ACCESS NUMBER: 30919 

STUDY TITLE: Boundary Layer Study in the Western and Central Gulf of Mexico

REPORT TITLE: Boundary Layer Study in the Western and Central Gulf of Mexico

CONTRACT NUMBER(S): 1435-01-99-CT-30919 

SPONSORING OCS REGION: Gulf of Mexico 

APPLICABLE PLANNING AREA(S): Western and Central Gulf of Mexico 

FISCAL YEAR(S) OF PROJECT FUNDING: 1998-2004 

COMPLETION DATE OF REPORT: October 2004 

COST(S): $554,281 

PROJECT MANAGERS(S): P. Roberts, C. MacDonald 

AFFILIATION: Sonoma Technology, Inc. 

ADDRESS: 1360 Redwood Way, Suite C, Petaluma, California 94954 

PRINCIPAL INVESTIGATOR: P. Roberts 

KEY WORDS: boundary layer; Gulf of Mexico; transport; pollution; CALMET; 

CALPUFF; trajectories; latent heat flux; sensible heat flux; wind; Eta Data Assimilation 

System; radar wind profiler; radio acoustic sounding system; friction velocity; dispersion; 

COARE; mixing depth 

BACKGROUND: The vertical and horizontal variability of the atmospheric boundary 

layer (ABL) in the Gulf of Mexico has been an uncertainty. The depth of the ABL and its 

vertical stability and wind and turbulence structure can vary greatly in Outer Continental 

Shelf (OCS) zones due to horizontal variations in water skin temperature and the 

overlying air mass. The Minerals Management Service (MMS) now has available new 

observations of vertical profiles in the ABL for the OCS from six meteorological stations 

in the Gulf of Mexico. Two stations collected ABL observations for three years from 

May 1998 through October 2001, and four stations collected observations from 

September 2000 through October 2001. For the first time in the Gulf of Mexico region, 

the required full vertical profiles of meteorological variables can be input into transport 

and dispersion models. 

OBJECTIVES: To characterize the ABL structure and evaluate how the ABL structure 

influences the transport and dispersion of pollutants in the western and central Gulf of 

Mexico to support techniques for evaluating the effects of oil and gas exploration, 

development, and production activities in the OCS on air quality over coastal areas 
ACCESS NUMBER: 30919 

DESCRIPTION: Observations of vertical profiles in the ABL were collected from six 

meteorological stations in the Gulf of Mexico. These stations were equipped with 915-

MHz radar wind profilers (RWP), 2-KHz Radio Acoustic Sounding Systems (RASS), and 

surface meteorological stations. Two stations collected ABL data for three years from 

May 1998 through October 2001, and four stations collected data from September 2000 

through October 2001. The RWPs and RASS measure winds and virtual temperatures, 

respectively, from near the surface to heights of a few kilometers, and the surface 

stations measure skin temperature as well as wind speed, wind direction, air 

temperature, and water vapor mixing ratio at an elevation of about 25 m on an oil 

platform. In addition to the new data from the vertical profilers, routine meteorological 

observations from buoys and from shoreline stations were included in the analysis. 

The new and routine data collected were analyzed to investigate the over-water surface 

energy balance, the climatology of latent heat versus sensible heat fluxes, mixing 

depths, the frequency of occurrence of very stable conditions, and the horizontal spatial 

variability of wind speed and direction. Estimates of the scaling velocity and scaling 

temperature were studied. Three-dimensional prediction fields of surface winds, heat 

and momentum fluxes, and wind profiles from the National Center for Environmental 

Protection’s Eta model were compared with the observations from the RWPs and 

buoys. Annual, seasonal, and diurnal variations of the ABL characteristics were 

determined. Using the new data as inputs, test runs with CALMET and CALPUFF were 

made for seven case study periods, covering a range of representative synoptic 

conditions and seasons, such as strong-wind January days and light-wind July days, to 

determine typical plume trajectories and relative dispersion rates. Arbitrary 

assumptions were used for hypothetical tracer releases from three oil platforms: Breton 

Island Platform (BIP), South Marsh Island (SMI), and Shallow Water Platform (SWP). 

Twenty-four-hour trajectories were calculated for releases at heights of 10 m, 75 m, and 

350 m using (1) CALMET wind fields and a trajectory model (TRAJMOD) and (2) Eta 

Data Assimilation System (EDAS) wind fields and the HYSPLIT trajectory model. The 

various data sets were collected, and quality assurance/quality control procedures were 

applied to produce a single, user-friendly database. 

SIGNIFICANT CONCLUSIONS: The fluxes and scaling parameters calculated by the 

COARE algorithm in the Gulf of Mexico are physically consistent with expectations and 

are similar in magnitude to the observations and COARE calculations for the Tropical 

Ocean-Global Atmosphere (TOGA) research project, which took place in the warm 

western Pacific Ocean near the equator. The COARE-calculated monthly average 

friction velocity using data from the buoys, the C-MAN sites at the shoreline, and the 

SMI platform near shore shows agreement among these sites well within a factor of two 

and often within 20%. This agreement is important because the monthly average 

friction velocity is the key scaling velocity for estimating transport speeds and dispersion 

rates. 

The EDAS-simulated wind fields can be improved if EDAS ingests offshore 

measurements of aloft wind data; such data can be obtained from offshore RWPs. 
ACCESS NUMBER: 30919 

The CALMET diagnostic wind fields and EDAS-HYSPLIT trajectories agree within 20° to 

30° most of the time, although the speeds of the EDAS trajectories are larger by as 

much as a factor of two. The CALPUFF-simulated plumes from three oil platforms 

sometimes impact the shoreline or offshore islands, depending on wind direction. The 

simulated concentrations are higher during light winds, when dilution is less. It is 

fortunate that the most persistent winds, associated with onshore impact near the same 

location for several hours, are nearly always strong; consequently, simulated 

concentrations are lower. When winds are light and variable, the local centerline 

concentration may be higher, but the plume does not remain for long over a specific 

point. 

The CALMET-CALPUFF estimates of over-water mixing depth were low, about 100 to 

200 m, in contrast to observed mixing depths of about 600 m. This factor of three to six 

differences causes model overpredictions in concentrations, because the plume is 

constricted to the mixing layer. The underpredictions of mixing depth appear to be due 

to the neglect of convective mixing processes offshore, where CALMET currently 

assumes that the mixing depth is due solely to mechanical mixing and is therefore 

proportional to wind speed. 

STUDY RESULTS: The EDAS-simulated wind fields and the observed RWP winds 

from six sites were compared. The mean wind speed (WS) bias was near zero close to 

the shore but increased with offshore distance, so that the EDAS mean WS exceeded 

the RWP mean WS by 1 to 2 m/s at 50 km offshore and by 2 to 6 m/s at 100 to 200 km 

offshore. Mean wind direction (WD) bias was small, a difference of about 10° to 20° 

(e.g., if the RWP WD was 180°, then the EDAS WD would be 160°). Standard 

deviations of the differences (with mean bias removed) were 1 to 2 m/s for WS and 20° 

to 40° for WD, in agreement with findings for other domains and models. 

Calculated monthly average sensible heat fluxes in the Gulf of Mexico ranged from 5 to 

30 W/m2

, typical of other over-water areas. Similarly, calculated monthly average latent 

heat fluxes ranged from 50 to 150 W/m2

, also typical of other over-water areas. Both 

the latent and sensible heat fluxes were highest in the late fall and early winter and 

lowest in the late spring and summer. Sensible heat flux is maximized for post-trough 

synoptic conditions, which are likely to be marked by above average wind speeds and 

by low air temperature. The latent heat flux is consistently large during the post-trough 

synoptic condition, due to higher wind speeds and low dew points that follow a cold 

front. The calculated fluxes are generally in good agreement with the monthly average 

Eta model latent and sensible heat fluxes. 

COARE-calculated monthly average friction velocities using data from the platform sites 

further from shore are 30% to 40% less than at the other sites. These platforms 

(Vermillion offshore oil platform [VRM], BIP, Deep Water Platform [DWP], Mid-Buoy 

Platform [MBP], SWP, and West Delta Platform [WDP]) are typically in deeper water. A 

possible explanation is that the wave height and frequency are estimated from empirical 

relations given observations of wind speed at the platforms, whereas they are directly 
ACCESS NUMBER: 30919 

measured at the buoys. The monthly average Eta model friction velocity was usually 

within about 10% to 20% of the COARE-calculated friction velocity. 

The differences between the observed water skin and air temperatures were, on 

average, +1 to +3°C at most sites all year. The differences were lower in late spring 

and greater in late fall and early winter. This persistent positive temperature difference 

suggests that the ABL is usually well-mixed and unstable. 

STUDY PRODUCTS: MacDonald C.P., P.T. Roberts, M.R. Lilly, C.A. Knoderer, D.S. 

Miller, and S.R. Hanna. 2003. Boundary Layer Study in the Western and Central Gulf of 

Mexico: A final report by Sonoma Technology, Inc. for the U.S. Dept. of the Interior, 

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

Study MMS 2004-060. MMS Contract Number 1435-01-99-CT-30919. 588 pp. 

MacDonald, C.P, J.D. Prouty, D.S. Miller, T.S. Dye, C.A. Knoderer, E.A. Gray, and D.T. 

Nguyen. 2003. EDAT User Guide [Environmental Data Analysis Tool User Guide]. A 

User Guide prepared by Sonoma Technology, Inc., Petaluma, CA, STI-999475/998555-

2344-UG, for the Offshore Operators Committee, New Orleans, LA, and Minerals 

Management Service, U.S. Department of the Interior, New Orleans, LA. April. 

MacDonald, C.P, J.D. Prouty, E.A. Gray, T.S. Dye, D.T. Nguyen, and C.A. Knoderer. 

2003. Environmental Data Analysis Tool Software. Software written by Sonoma 

Technology, Inc., Petaluma, CA, STI-998555, for the Minerals Management Service, 

U.S. Department of the Interior, New Orleans, LA. April. 

MacDonald, C.P, J.D. Prouty, D.S. Miller, T.S. Dye, C.A. Knoderer, E.A. Gray, D.T. 

Nguyen, and M.A.R. Lilly. 2003. MMS Database. Complete database prepared by 

Sonoma Technology, Inc., Petaluma, CA, STI-998555 for the Minerals Management 

Service, U.S. Department of the Interior, New Orleans, LA. April. 
ACCESS NUMBER: 30919 

Map of the MMS study region depicting locations of C-MAN, buoy, and RWP platform 

monitors.