Diurnal Variations of Cloud Cover: Their Relationship to Climatological Conditions and Role in the Time-Mean Energy Budget.

From satellite observations, four important diurnal cloud categories are identified: high-cloud fraction over maritime convective locations, low-cloud fraction over maritime nonconvective locations, high-cloud fraction over continental locations, and low-cloud fraction over continental locations. In each cloud category, most of the cloud diurnal variation at individual locations is explained by the regression of diurnal variability onto only three climatological state variables, making cloud diurnal variations spatially coherent on large horizontal dimensions. Cloud diurnal contributions to time-mean energetics follow from the nonlinear dependence of radiative fluxes on diurnally-varying properties. An observationally-driven radiative transfer calculation reveals that contribution to be 5-15 W m^{-2} over continental and maritime subsidence regions. Furthermore, a shift in the local climate can alter the cloud diurnal contribution to time-mean energetics by as much as 20 W m^{-2}. The large-scale diurnal variability of cloud cover is derived from diurnal variations of temperature, density, water content, and static stability in a linearized calculation. Forced by the diurnal cycle of solar heating, the calculated cloud distribution agrees with observed diurnal variations in all four categories. The results suggests that large-scale cloud diurnal variations are an important component of climate and should be accounted for in climate models. However, the results also suggest that cloud diurnal variations, together with their impact on energetics, can be calculated directly from time-mean climatological conditions without explicitly resolving atmospheric diurnal variability.