Development of statistical cloud population models

 

Authors

Samson M Hagos — Pacific Northwest National Laboratory
Zhe Feng — Pacific Northwest National Laboratory
Casey Dale Burleyson — Pacific Northwest National Laboratory
Robert Houze — University of Washington

Category

Convective clouds, including aerosol interactions

Description

a) Size distribution of convective cells from the Darwin C-band radar as a function of the number of cells in the domain, (b) same (a) obtained from the master equation, and (c) the average echo top height of clouds.
Concepts from statistical mechanics are used to develop a hierarchy of models that represent the evolution of populations of convective clouds. C-Pol radar observations at Darwin, Australia and cloud-permitting model simulations are used to evaluate the statistical models. The radar observations show that, in scenes with small numbers of convective cells, the frequency of cells for a given cell area follows a Boltzmann distribution, i.e., the number of cells drops exponentially with cell area at a rate proportional to the inverse of the number of cells in the radar domain. This behavior is consistent with the simplest model that assumes equilibrium conditions involving weak cloud-cloud interactions. In contrast, in scenes with large number of convective cells, deviation from a Boltzmann distribution occurs with more small cells and fewer large cells than are predicted by the distribution. On the other hand, a non-equilibrium model that includes cloud-cloud interactions via damping captures the evolution of the cloud populations, including their diurnal cycle, well. Implications of these findings for the representation of cloud populations in stochastic parameterizations are discussed.