Large-Scale Heat and Moisture Budgets over the ASTEX Region

Rawinsonde data collected from the Atlantic Stratocumulus Transition Experiment (ASTEX) were used to investigate the mean and temporal characteristics of large-scale heat and moisture budgets for a 2-week period in June 1992. During this period a large apparent heat sink and an apparent moisture source were observed near inversion base. Analyses from other budget studies show that similar convective signatures occur in a wide variety of regimes when trade wind–type inversions are present. In the lowest kilometer the vertical eddy flux of moist static energy over the ASTEX domain (centered at 338N with an average sea surface temperature of 19.48C) is about 60% of that observed in the undisturbed trade wind regime of BOMEX (centered at 158N with an average sea surface temperature of 28.18C). The apparent heat source, apparent moisture sink, and convective flux of moist static energy over ASTEX were strongly modulated on a synoptic timescale by the passage of fronts and by fluctuations in the subsidence rate associated with changes in the strength and position of the subtropical high. The influence of midlatitude disturbances on convection over ASTEX further distinguishes this region from the trade wind and tropical regimes. Daily budgets are examined for three different convective regimes during ASTEX to determine the mechanisms contributing to the large synoptic variability over this region.