The role of stability and moisture in the diurnal cycle of convection overland

The diurnal cycle of convection over land is investigated by a cloud-resolving model simulation. Three regimes of convection-dry, shallow, and deep-successively take place during daytime under the presence of substantial convective available potential energy. The convective inhibition (CIN) and the normalized saturation deficit (NSD) in the cloud-base layer are identified as the major two variables that characterize the cycle of the convective regimes. The surface heating during daytime leads to the development of a quasi-dry well-mixed convective planetary boundary layer (PBL). This yields a decrease of CIN while NSD remains steady. Shallow convection is initiated as soon as the CIN becomes lower locally than the vertical kinetic energy in the PBL. This timing also marks the minimum of CIN, both in local and in domain-mean senses. Then, detrainment of moisture from the cloud layer gradually moistens the low free troposphere, resulting in a NSD decrease. Finally, deep convection is triggered when sufficient moistening is realized, as measured by a NSD minimum. During deep convection, NSD rapidly increases and CIN increases. Once CIN has exceeded the vertical kinetic energy in the PBL, deep convection ceases.