An analysis of the Northern Gulf of Mexico sea/land breeze and associated convective precipitation

Much of the summertime weather along the north central coast of the Gulf of Mexico is associated, either directly or indirectly, with the land-sea interface. A regional composite study of the summertime wind patterns and rainfall patterns was conducted. Some of the factors influencing convective rainfall were investigated. Rawinsonde data and NEXRAD base reflectivity data, each from Slidell, LA, along with data from surface observing stations at the north central Gulf coast, are examined in the study.

NEXRAD-scanned points returning a base reflectivity value greater than 30 dBZ (signifying convection) were tallied within four geographic scan quadrants at 4-hour intervals for each of the months of June, July, and August during the period of 2003-2005. These NEXRAD composites were filtered to only include days in which: 1) land observing stations reported wind speeds less than 3.4 m/s (7.5 mph) at 00 and 12 UTC, and 2) no synoptic-scale system or widespread air mass thunderstorm activity were discerned in the NEXRAD data; this reduced the 276-day study period to 96 days. For the 96 days of interest, known as “sea breeze” days, hourly averages of wind speed and direction are calculated by month to determine the prevalence of the sea breeze and the land breeze. Variables of station pressure, temperature, and dewpoint temperature are collected from select surface stations and used to compute air density gradients in the region. The resulting 3-year summertime climatologies of radar reflectivity, wind, and density gradient are presented, and the trends of each are compared. A climatology of parameters derived from rawinsonde data is also compared against the climatology of radar reflectivity.

A distinct diurnal cycle of precipitation distribution for the north central Gulf coast on “sea breeze” days is revealed. Precipitation typically begins during the late morning along the immediate Gulf coast, and spreads inland (northward) during the afternoon; the precipitation is triggered by the local sea breeze front, as evidenced through coinciding wind data and thermodynamic data. Precipitation dissipates during the night, and develops again over adjacent Gulf waters during around sunrise time. The early morning precipitation is typically concentrated over an enclosed region of the Gulf of Mexico known as the Mississippi-Alabama Shelf, and is likely a result of convergence in association with a land breeze front or multiple discrete land breeze fronts.

Regressional calculations were performed of regional rainfall coverage versus a variety of potential influences. The 00Z and 12Z Slidell sounding data were used in these computations. No correlation with CAPE was found. The synoptic wind direction was examined for influences on rainfall coverage. Rainfall coverage was noted to be greater with an 850-hPa south wind, and the lowest rainfall coverage was associated with a 850-hPa north wind. Inland precipitation coverage also increased (decreased) with an 850-hPa south (north) wind, consistent with other studies about sea breeze convection. Rainfall coverage was also correlated with the K-Index and precipitable water. A detailed analysis reveals that, in most situations, the 700-hPa dewpoint depression term of the K-Index contributed the most to this relationship by a factor of 2 to 8, followed by the 850-hPa dewpoint temperature term; the lapse rate term often contributed weakly (usually over land) or not at all.

Multiple regression analysis was completed using a modified 850-hPa wind term (wind direction value minus 180 degrees) and the K-Index. The variance was computed for both the whole radar region, as well as for quadrants of the radar region, for 4-h rain coverage. The largest variance explained was 23%. In most cases, the K-Index was 2-3 times more important than wind direction, but in some cases the two terms contributed equally to the variance.