Nonhydrostatic atmospheric modeling with a hybrid vertical coordinate

The primary research goal of CMMAP is to better understand the effect of cloud processes on the global climate through the development of new atmospheric modeling methods. One area we are exploring is the benefit of using quasi-Lagrangian (QL) vertical coordinates in cloud resolving models (CRMs). In a PhD thesis project at Colorado State University, a model has been developed to examine the feasibility of using such a coordinate, and the possibility that it will lead to improved cloud simulations when implemented in a CRM. CRMs typically use a fixed-in-space Eulerian vertical coordinate such as geometric height. The accurate representation of the vertical transport of water vapor, aerosols and chemical species across model surfaces can be difficult with such coordinates. The use of a QL coordinate virtually eliminates the computational error associated with vertical transport because coordinate surfaces approximate fluid material surfaces, and therefore the mass transport across model surfaces is minimal. There has been much success with QL coordinates in low-resolution, hydrostatic models that simulate large-scale motion (e.g., Konor and Arakawa 1997; Benjamin et al. 2004). However, the coordinate surfaces become very irregular when they are used in nonhydrostatic models that simulate the fine scale, turbulent motion characteristic of cloud-scal