Factors Controlling the Vertical Extent of Fair-Weather Shallow Cumulus Clouds over Land: Investigation of Diurnal-Cycle Observations Collected at the ARM Southern Great Plains Site

Summertime observations for 13 yr at the Atmospheric Radiation Measurement Southern Great Plains site are used to study fair-weather shallow cumuli (ShCu). To roughly separate forced from active ShCu, days are categorized into thin- or thick- ShCu days according to whether the daytime-average cloud depth exceeds 300 m. By comparing diurnal-cycle composites of these two regimes, the authors document differences in cloud properties and their radiative impacts. The differences in environmental conditions provide clues as to what controls ShCu vertical extent. Higher boundary layer (BL) relative humidity (RH) is found on thick-cloud days, associated with large-scale moisture advection before sunrise. This higher BL RH not only contributes to a lower cloud base but also to the penetrating ability of an air parcel to reach higher levels, and thus leads to larger cloud vertical extent. Although not as significant as BL RH, ShCu vertical extent also varies with thermal stability and surface fluxes. Enhanced stability above cloud on thin-cloud days may limit cloud vertical extent. A larger sensible heat flux on thin-cloud days encourages greater entrainment of dry air into the BL, whereas a larger latent heat flux on thick-cloud days helps sustain higher afternoon BL RH. These heat flux differences help maintain the BL RH differences that appear to control cloud vertical extent. This study provides observational evidence that forced clouds are related to BL large-eddy overshoots limited by a stronger inversion whereas higher moisture and a weaker stability above favor active cumuli with greater vertical extent.