A Numerical Investigation of Cloud Diurnal Variations

The large-scale diurnal variation 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 is broadly consistent with observed diurnal variations under maritime nonconvective, maritime convective, and continental convective conditions. The calculated diurnal variation of low-cloud fraction follows primarily from the diurnal variation of temperature, which creates a diurnal variation of saturation vapor pressure. The calculated diurnal amplitude of low-cloud fraction is large under maritime nonconvective conditions, in which a well-mixed boundary layer promotes the transition between cloudy and clear conditions. The amplitude is further enhanced under continental conditions by the diurnal variation of vertical heat transport from the surface. The diurnal variation of high-cloud fraction under continental conditions follows primarily from the diurnal variation of low-level stability, which is large if the diurnal amplitude of surface temperature is large. The diurnal variation of high-cloud fraction under maritime convective conditions follows primarily from the diurnal variation of stability at cloud top, which controls the probability that convective cloud top exists in a height interval Δz. The role of clouds in radiative heating is then important because high clouds concentrate shortwave heating in the upper troposphere, which enhances the diurnal variation of stability there.