ACCESS NUMBER: 30951-17810 

STUDY TITLE: Determining Overwater Visibility and Mixing Height Using Satellite and In-Situ 

Measurements Over the Gulf of Mexico 

REPORT TITLE: Visibility and Atmospheric Dispersion Capability over the Northern Gulf of 

Mexico: Estimates and Observations of Boundary Layer Parameters 

CONTRACT NUMBER: 1435-01-99-CA-30951-17810 

SPONSORING OCS REGION: Gulf of Mexico 

APPLICABLE PLANNING AREA: Northern Gulf of Mexico 

FISCAL YEAR(S) OF PROJECT FUNDING: 2001, 2002, 2003 

COMPLETION DATE OF REPORT: February 2005 

COSTS: FY 2001: $94,407, FY 2002: $98,597, FY 2003: $101,098; CUMULATIVE : $294,102 

PROJECT MANAGER: S. A. Hsu 

AFFILIATION (OF PROJECT MANAGER): Louisiana State University, Coastal Studies Institute 

ADDRESS: Howe-Russell Geoscience Bldg., Louisiana State University, Baton Rouge, 

Louisiana, 70803 

PRINCIPAL INVESTIGATOR(S): S. A. Hsu, O. K. Huh 

KEY WORDS: Gulf of Mexico, mixing height, stability, ventilation factor, visibility, fog, haze, 

plumes, satellite imagery 

BACKGROUND: The National Haze Program was enacted to address concerns about the 

effects of atmospheric pollutants on the visibility of National Parks and Wilderness Areas. Many 

of these designated areas are located in coastal zones, and are therefore under Minerals 

Management Service oversight. In the northern Gulf of Mexico, there are many energy 

production platforms in operation, which are supported daily by both marine vessels and 

airborne transports. Emissions from these stationary and transient sources, combined with 

natural emissions from the marine environment (such as sea salts) could potentially degrade the 

visibility over coastal park areas. Under certain conditions, land-based emissions could also 

impact these areas. This study was designed to monitor offshore visibility and to investigate the 

atmospheric boundary layer characteristics associated with impaired visibility events. 

OBJECTIVES: (1) To deploy two offshore visibility and meteorological stations for continuous 

monitoring; (2) to integrate captured data from these stations into the LSU WAVCIS webpage so 

that near-real time information was available over the internet; (3) to determine whether wavebreaking was contributing to reduced visibility offshore; (4) to derive formulas which describe the 

offshore boundary layer characteristics (such as stability, mixed height, and ventilation factor 

index) using only easily and routinely acquired input data; (5) to apply the formulas from (4) to 

describe the mean atmospheric boundary layer conditions over the northern Gulf of Mexico 

using our offshore stations and existing NDBC C-MAN and buoy records; and (6) to identify lowvisibility events from our offshore stations and analyze employing pertinent weather maps and 

available satellite imagery. 
ACCESS NUMBER: 30951-17810 

DESCRIPTION: Belfort Model 6100 visibility sensors and Rotronics MP101A temperature / 

relative humidity probes were added to the existing suite of instrumentation on two LSU 

WAVCIS platforms; CSI-3 (92°03.68' 29°26.47') and CSI-6 (90°29' 28°52'). Hourly data was 

recorded beginning in November 2001 at CSI-3 and June 2003 at CSI-6. The near-real time 

data was posted on the WAVCIS webpage (www.wavcis.lsu.edu). Simplified formulas were 

derived for computing atmospheric boundary layer parameters in the offshore environment. 

These new formulas, along with a NOAA algorithm for determining whether fog or haze is 

occurring, were applied to the WAVCIS platform hourly data along with longer-term records from 

NDBC coastal and offshore stations. Mean values of stability, mixing height, and ventilation 

factor index are described for the northern Gulf of Mexico. Episodes of reduced visibility 

recorded at our CSI-3 station were selected and investigated using weather map analysis and 

available satellite imagery. 

SIGNIFICANT CONCLUSIONS: On average, sea temperatures are mostly warmer than air 

temperatures; however Stability Class D (neutral) prevails due to the wind contribution. Class C 

(free convective) is occasionally observed. Computed mixed heights ranged between 400 and 

800 m, with higher values over the deeper Gulf. Areas of poor ventilation factor index often 

develop near shore, due to lower mixed heights in winter and lower wind speeds in summer. In 

these areas, the atmosphere's capability to disperse pollutants would be reduced. It is found 

that impaired visibility along the Louisiana coast and offshore is mostly due to naturally 

occurring fog conditions. Fog frequently and significantly reduces horizontal visibility. Haze can 

develop in each month, but generally less than 5% of the time (about 1.5 day). Reductions in 

visibility caused by haze conditions are usually less severe and are of shorter duration than 

those due to fog. Reductions in visibility at Boothville, Louisiana and CSI-3 were mostly 

associated with easterly winds. Satellite imagery revealed that plumes generated by landbased sources can impact and temporarily degrade offshore visibility. 

STUDY RESULTS: In the fall and winter seasons particularly, the shelf waters off Louisiana 

cool down. When warm, moist Gulf air moves over the shelf, fog can form rapidly even in 

moderate winds. This effect is enhanced in the Mississippi River Delta region and over coastal 

wetlands and marshes. Fog is the dominant cause of impaired visibility in this region. Haze 

conditions were observed in most months, but usually for short duration (hours) and with lesser 

degradation of visibility. Salt particulates ejected by sea spray are known to contribute to haze, 

however episodes of impaired visibility at CSI-3 were mostly associated with light winds (hence 

limited wave breaking). Neutral to free convective stability conditions prevail over the northern 

Gulf of Mexico and average computed mixed heights range between 400 and 800 m. Nearer to 

shore, stable conditions sometimes occur, and very low mixed heights can develop. In summer 

months, light to near-calm winds can exist. Under these conditions, the atmosphere's 

dispersion capability can be very limited. Offshore visibility can temporarily be affected by 

smoke plumes generated from land-based fires. 

STUDY PRODUCTS: 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. OCS Study MMS 2005-008. U.S. Dept. of the 

Interior, Minerals Management Service, Gulf of Mexico OCS Region, New Orleans, LA. 184 pp.