The Role of Cloud Diurnal Variations in the Time-Mean Energy Budget

Abstract

The contribution to time-mean energetics from cloud diurnal variations is investigated. Cloud diurnal contributions to radiative fluxes follow as the differences between time-mean radiative fluxes based on diurnally varying cloud properties and those based on fixed cloud properties. Time-mean energetics under both conditions are derived from an observationally driven radiative transfer calculation in which cloud cover, temperature, and moisture are prescribed from satellite observations. Cloud diurnal contributions to time-mean energetics arise from the nonlinear dependence of radiative fluxes on diurnally varying properties. Diurnal variations of cloud fractional coverage and solar flux are the main factors of the cloud diurnal contributions to shortwave (SW) flux, although the diurnal variation of cloud type is also important. The cloud diurnal contribution to longwave (LW) flux at the top of the atmosphere (TOA) is produced by diurnal variations of cloud fractional coverage, cloud-top height, and surface temperature. The cloud diurnal contribution to LW flux at the surface is produced by diurnal variations of cloud fractional coverage and cloud-base height. Cloud diurnal contributions to SW fluxes at the surface and TOA are much larger than the contribution to SW atmospheric absorption. The contribution to radiative heating in the atmosphere is concentrated inside the cloud layer. Its vertical profile changes sign, so the cloud diurnal contribution to atmospheric energetics is significantly larger than is implied by the column average. Cloud diurnal contributions to SW flux at the surface and TOA are 5?15 W m?2 over continental and maritime subsidence regions, where the diurnal variation of cloud fractional coverage is large. The contributions to LW fluxes are 1?5 W m?2 over continental regions, where diurnal variations of cloud fractional coverage and surface temperature are large. A cancellation between contributions of opposite sign makes the cloud diurnal contributions to globally averaged energetics much smaller than regional contributions. However, a shift in regional climate from one dominated by high clouds to one dominated by low clouds can alter time-mean surface energetics by as much as 20 W m?2.