is said to be stable. The mixing height under stable conditions, 

hstable, is (see Garratt, 1992) 

f stable h c

u L = * * (28)

where f if the Coriolis parameter. From limited measurements under stable conditions, the 

coefficient c*

 is found to be 0.11 as shown by Hsu and Blanchard (2003). Certainly, more field 

experiments are needed to further substantiate Eq. (28) and the value of c*

. 

Finally, the ventilation factor is given by the product of the computed mixed height and 

the wind speed measured by offshore buoys and platforms. The ventilation factor is related to 

pollution dispersion capacity as shown in Table 3. 

Table 3. 

Pollution dispersion forecast categories related to atmospheric ventilation (product of wind speed 

and mixing depth) (after Eagleman, 1996, based on the air pollution dispersal index used by the 

State of Colorado Department of Health in Denver). 

Pollution dispersion Ventilation (m2

 / sec) 

Bad 0 - 2000 

Fair 2001 - 4000 

Good 4001 - 6000 

Excellent 6001 or more 

73 
IV. APPLICATIONS 

As discussed in the previous section, spatial and temporal variations of the mixed layer 

over the Gulf of Mexico region are a function of the prevailing atmospheric stability. In this 

analysis, monthly mean values were obtained for pertinent NDBC C-MAN and buoy stations in 

the Gulf (see Fig. 65 and Table 4 for approximate locations). Parameters included air and sea 

temperatures, dew point temperatures, and average wind speeds. Table 4 includes the stations 

and the period of record for each parameter used here. Note that the dew point records are 

generally of much shorter duration. For our CSI-3 platform, monthly averages were computed. 

From sea-air temperature differences, mean L values were derived using Eqs. (16) and 

(17). Setting z = 10 m, z/L values were found to determine whether the stability was unstable 

(z/L < -0.4), neutral (|z/L| < 0.4), or stable (z/L > 0.4). Mixed height values were then computed 

using Eqs. (26), (22), and (28) for unstable, neutral, or stable, respectively. Finally, from the 

mean wind speed values, an estimated ventilation factor for each station was found. If a station 

reported insufficient or missing data so that a monthly mean was not available, then mixing 

height and ventilation factor were not computed. Note that no correction was made for 

anemometer height. The results are shown in Figs. 66 through 89. 

The seasonal progression of air and sea temperatures is clear, with cool shelf waters 

gradually warming in the springtime to near homogeneous conditions by late summer. In the 

fall, the cycle repeats as returning cold fronts again chill the shallower shelf.

Slightly unstable (z/L < 0) conditions are found at almost every station throughout the 

year. Stability Class D (or neutral) prevails; in the summer months several of the C-MAN 

stations achieve Class C (free convective). Buoys in the northeastern Gulf also approach Class C 

in the winter months, with buoy 42040 surpassing the mark in December. 

Mean mixed heights over the open Gulf typically range between 400 - 800 m, with higher 

values over the deeper waters. Areas of low ventilation factor index (hence, poor dispersion 

capability) are seen almost every month, mostly along the coastal areas. In the winter months, 

the low ventilation factor is mainly due to lower mean mixed heights, while in summer light 

average winds are the dominant factor. 

75 
Table 4. 

Locations and pertinent periods of record for coastal and offshore stations used in this report. 

Station Type

Location 

Lat N Lon W

Air 

Temperature 

Sea 

Temperature 

Dew 

Temperature Wind Speed 

BVILE ASOS 29.20 84.24

BUSL1 C-MAN 27.88 90.90 1/91 - 4/93 1/91 - 12/93 1/91 - 12/93 

CSI-3 WAVCIS 29.44 92.06 11/01 - 10/03 11/01 - 10/03 11/01 - 10/03 11/01 - 10/03 

CSI-6 WAVCIS 28.87 90.48

DPIA1 C-MAN 30.25 88.07 1/87 - 12/01 1/87 - 12/98 2/97 - 7/97 1/87 - 12/01 

DRYF1 C-MAN 24.64 82.86 12/92 - 12/01 12/92 - 12/01 12/92 - 12/01 

GBCL1 C-MAN 27.80 93.10 9/89 - 9/92 9/89 - 9/92 9/89 - 9/92 

GDIL1 C-MAN 29.27 89.96 12/84 - 12/01 12/84 - 12/01 5/89 - 12/01 12/84 - 12/01 

LONF1 C-MAN 24.84 80.86 11/92 - 12/01 11/92 - 12/01 11/92 - 12/01 

MLRF1 C-MAN 25.01 80.38 12/87 - 12/01 12/87 - 12/01 12/87 - 12/01 

MPCL1 C-MAN 29.40 88.60 2/88 - 7/92 2/88 - 7/92 2/88 - 7/92 

PTAT2 C-MAN 27.83 97.05 3/84 - 12/01 10/89 - 12/01 4/97 - 12/01 3/84 - 12/01 

SANF1 C-MAN 24.46 81.88 1/91 - 12/01 5/91 - 12/01 1/91 - 12/01 

SMKF1 C-MAN 24.63 81.11 2/88 - 12/01 2/88 - 12/01 3/97 - 12/01 2/88 - 12/01 

SRST1 C-MAN 29.67 94.05 2/84 - 12/01 4/97 - 12/01 2/84 - 12/01 

VENF1 C-MAN 27.07 82.45 1/94 - 12/01 1/94 - 12/01 4/97 - 12/01 1/94 - 12/01 

42001 Buoy 25.84 89.66 8/75 - 12/01 8/75 - 12/01 8/75 - 12/01 8/75 - 12/01 

42002 Buoy 25.17 94.42 6/73 - 12/01 9/76 - 12/01 6/73 - 12/01 6/73 - 12/01 

42003 Buoy 26.01 85.91 11/76 - 12/01 11/76 - 12/01 6/00 - 12/01 11/76 - 12/01 

42005 Buoy 30.00 85.90 12/78 - 5/80 12/78 - 5/80 12/78 - 5/80 

42007 Buoy 30.09 88.77 1/81 - 12/01 1/81 - 12/01 12/98 - 12/01 1/81 - 12/01 

42008 Buoy 28.70 95.30 10/80 - 7/84 10/80 - 11/83 10/80 - 7/84 

42019 Buoy 27.91 95.36 5/90 - 12/01 5/90 - 12/01 5/93 - 12/01 5/90 - 12/01 

42020 Buoy 26.95 96.70 5/90 - 12/01 5/90 - 12/01 12/98 - 12/01 5/90 - 12/01 

42035 Buoy 29.25 94.41 5/93 - 12/01 5/93 - 12/01 8/98 - 12/01 5/93 - 12/01 

42036 Buoy 28.51 84.51 1/94 - 12/01 1/94 - 12/01 11/98 - 12/01 1/94 - 12/01 

42039 Buoy 28.80 86.06 12/95 - 12/01 12/95 - 12/01 5/97 - 12/01 12/95 - 12/01 

42040 Buoy 29.21 88.20 12/95 - 12/01 12/95 - 12/01 12/95 - 12/01 12/95 - 12/01 

42041 Buoy 27.50 90.46

*ASOS - Automated Surface Observing System (NOAA / FAA) - Boothville, LA (BVILE) 

 WAVCIS - Wave Current Information System (LSU) 

 C-MAN - Coastal-Marine Automated Network (NDBC) - Bullwinkle, LA (BUSL1), Dauphin 

Island, AL (DPIA1), Dry Tortugas, FL (DRYF1), Garden Banks, LA (GBCL1), Grand Isle, LA 

(GDIL1), Long Key, FL (LONF1), Molasses Reef, FL (MLRF1), Main Pass, LA (MPCL1), Port 

Aransas, TX (PTAT2), Sand Key, FL (SANF1), Sombrero Key, FL (SMKF1), Sabine, TX 

(SRST2), and Venice, FL (VENF1) 

 Buoy - offshore moored stations (NDBC) 

76 
77 

Figure 65. Approximate locations of National Data Buoy Center Coastal-Marine Automated Networ along with WAVCIS platforms CSI-3 and CSI-6 superimposed on a MODIS satellite ima
Figure 66. January monthly mean values of air temperature, sea temperature, and stability (z/L). 

78 
Figure 67. January monthly mean mixed heights (m) and ventilation factor index. 

79 
Figure 68. February monthly mean air temperature, sea temperature, and stability (z/L). 

80 
Figure 69. February monthly mean mixed heights (m) and ventilation factor index. 

81 
Figure 70. March monthly mean air temperature, sea temperature, and stability (z/L). 

82 
Figure 71. March monthly mean mixed height (m) and ventilation factor index. 

83 
Figure 72. April monthly mean air temperature, sea temperature, and stability (z/L). 

84 
Figure 73. April monthly mean mixed height (m) and ventilation factor index. 

85 
Figure 74. May monthly mean air temperature, sea temperature, and stability (z/L). 

86 
Figure 75. May monthly mean mixed height (m) and ventilation factor index. 

87 
Figure 76. June monthly mean air temperature, sea temperature, and stability (z/L). 

88 
Figure 77. June monthly mean mixed height (m) and ventilation factor index. 

89 
Figure 78. July monthly mean air temperature, sea temperature, and stability (z/L). 

90 
Figure 79. July monthly mean mixed height (m) and ventilation factor index. 

91 
Figure 80. August monthly mean air temperature, sea temperature, and stability (z/L). 

92 
Figure 81. August monthly mean mixed height (m) and ventilation factor index. 

93 
Figure 82. September monthly mean air temperature, sea temperature, and stability (z/L). 

94 
Figure 83. September monthly mean mixed height (m) and ventilation factor index. 

95 
Figure 84. October monthly mean air temperature, sea temperature, and stability (z/L). 

96 
Figure 85. October monthly mean mixing height (m) and ventilation factor index. 

97 
Figure 86. November monthly mean air temperature, sea temperature, and stability (z/L). 

98 
Figure 87. November monthly mean mixed height (m) and ventilation factor index. 

99 
Figure 88. December monthly mean air temperature, sea temperature, and stability (z/L). 

100
Figure 89. December monthly mean mixing height (m) and ventilation factor index. 

101
With the deployment of our second visibility station on WAVCIS platform CSI-6, we 

were able to record three months of simultaneous measurements during August through October 

2003. Our two stations are added to NDBC station GDIL1, 42041, and 42001 here to create a 

"line" extending from the coast across the shelf to the deep Gulf. 

Table 5 lists the frequency of occurrence of stability classes at each station. In August, 

free convective conditions dominate the near shore areas (GDIL1 and CSI-3), but become less 

frequent offshore. This is particularly evident at CSI-6, where smaller sea-air temperature 

differences produce more near-neutral conditions. Free convective classes decline further at all 

stations in September and October. 

Monthly mean mixed heights are shown in Fig. 90. Generally good agreement is found 

amongst all stations except CSI-6. This is found to be due to the greater observance of Class D 

and the relatively small mean dew point depression at this location. Station 42001 exhibits a 

continuous increase, again due to more numerous Class D and slightly larger dew point 

depression in October. 

All available data from CSI-3 was combined to produce an annual distribution of stability 

and mixed height. From Table 6, the seasonal change of stability regime from near neutral to 

free convective in the summer months is clear. It should also be noted that stable conditions are 

observed in the winter months, but normally less than 5% of the time. Nevertheless, extremely 

low mixed heights can occur. 

Figure 90. August to October 2003 computed mean mixed heights from the Louisiana coast to 

deep Gulf. 

102
Table 5. 

Frequency of occurrence (%) of stability classes, August to October 2003 

Month Category GDIL1 CSI-3 CSI-6 42041 42001

< 23.7 16.4 6.9 22.2 19.8

B 16.7 16.6 7.6 13.5 14.9

C 30.2 31.9 15.8 20.6 22.6

D 29.3 34.3 66.3 43.7 42.6

E 0.1 0.1 1.1 0 0

F 0 0.1 0.8 0 0

August 

> 0 0.5 1.5 0 0.1

< 11.0 19.0 5.6 8.6 11.1

B 11.3 13.8 5.9 7.8 6.5

C 20.6 28.8 14.3 13.4 15.6

D 56.9 38.3 73.5 70.2 66.4

E 0.3 0 0.2 0 0

F 0 0.1 0.2 0 0

September 

> 0 0 0.4 0 0.4

< 8.9 9.3 3.7 5.9 4.9

B 6.1 11.8 8.1 4.2 5.5

C 21.6 27.4 18.2 20.8 16.8

D 61.6 50.5 68.2 68.9 72.8

E 1.2 0.3 0.9 0 0

F 0.3 0 0.4 0 0

October 

> 0.3 0.7 0.6 0.1 0

Table 6. 

Monthly frequency of occurrence (%) of stability class at CSI-3 

Month < B C D E F >

JAN 6.0 7.3 20.9 59.1 4.6 1.4 0.6

FEB 9.4 6.7 18.4 62.4 2.1 0.4 0.5

MAR 5.1 5.0 14.8 69.5 2.0 0.9 2.6

APR 5.6 5.6 21.0 66.8 0.2 0.1 0.5

MAY 6.6 7.9 26.1 59.3 0 0 0

JUN 9.5 11.6 31.4 47.3 0.1 0 0.1

JUL 16.5 15.0 36.6 31.6 0.1 0 0.1

AUG 15.8 17.2 30.7 35.8 0.1 0 0.3

SEP 13.4 9.9 23.3 52.7 0.3 0.1 0.2

OCT 9.7 11.7 27.0 50.6 0.1 0 0.3

NOV 6.8 7.4 18.3 64.7 0.8 0.7 1.3

DEC 8.1 8.3 19.4 63.0 0.1 0.4 0.5

103
Records from NDBC GDIL1, 42041, and 42001 are again employed to compare with 

CSI-3 mean annual mixed heights. Results are listed in Table 7 and depicted in Fig. 91. Mean 

annual mixed height at CSI-3 is 445 m with a standard deviation of 81 m. As anticipated, the 

near-shore stations exhibit the highest heights in summer months and lowest in winter, while the 

pattern for the deeper water stations is opposite. 

Table 7. 

Annual variation of average computed mixed height extending from shoreline to deep Gulf 

during the approximate period of November 2001 through October 2003. 

Month GDIL1 CSI-3 42041 42001

January 550 426 606 666

February 316 435 448 556

March 340 340 342 459

April 456 434 488 498

May 532 479 480 417

June 494 504 448 488

July 459 485 462 508

August 494 507 484 509

September 525 517 523 534

October 489 490 517 565

November 485 483 624 712

December 421 424 637 710

GDIL1 March 2002 - December 2003 

42041 May 2002 - December 2003 

42001 January 2002 - December 2003 

104
Figure 91. Annual distribution of computed mixed heights from the Louisiana coast to de

105
V. SOME CHARACTERISTICS OF FOG AND HAZE ALONG THE LOUISIANA COAST 

Automated visibility sensors such as the ones deployed at CSI-3 and CSI-6 provide 

invaluable point measurements of visual range, however they cannot describe the conditions 

which may be causing a reduction of that range. To address this problem, NOAA (1998) has 

developed an algorithm for its Automated Surface Observing System (ASOS) stations which 

relies on temperature, dew point, and present weather in addition to visibility to differentiate 

prevailing conditions. 

When the surface visibility drops below 7 statute miles, the algorithm obtains the current 

dew point depression to distinguish between fog, mist, or haze. If the dew point depression is 

less than or equal to 4°F (about 2°C), then mist is reported if the visibility is between 5/8 and 7 

miles; if less than 5/8 mile, fog is reported. If these conditions are met and precipitation is 

occurring, fog or mist will still be reported. If the dew point depression is greater than 4°F 

(about 2°C) and no precipitation is observed, then haze is reported. While other sources (such as 

smoke, dust, or blowing sand) may certainly be the cause of reduced surface visibility, only fog, 

mist, or haze are indicated by the automated system. 

Fog is a visible aggregate of many minute water droplets that are suspended in the 

atmosphere near the earth's surface. Fog differs from cloud only in that the base is at the surface, 

while clouds are above the surface (Huschke, 1959). According to Hsu (1988), fog can occur 

along the Louisiana coast in winter months when warm, moist air flows northward over the cool 

shelf waters. The air gives off heat to the surface waters, which eventually reduces the 

temperature to the dew point and condensation occurs. Due to lesser frictional effect over water, 

fog can occur even if winds are relatively strong. This effect is further enhanced by drainage of 

cold, fresh water from the Mississippi River and bays and estuaries. Due to the potential hazards 

to marine operations and surface traffic, the National Weather Service offices at Lake Charles 

and Slidell have conducted research into Louisiana coastal fog, including a sea fog forecasting 

decision tree (see, e.g., Erickson (2001), and National Weather Service (2003)). 

Haze, on the other hand, is a suspension in the air of extremely small dry particles or 

aerosols invisible to the naked eye but sufficiently numerous to give the air an opalescent 

appearance (Byers, 1974). Growing concerns about reductions in visibility at scenic and 

wilderness (Class I) areas caused by haze prompted the Regional Haze Rule (Federal Register, 

1999), which requires monitoring in order to track progress toward the national visibility goal. 

The Breton National Wildlife Refuge is a Class I area located north of the Mississippi River 

Delta. Since the EPA defines regional haze as "...visibility impairment that is produced by a 

multitude of sources and activities which emit fine particles and their precursors and which are 

located across a broad geographic area" (Malm et al., 2000), we extend our study westward 

along the Louisiana coast to incorporate available NOAA stations and our instrumented 

platforms. The potential for haze formation in the OCS environment may be enhanced due to the 

contribution of both land-based and oceanic sources. Natural and anthropogenic aerosols may 

drift seaward from land or from numerous production platforms. Over oceans, aerosols 

composed of sea salts continuously originate from drops ejected into the air when air bubbles in 

breaking waves burst at the ocean surface. Smaller aerosols are produced when the upper part of 

an air bubble film bursts. When the relative humidity rises, aerosols may increase in size, 

107
forming a haze which reduces visibility by scattering light. For example, the amount of light that 

sea-salt particles remove from a beam by scattering increases by about a factor of three as the 

relative humidity increases from 60 to 80% (Wallace and Hobbs, 1977). 

A simplified version of the ASOS visibility algorithm was applied to the hourly data 

records from stations CSI-3, CSI-6, Boothville, and GDIL1 during the measurement period of

November 2001 through October 2003. For this analysis, any hourly observation of visibility 

less than 7 miles was designated as "reduced" and retained. The dew point depression was then 

applied to separate into either fog or haze; no distinction was made for mist. Furthermore, since 

present weather observations are typically unavailable at most stations, no effort was made to 

identify or remove events associated with precipitation. Our results are shown in Figs. 92 

through 98. Note that the bar graphs showing both fog and haze depict the potential occurrence 

of these conditions based on the criteria previously described, while the bar graphs showing 

visibility alone represent actual hours of severely reduced visibility recorded, regardless of cause. 

At each station, the potential for fog formation is most frequently observed during the winter 

season months of January through March. Reduced visibility due to fog can be expected from

about 20 to 40% of the time (or about 6 to 12 entire days); with slightly higher frequency at the 

coastal stations than offshore. Surface visibility can become very restricted, being equal to or 

less than one mile about 7 - 10% of the time (2 - 3 entire days). Fog conditions can occur in 

every month. 

Impaired visibility attributed to haze is also observed year round, however the frequency 

of occurrence is much less, generally less that 5% (about 1.5 day) per month. Peaks occur in the 

winter months and late summer (August - September), and more often along the coast. 

Reductions in surface visibility associated with haze are smaller than those that can result from

fog conditions. 

108
Figure 92. Frequency of occurrence of fog and haze as defined by the ASOS at CSI-3

 records for May and October 2002 are less than 75% complete. Fog can o month. 

109
Figure 93. Frequency of occurrence of restricted visibility at CSI-3. 

110
Figure 94. Frequency of occurrence of fog and haze as defined by the ASOS at st

111
Figure 95. Frequency of occurrence of fog and haze as defined by the ASOS at 

 station. 

112
Figure 96. Frequency of occurrence of restricted visibility at the Boothville station. 

113
Figure 97. Frequency of occurrence of fog and haze as defined by the ASOS a

114
Figure 98. Frequency of occurrence of restricted visibility at NDBC GDIL1. 

Monthly wind and visibility 'roses' are shown for CSI-3 and Boothville in Figs. 99 

through 110. Reduced visibility at both stations is most often associated with wind flow from

the east; southeast and east during January through May, and northeast to southeast during 

September through December. The summer months of June through August have a more even 

distribution with a slight westerly peak, but the frequencies are less than about 2%. Since we 

have shown that reduced visibility along the Louisiana coast is almost entirely caused by fog 

conditions, the dominance of east-southeast wind flow supports Hsu's (1988) scenario as 

described previously. Note that the analysis of Boothville data only considered hourly records in 

which both visibility and wind direction were reported; calm (no wind) reports were not 

included. 

115
Figure 99. January wind rose for CSI-3 (top), and visibility 'roses' for the CSI-3 and Boothville 

stations (bottom). 

116
Figure 100. February wind rose for CSI-3 (top), and visibility 'roses' for the CSI-3 and 

Boothville stations (bottom). 

117
Figure 101. March wind rose for CSI-3 (top), and visibility 'roses' for the CSI-3 and Boothville 

stations (bottom). 

118
Figure 102. April wind rose for CSI-3 (top), and visibility 'roses' for the CSI-3 and Boothville 

stations (bottom). 

119
Figure 103. May wind rose for CSI-3 (top), and visibility 'roses' for the CSI-3 and Boothville 

stations (bottom). 

120
Figure 104. June wind rose for CSI-3 (top), and visibility 'roses' for the CSI-3 and Boothville 

stations (bottom). 

121
Figure 105. July wind rose for CSI-3 (top), and visibility 'roses' for the CSI-3 and Boothville 

stations. 

122
Figure 106. August wind rose for CSI-3 (top), and visibility 'roses' for the CSI-3 and Boothville 

stations (bottom). 

123
Figure 107. September wind rose for CSI-3 (top), and visibility 'roses' for the CSI-3 and 

Boothville stations (bottom). 

124
Figure 108. October wind rose for CSI-3 (top), and visibility 'roses' for the CSI-3 and Boothville 

station. 

125
Figure 109. November wind rose for CSI-3, and visibility 'roses' for the CSI-3 and Boothville 

stations (bottom). 

126
Figure 110. December wind rose for CSI-3 (top), and visibility 'roses' for the CSI-3 and 

Boothville stations (bottom). 

127
VI. APPLICATIONS OF SATELLITE IMAGERY DURING PERIODS OF REDUCED 

VISIBILITY OVER THE NORTHERN GULF OF MEXICO 

Very few surface observing stations in the Gulf of Mexico region monitor visibility. 

Therefore, one task of this project as originally proposed was to take advantage of satellite 

imagery, in particular that provided by the MODIS platform, to help differentiate natural from

anthropogenic sources as well as the spatial distribution of overwater haze. Unfortunately, over 

the course of our measurement period, several factors were encountered which greatly limited 

our use of satellite data in this analysis. 

As discussed in the previous section, episodes of reduced visibility attributed to haze (by 

ASOS definition) over the Louisiana coastal region occur fairly infrequently. In most cases, 

haze conditions were also of short duration, on the order of a few hours. High resolution 

imagery, such as provided by MODIS and other NOAA polar orbiting satellites, was only 

available for one or two passes per day, and rarely coincided with our identified haze times. 

Some of the haze cases occurred during night hours. Visual imagery, including the 250 m

resolution MODIS, is not discernable at night. In many cases for which imagery was available, 

it was found that near-surface (low atmosphere) features over the northern Gulf were obscured 

by higher level cloud. 

It should also be noted that the development and implementation of image processing 

algorithms for the MODIS data was delayed; becoming routinely available on the LSU Earth 

Scan system between March and June 2003. Regardless, some imagery was captured and 

incorporated into our analysis, and a few examples are now presented. All satellite imagery was 

captured, processed, and analyzed by the Earth Scan Lab, LSU. The weather charts presented 

were obtained via the web through the NOAA National Data Center webpage 

(http://nndc.noaa.gov). 

In almost all of the following cases, the observed wind speeds were less than about 6 - 7 

m s-1. This is below the threshold necessary to produce numerous breaking waves, therefore the 

reduction in visibility contributed by sea spray is likely negligible. 

A. November 25, 2001 

A weak, stationary frontal boundary extends over the southeastern U.S. to the Mississippi 

River Delta during 00 - 03 UTC 26 November (Fig. 112). Surface flow over southern Louisiana 

and the northwest Gulf is light (about 10 kts) easterly. During the hours of 00 - 02 UTC (18 - 20 

CST 25 November), visibility at CSI-3 drops to an average of 6.0 miles, while the dew point 

depression is 5.2°C (see Fig. 111). Areas of Marginal Visual Flight Rules (MVFR) are indicated 

along the Texas and Louisiana coasts (Fig. 113), and haze is indicated. GOES satellite imagery 

depicts a large area of low cloud extending from the Texas coastal region into southwest 

Louisiana and over the shelf waters (Fig. 114). Note that the earlier drop in visibility recorded at 

BVE (Fig. 111, 04-05 CST) was likely due to a line of precipitation which developed along the 

front and extended southward to the Mississippi River Delta. 

129
Figure 111. Time series (CST) of observed parameters on 25 November 2001. 

130
Figure 112. NWS surface weather charts for 0000Z and 0300Z 26 November 2001. 

131
Figure 113. 0400Z 26 November 2001 NWS Weather Depiction chart. 

Figure 114. 1910 CST 25 November 2001 GOES-8 infrared image. 

132
Figure 115. Time series (CST) of observed parameters on 4 - 5 March 2002. 

B. 4 - 5 March 2002 

Weather over the northwestern Gulf of Mexico is controlled by a strong center of high 

pressure which migrates to the east-northeast over the period of 2100 UTC 4 March to 0600 

UTC 5 March (Fig 116). Light winds and large air-sea temperature differences produce free 

convective conditions over the Louisiana shelf. During the hours of 01 to 06 UTC 5 March, 

average visibility at CSI-3 drops to 3.8 mile, while the average dew point depression is 10.0°C 

(Fig. 115). Computed mixed height is approximately 443 m. No restrictions are indicated on the 

weather charts, however patches of low cloud are seen over eastern Texas, and radar shows an 

area of rain (Fig. 117). No significant cloud or feature is found directly over CSI-3; on the other 

hand large fires on the southwest Louisiana coast and their associated smoke plumes are detected 

in the satellite imagery (Figs. 118 - 120). Note that the low visibility appears to coincide with a 

wind shift; clear with northerly flow, drops when direction becomes westerly, then clears again 

with shift to stronger easterly flow. Since air temperature and dew point depression remain fairly 

constant, the reduced visibility appears to be due to a local source 

133
Figure 116. NWS surface weather charts for 0000, 0300, and 0600 Z 5 March 2002. 

134
Figure 117. NWS Radar Summary chart for 0314 Z 5 March 2002. 

Figure 118. 1400 CST 4 March 2002 NOAA-16 visible image. 

135
Figure 119. 1637 CST 4 March 2002 NOAA-12 visible image (top) and near-infrared image 

(bottom). The dark areas along the southwest Louisiana coast are fires, and a large 

plume is evident trailing off to the south-southeast. 

136
Figure 120. 1813 CST 4 March 2002 NOAA-15 near-infrared image. 

C. 27 - 28 March 2002 

Louisiana coastal weather is largely controlled by a high pressure ridge which extends 

southwestward from a center over the eastern U.S. (Fig. 122). During the period, the high moves 

eastward, allowing greater return flow of moist Gulf air over the state. Winds are easterly 

becoming more southerly, and light to nearly calm (Fig. 121). Areas of MVFR and Instrument 

Flight Rules (IFR) are depicted along the northwest Gulf coast (Fig. 123), attributed to fog; 

coastal stations become mostly overcast. Satellite imagery shows a large mass of low and high 

cloud progressing east-northeastward over the shelf waters (Figs. 124 - 128). Unstable 

conditions prevail at CSI-3, but moderate to Class C and D as winds increase and sea-air 

temperature difference decreases. Average computed mixed height is 403 m.

137
Figure 121. Time series (CST) of observed parameters on 27 - 28 March 2002. 

138
Figure 122a and b. NWS Surface Weather Charts for 0300 and 0600 Z 28 March 2002. 

139
Figure 122 c and d. NWS Surface Weather charts for 0900 and 1200 Z 28 March 2002. 

140
Figure 123. NWS Weather Depiction charts for 07, 10, and 13 Z 28 March 2002, top to bottom,

respectively. 

141
Figure 125. 2039 CST 27 March 2002 GOES-8 infrared image. 

Figure 124. 1917 CST 27 March 2002 NOAA-15 infrared image. 

142
Figure 126. 0136 CST 28 March 2002 NOAA-16 infrared image. 

Figure 127. 0609 CST 28 March 2002 GOES-8 infrared image. 

143
Figure 128. 0739 CST 28 March 2002 NOAA-15 visible image (top) and infrared image 

(bottom). 

144
D. 10 April 2002 

A stationary frontal boundary is positioned near New Orleans and extends southsouthwestward into the Gulf (Fig. 130). Winds over the state are mostly light and variable to 

calm. An area of MVFR appears briefly along the coast; but there are no other indications of 

either fog or haze (Fig. 131). Considerable cloud coverage exists over the entire region (Figs. 

132 - 136). Note onset of reduced visibility at CSI-3 occurs when winds shift from northeasterly 

to southerly and speeds decrease (Fig. 129). Unstable conditions become neutral as winds 

strengthen and sea-air temperature difference decreases. Average computed mixed height is 430 

m.

Figure 129. Time series (CST) of observed parameters on 10 April 2002. 

145
Figure 130. NWS Surface Weather charts for 21 Z 10 April (top) and 00 Z 11 April 2002 

(bottom). 

146
Figure 131. NWS Weather Depiction chart for 19 Z 10 April 2002. 

Figure 132. 1221 CDT 10 April 2002 MODIS true color image. 

147
Figure 133. 1456 CDT 10 April 2002 NOAA-16 visible image (top) and infrared image 

(bottom). 

148
Figure 134. 1751 CDT 10 April 2002 NOAA-16 visible image (top) and infrared image 

(bottom). 

149
Figure 135. 1939 CDT 10 April 2002 GOES-8 infrared image. 

Figure 136. 2039 CDT 10 April GOES-8 infrared image. 

150
E. 30 August 2002 

A low and trough extend through the central Gulf of Mexico, but almost the entire U.S. is 

under the influence of high pressure (Fig. 138). Flow over Louisiana is northeast to east, and 

widespread haze and fog are indicated, producing MVFR and IFR over the shelf waters (Fig. 

140). Scattered cloudiness appears over the Gulf, and extensive cumulus develops over land 

areas, however CSI-3 appears to remain mostly clear (Figs. 141 - 144). Class C conditions 

moderate to just barely Class D, and average computed mixed height is 531 m.

Figure 137. Time series (CST) of observed parameters on 30 August 2002. 

151
Figure 139. NWS Radar Summary for 1515 Z 30 August 2002. 

Figure 138. NWS Surface Weather chart for 1500 Z 30 August 2002. 

152
Figure 140. NWS Weather Depiction charts for 13 Z and 16 Z 30 August 2002. 

153
Figure 141. 0945 CDT 30 August 2002 GOES-8 visible image (top) and infrared image 

(bottom). 

154
Figure 142. 1015 CDT 30 August 2002 GOES-8 visible image (top) and infrared image 

(bottom). 

155
Figure 143. 1146 CDT 30 August 2002 NOAA-17 visible image (top) and infrared image 

(bottom). 

156
F. 14 - 15 September 2002 

Figure 144. 1234 CDT 30 August 2002 MODIS true color image. 

Louisiana is situated between the remnants of Tropical Depression Hanna over Alabama 

and an advancing low center / frontal boundary over Texas (Fig. 146). Winds are mostly 

westerly and light to calm (Fig. 145). There are widespread indications of haze and fog, the 

latter along the Louisiana coast produces MVRF and IFR restrictions (Fig. 147). There is 

considerable cloud cover moving into Louisiana from eastern Texas, however CSI-3 appears 

mostly clear during the period (Figs. 149 - 152). September 15 begins as neutral but becomes 

free convective as wind speeds decrease. Average computed mixed height during hours 23 - 02 

CST is 536 m, lowering to 395 m during hours 07 - 09 CST. 

157
Figure 145. Time series (CST) of observed parameters on 14 - 15 September 2002. 

158
Figure 146. NWS Surface Weather Charts for 0600, 1200, and 1500 Z 15 September 2002 (top 

to bottom, respectively). 

159
Figure 147. NWS Weather Depiction charts for 07Z (top) and 13Z (bottom) 15 September 2002. 

Figure 148. NWS Radar Summary chart for 1415Z 15 September 2002. 

160
Figure 149. 0145 CDT 15 September 2002 GOES-8 infrared image. 

Figure 150. 0309 CDT 15 September 2002 NOAA-16 infrared image. 

161
Figure 151. 0915 CDT 15 September 2002 GOES-8 visible image (top) and infrared image 

(bottom). 

162
Figure 152. 1049 CDT 15 September 2002 NOAA-16 visible image (top) and infrared image 

(bottom). 

163
G. 14 - 15 December 2002 

High pressure is centered over the Louisiana coast. Surface winds begin light northerly 

and shift to southerly by the end of the period as the high moves eastward (Figs. 154 - 155). 

Wind speeds are extremely light and widespread dense fog is indicated along southern Louisiana 

(Figs 153, 156, 157). Satellite reveals patches of low cloud in the vicinity of CSI-3 (Figs. 158 - 

161). Weak winds combined with large air-sea temperature differences produce unstable 

conditions, with average computed mixed height of 414 m.

Figure 153. Time series (CST) of observed parameters on 14 - 15 December 2002. 

164
Figure 154. NWS Surface Weather charts for 0000 Z (top) and 0600 Z (bottom) 15 December 

2002. 

165
Figure 155. NWS Surface Weather charts for 1200 Z (top) and 18 Z (bottom) 15 December 

2002. 

166
Figure 156. NWS Weather Depiction charts for 07 Z (top) and 10 Z (bottom) 15 December 

2002. 

167
Figure 157. NWS Weather Depiction charts for 13 Z (top) and 16 Z (bottom) 15 December 

2002. 

168
Figure 158. 1918 CST 14 December 2002 NOAA-15 infrared image. 

Figure 159. 2015 CST 14 December 2002 GOES-8 infrared image. 

169
Figure 160. 0615 CST 15 December 2002 GOES-8 infrared image. 

170
Figure 161. 0740 CST 15 December 2002 NOAA-15 visible image (top) and infrared image 

(bottom). 

171
H. 26 April 2003 

Weather over Louisiana is dominated by a high pressure ridge building in behind a cold 

frontal boundary (Figs. 163 - 164). Winds are mostly variable from north and light (Fig. 162). 

There are no indications of either fog or haze at any reporting stations in southern Louisiana. At 

CSI-3, stability improves from Class B to D, and average computed mixed height is 983 m. 

Satellite depicts clear skies over the northwestern Gulf of Mexico; however closer inspection 

reveals what appears to be marsh fires to the southeast of White Lake, with plumes extending 

towards CSI-3 (Figs. 165- 169). Note that when the wind shifts to the northeast and strengthens, 

visibility improves. 

Figure 162. Time series (CST) of observed parameters on 26 April 2003. 

172
Figure 163. NWS Surface Weather Chart for 2100 Z 26 April 2003. 

Figure 164. NWS Surface Weather charts for 0000 Z (top) and 0300 Z (bottom) 27 April 2003. 

173
Figure 165. 1149 CDT 26 April 2003 MODIS true color image (top); close-up (bottom)

identifies fires and smoke plume originating on the central Louisiana coast. 

174
Figure 166. 1459 CDT 26 April 2003 NOAA-16 visible (top) and infrared (middle) images; 

bottom panel is close-up of middle IR image. 

175
Figure 167. 1545 CDT 26 April 2003 GOES-12 visible image (top) and infrared image 

(bottom). 

176
Figure 169. 2015 CDT 26 April 2003 GOES-12 infrared image. 

Figure 168. 1958 CDT 26 April 2003 NOAA-16 infrared image. 

177
178

σ y

Total Width of Plume = 7 8.

I. Satellite Detection of Plume Geometry 

Under certain conditions, the coastal and shelf waters region of the northern Gulf of 

Mexico can be influenced by emitting sources located further inland. This was demonstrated in 

two of the case studies presented, in which it appeared that offshore visibility was temporarily 

reduced due to smoke plume drift from an onshore fire. If the fires cover a large area (such as a 

controlled marsh burn) or are sustained for some time, the resultant plumes can extend a 

considerable distance over the shelf waters before significantly dissipating. Since such a feature 

can easily be detected by satellite (in the daytime), we will now show that lateral plume 

geometry may be determined through remotely-sensed imagery. 

On 6 December, 1999 at 2134 Z (1534 CST), a large plume was detected during the 

overpass of the NOAA-14 satellite (Fig. 170). High pressure was centered over western 

Louisiana (see Figs. 171 and 172), and winds were fairly brisk out of the north-northwest. Cool, 

dry air prevailed as the high eventually moved northeastward and wind diminished. 

Channel 2 of the NOAA-14 Advanced Very High Resolution Radiometer data was 

analyzed as shown in Fig. 170. Note that the pixel width and height are 1.1 km. This spectral 

channel was selected because it provides a measure of reflectance, hence a better contrast 

between the higher cloud and the water surface beneath. XVU software was used to analyze the 

image and find a mean, or threshold, value for the water area outside of the plume. The 

threshold value was input into a C program, which then examined each pixel in the image to 

identify the latitude and longitude of the plume boundaries (by difference from the threshold). 

Horizontal distances between the endpoints were computed, and the coordinates of the plume 

centerline position found. Finally, the distance from the fire to each point in the centerline was 

generated (Arnone, personal communication). 

According to statistics (see, e.g., Spiegel, 1961, p. 343), the total width (i.e., 100% 

coverage) of the crosswind standard deviation (σy) is (3.9 σy + 3.9 σ σy. Therefore, 

From the plume analysis conducted, at 20 km downwind of the burning, the total plume 

width was approximately 12 km. Dividing by 7.8, one gets σy = 1.5 km, which is in excellent 

agreement with that of 1.5 km under stability C condition (after cold-air outbreak) (see, p. 2-48, 

Table 2.5 in Turner, 1994). Similarly, at 30 km, σy = 2.1 km (satellite) vs. 2.2 km (Turner), and 

at 40 km, σy = 2.5 km (satellite) vs. 2.7 km (Turner). Certainly, more cases are needed to further 

verify this approach. 

y) or 7.8 
Figure 170. 2134 Z 6 December 1999 NOAA-14 near-infrared image showing a smoke plume 

extending from the central Louisiana coast out over the Gulf of Mexico shelf waters. 

Blue line is estimated center of plume, while dots are the plume crosswind 

boundaries (image and analysis courtesy of Robert Arnone, Earth Scan Lab, LSU). 

179
Figure 171. NWS Surface Weather chart for 2100 Z 6 December 1999. 

Figure 172. NWS Surface Weather chart for 0000 Z 7 December 1999. 

180
VII. SUMMARY 

Two platforms in the Louisiana coastal waters with meteorological and oceanographic 

monitoring capability were further outfitted with additional meteorological and visibility sensors 

beginning in November 2001. Hourly data recorded at these stations (part of the LSU WAVCIS 

chain) is available in near real time via internet; the historical archive can also be requested. 

This data source has been combined with that available from NDBC C-MAN stations and 

offshore buoys so that nearly the entire northern Gulf of Mexico is covered. 

Regional visibility is influenced by the atmospheric mixed layer characteristics. 

Simplified air-sea interaction formulas which require only routinely observed parameters are 

derived for the determination of stability (M-O length), Bowen ratio, and mixed height under 

unstable (free convection), neutral, and stable conditions. Since very few point measurements of 

visibility are available over the Gulf, we employ the ventilation factor index to describe the local 

atmosphere dispersion capability. 

Monthly mean conditions are presented for all pertinent stations in the northern Gulf. 

Stability Class D (neutral) is found to prevail, with Class C (free convective) occasionally 

observed. Sea temperatures are typically greater than air temperatures, however in winter 

months nearer to shore the opposite does occur with very low mixed heights resulting. On 

average, computed mixed heights were between 400 - 800 m, with higher values for offshore 

stations. Areas of poor ventilation factor index often develop near the shoreline, due to low 

mixed heights in winter months and low average wind speeds during the summer. 

Using the record from CSI-3, several episodes of reduced visibility are identified and 

discussed with weather charts and satellite imagery. In two cases, it appears that smoke plumes 

from coastal fires may have drifted offshore and degraded surface visibility at CSI-3. A 

historical case is presented in which the lateral smoke plume geometry is verified through 

satellite imagery. 

The algorithm used by the ASOS for determining whether fog or haze is impairing 

visibility was applied to our measurements as well as those from the NOAA Boothville and 

Grand Isle stations. By this definition, it is shown that fog occurs with much greater frequency 

than haze, and that fog can severely restrict visibility as much as 10% of the time. Haze can 

occur year-round, but less than about 5% per month (about 1.5 day). Episodes of haze were 

usually of shorter duration (several hours) than those of fog, and reductions in surface visibility 

were of lesser magnitude. Reductions in visibility at the Boothville and CSI-3 stations were 

mostly associated with winds from an easterly quadrant. 

181
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conditions of wind sea and swell. J. Phys. Oceanogr. 27:2087-2099. 

Erickson, T. A. 2001. Louisiana and southeast Texas fog research and modeling. NWS Lake 

Charles, Louisiana. Internet website: http://www.srh.noaa.gov/lch/research/fogres.htm.

Federal Register, 1999. Regional Haze Regulations. Vol. 64, Number 126, July 1999, pp. 

35713-35774. 

Garratt, J. R. 1992. The Atmospheric Boundary Layer. Cambridge: Cambridge University 

Press. 

Geer, Ira W., ed.. 1996. Glossary of Weather and Climate. Boston, MA:American 

Meteorological Society. 

Hanna, S. R., L. L. Schulman, R. J. Paine, and J. E. Pleim. 1984. The Offshore and Coastal 

Dispersion (OCD) model user's guide, revised. OCS Study MMS 84-0069. 

Environmental Research and Technology, Inc., Concord, Massachusetts (NTIS PB86-

159803). 

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The Department of the Interior Mission

As the Nation's principal conservation agency, the Department of the Interior has responsibility 

for most of our nationally owned public lands and natural resources. This includes fostering 

sound use of our land and water resources; protecting our fish, wildlife, and biological diversity; 

preserving the environmental and cultural values of our national parks and historical places; 

and providing for the enjoyment of life through outdoor recreation. The Department assesses 

our energy and mineral resources and works to ensure that their development is in the best 

interests of all our people by encouraging stewardship and citizen participation in their care. 

The Department also has a major responsibility for American Indian reservation communities 

and for people who live in island territories under U.S. administration. 

The Minerals Management Service Mission 

As a bureau of the Department of the Interior, the Minerals Management Service's (MMS) 

primary responsibilities are to manage the mineral resources located on the Nation's Outer 

Continental Shelf (OCS), collect revenue from the Federal OCS and onshore Federal and Indian 

lands, and distribute those revenues. 

Moreover, in working to meet its responsibilities, the Offshore Minerals Management Program

administers the OCS competitive leasing program and oversees the safe and environmentally 

sound exploration and production of our Nation's offshore natural gas, oil and other mineral 

resources. The MMS Minerals Revenue Management meets its responsibilities by ensuring the 

efficient, timely and accurate collection and disbursement of revenue from mineral leasing and 

production due to Indian tribes and allottees, States and the U.S. Treasury. 

The MMS strives to fulfill its responsibilities through the general guiding principles of: (1) being 

responsive to the public's concerns and interests by maintaining a dialogue with all potentially 

affected parties and (2) carrying out its programs with an emphasis on working to enhance the 

quality of life for all Americans by lending MMS assistance and expertise to economic 

development and environmental protection.