A simplified model for understanding the evolution of Cirrus clouds

Developing an understanding of cloud evolution is central to understanding the climate system as a whole. Stratiform cirrus layers play a significant role in the radiative interaction with the climate system. Radiational effects are a driving force in the dynamic evolution of these layers, particularly in determining the areal coverage, vertical distribution, and microphysical properties of the stratiform clouds. The deposition of energy by radiative flux divergence in a cloud layer provides potential energy to drive cloud evolution. This work uses a large eddy simulation model (LESM) to investigate a number of parameters that can be used to easily predict how a cirrus cloud will evolve. This work also includes a study of the sensitivity of formation of mammatus-like features in clouds to the below cloud layer relative humidity. Three distinct modes of cloud evolution were found to occur due to the radiative processes simulated in this study. These modes include isentropic adjustment, mixing, and evaporation/condensation. These modes of evolution were found to be independent of each other in the sense that one mode did not always occur with either of the other two modes. Similarly, the modes of evolution did not always occur in isolation and were found to simultaneously occur in many cases. Two dimensionless numbers are derived in this work that provide a basic framework for understanding the modes of evolution that can be expected of an initial cloud. These dimensionless numbers are found to have strong predictive power for cloud evolution. The simulations of several clouds produced mammatus-like structures forming at the base of the simulated clouds at the end of the model runs. These formations prompted further investigation of the processes that influence mammatus formation. The theory proposed by this work is that the mammatus cloud formation is a radiative process, mediated by the below cloud layer relative humidity. Several simulations were performed to test this theory. The analysis of these simulated clouds determined the varying degree of mammatus formation through a variety of methods. These analytical methods indicate that the below cloud layer relative humidity plays a substantial role in the formation of mammatus clouds by mediating the radiative energy from the warm surface that is available to the cloud.