Diurnal Variability of the Marine Boundary Layer during ASTEX

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Abstract
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 s−1 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 verti... Abstract 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 s−1 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 verti...