Evolution of shallow cumulus convection and its parameterization

Shallow cumulus convection is important not only because of its role in mediating local turbulent transport in the boundary layer (BL), but also because of its ability to affect large-scale atmospheric circulations at a climate scale. Representing the effects of shallow cumuli appropriately in large-scale models presents a challenge to both climate modeling and weather forecasting. The goal of this study is to improve the treatment of shallow cumulus convection in large-scale models through a combination of theoretical analyses and modeling studies. All these analyses and simulations rely primarily on the observations from the Atmospheric Radiation Measurement (ARM) project and focus on continental fair-weather cumuli (FWC). To fulfill its principal objectives, three different but related analyses are made in this study. These include: (1) A theoretical analysis based on a simple mixed layer model focuses on exploring the physics underlying the formation of FWC. The analyses indicate that the effects of the entrainment of moisture process and the surface processes on cloud formation can be evaluated by a combination of the surface Bowen ratio and the ratio between the entrainment moisture flux and the surface moisture flux. The stratification above the inversion and large-scale subsidence are the other keys affecting cloud formation. The weaker the stability, the greater the potential for cloud formation. Generally, subsidence tends to reduce the chances of cloud formation, but the intensity of this reduction may vary depending on conditions of the BL. For a moist BL with a small surface Bowen ratio, subsidence may be a minor factor affecting cloud formation even though it can reduce the BL depth substantially. (2) Large eddy simulations (LESs) are used to study the evolution of shallow cumulus convection in response to the change of external forcings and conditions. It is found that the surface sensible heat flux can significantly affect the cloud initiation and cloud base height, but has limited influence on cloud cover fraction and liquid water content. By contrast, the surface latent heat flux may have strong impact on these two cloud properties. The other two parameters that can substantially affect the cloud development and cloud radiative properties are the moisture difference across the top of the BL and the stratification above the BL. Horizontal winds, however, have little influence on changing cloud radiative properties even though they have a strong effect on cloud organization. (3) Using the LES data, this study verified the basic assumptions underlying the current cumulus parameterization schemes. The results indicate that the commonly used assumptions may not be appropriate for shallow cumulus convection, especially when diurnal variation is significant and surface forcing is strong. The analysis finally leads to the construction of a physically based parameterization framework for shallow cumulus convection. It includes two parameterizations: one for estimating the cloud-induced fluxes of heat, moisture, and momentum; and the other for parameterizing the cloud cover fraction and the cloud water content. A primitive verification indicates that the proposed schemes alleviate some problems with the old schemes and can be applied to a broad range of situations.