Effects of cloud-top processes on convection in the cloud-topped boundary layer

Abstract

Cloud-top processes-s studies in this paper include cloud-top radiative cooling and entrainment (entrainment mixing warming and evaporative cooling due to the mixing). We have studied how they drive and/or regulate convection in the stratocumulus-topped boundary layer (STBL) analytically, numerically, and through analysis of observational data and data from large-eddy simulations (LES). An analytical second-order bulk boundary-layer model has been built in an attempt to parameterize the planerary boundary layer (PBL) for large-scale models, as well as to understand the complex physics of entrainment in a relatively simple framework. Cloud­top processes are parameterized in terms of "bulk" properties, and are related to the con­vection inside the PBL by the matching conditions developed. The model is able to deter­mine the fractional cloudiness, and relaxes the "well mixed" assumption. The vertical structures of the mean state and the turbulent fluxes are determined analytically. Several aspects of this simple model's formulation are evaluated using results from LES. For the further analysis of cloud-top processes methods which can be used to evaluate the radiative cooling, and entrainment warming of individual parcels are systematically discussed. These methods are applied to study an LES-generated STBL field, as well as a set of tethered balloon data observed during FIRE. By applying these methods to the LES-generated STBL, some parameters used in the earlier analytical second-order bulk boundary-layer model are further investigated. Moreover. as a case study, the relative importance of radiative cooling and evaporative cooling is investigated based on the LES data. The effects of cloud-top processes on mesoscale cellular convection (MCC) are studied both analytically and numerically by means of a two-dimensional nonlinear Boussinesq model. It is found that strong cloud-top cooling can generate closed MCC. Nonlinear processes. which are shown as mesoscale advection and interactions between convection and the basic state, are essen­tial for generating and maintaining mesoscale convection. A conceptual model is constructed to suggest a mechanism for the formation of closed MCC. This model appears to be applicable to the atmosphere.