A Comparison of Key Forecast Variables Derived from Isentropic and Sigma Coordinate Regional Models

Abstract

When diabatic heating, frictional forces and radiative effects within the atmosphere are negligible then a parcel's entropy will remain nearly constant and its motion will be along isentropic surfaces. Because such conditions may extend over large areas on the synoptic scale, use of the isentropic coordinate in a regional forecast model offers the potential of more accurate predictions since numerical integration can largely be carried out in two rather than three dimensions. Forecasts from an isentropic-coordinate model employing a sigma-coordinate lower boundary are compared with those from two of the National Weather Service's operational models, the Limited-Area Fine Mesh Model and the Nested Grid Model. Five significant weather events are considered with attention given to those predicted quantities most important to each case. The operational models produced the best overall precipitation forecasts. The isentropic model displayed some advantages in its wind, specific humidity, and surface pressure forecasts which are likely to be attributable in part to the quasi-horizontal nature of the mass, momentum, and water vapor transports in that coordinate system.