Preliminary Results from Long-Term Measurements of Atmospheric Moisture in the Marine Boundary Layer in the Gulf of Mexico*

Abstract

Measurements of boundary layer moisture have been acquired from Rotronic MP-100 sensors deployed on two National Data Buoy Center (NDBC) buoys in the northern Gulf of Mexico from June through November 1993. For one sensor that was retrieved approximately 8 months after deployment and a second sensor that was retrieved about 14 months after deployment, the pre- and postcalibrations agreed closely and fell within WMO specifications for accuracy. A second Rotronic sensor on one of the buoys provided the basis for a detailed comparison of the instruments and showed close agreement. A separate comparison of the Rotronic instrument with an HO-83 hygrometer at NDBC showed generally close agreement over a 1-month period, which included a number of fog events. The buoy observations of relative humidity and supporting data from the buoys were used to calculate specific humidity. Specific humidities from the buoys were compared with specific humidities computed from observations obtained from nearby ship reports, and the correlations were generally high (0.7?0.9). Uncertainties in the calculated values of specific humidity were also estimated and ranged between 0.27% and 2.1% of the mean value, depending on the method used to estimate this quantity. The time series of specific humidity revealed three primary scales of variability: small scale (of the order of hours), synoptic scale (several days), and seasonal (several months). The synoptic-scale variability was clearly dominant; it was eventlike in character and occurred primarily during September, October, and November. Most of the synoptic-scale variability was due to frontal systems that dropped down into the Gulf of Mexico from the continental United States, followed by air masses that were cold and dry. One particularly intense event on 30 October 1993 was chosen for a more detailed analysis in terms of the characteristic return-flow cycles that occur in the northern Gulf of Mexico during fall and winter. Finally, cross-correlation analyses of the buoy data indicated that the prevailing weather systems generally entered the buoy domain from the south prior to September; thereafter, they became more coherent and tended to enter the region from the north.