Diurnal Variability of the Marine Boundary Layer during ASTEX

The diurnal cycle of the marine boundary layer over the Atlantic Stratocumulus Transition Experiment (ASTEX) domain is examined during a two-week period of June 1992 in several fields, including: fractional low cloudiness, various parameters that enter into the heat and moisture budgets (e.g., temperature, water vapor mixing ratio, vertical motion), the budgets themselves, radiative heating, and the vertical eddy flux of moist static energy. Results show fractional low cloudiness varies over this region from a maximum of 54% in the predawn hours to a minimum of 39% in the midafternoon. These changes in low cloudiness are accompanied by an opposite trend in the boundary layer moisture, which shows a predawn drying and an afternoon moistening. Large-scale vertical motion also varies diurnally with a peak amplitude of 0.12 cm s21 at 1.8 km, which is a ;20% variation from the mean subsidence rate. Cloud-radiative processes play an important role in determining the diurnal characteristics of the vertical eddy flux of moist static energy, with a peak amplitude in the cloud layer at 1.2 km and a variation at this level from 24 W m22 at 1300 LST to 100 W m22 at 0400 LST. These diurnal changes suggest that the early morning peak in eddy flux results in a drying of the subcloud layer as moisture transport into the cloud layer is enhanced, leading to the observed low-cloud maximum. In contrast, during the daylight hours, solar heating within the cloud leads to a well-mixed cloud layer, a stabilizing of the subcloud layer with respect to the cloud layer, a decoupling of the cloud and subcloud layers, and ultimately, a thinning or reduction of the low clouds. Comparison of the diurnal cycle of summertime low-level divergence and vertical motion from this study with earlier published results from other regions in the North Atlantic shows a complex diurnal pattern with alternating phase in adjacent latitude belts across this region. The sense of these phase changes suggests that the northern branch of the Hadley circulation from roughly 58N to 258N over the eastern Atlantic is pulsing diurnally with enhanced mass flow during the early morning hours.