Mesoscale Predictability of Terrain Induced Flows

Abstract

Abstract : The long-term goal of this research is to develop an understanding of the predictability of small-scale atmospheric circulations appearing in forecasts generated by state-of-the-art, high-resolution mesoscale models. Using previously collected observations and archived simulations performed using the Navy's Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS) model, as well as new simulations, we focus on assessing the predictability of winds, mountain waves, and clear air turbulence (CAT) in the lee of the Sierra Nevada. The specific questions addressed in our research are as follows: (1) How sensitive are downslope winds to atmospheric conditions upstream of the mountain barrier?; (2) When such sensitivity is not extreme, can forecast errors in downslope winds and mountain-wave structure be linked to large characteristic errors in the atmospheric conditions forecast to occur on the upstream side of the mountains?; (3) Can systematic improvements in COAMPS be identified to remove these errors?; and (4) What do ensemble forecasts indicate about the sensitivity of downslope winds and mountain waves to the upstream conditions, and how can such forecasts be best used to predict these events? The answers to these questions are of direct benefit to operational forecasters using COAMPS to produce aviation and other forecasts over complex terrain. Although the focus is on the forecasting of terrain-induced mesoscale disturbances, these findings are likely to be relevant to the predictability of other mesoscale phenomena. The investigators used the COAMPS model to conduct a series of 70-member ensemble simulations of high-wind events observed during the Terrain-Induced Rotors Experiment (T-REX). By examining the ensemble spread, they obtained an unprecedentedly complete description of the sensitivity of mountain waves, CAT, and downslope winds to small variations in the initial conditions.