Effects of Terrain Height and Blocking Initialization on Numerical Simulation of Alpine Lee Cyclogenesis

Abstract

A series of numerical experiments were performed to test the evolution of lee cyclones with various terrain representations, and also to test forecast sensitivity to initial wind fields. Using the data set collected during the operational phase of the ALPEX field experiment, a mesoscale model was initialized with data from two markedly different cases, 12 hours prior to formation of a cyclone in the Mediterranean. The terrain used ranged from flat, sea level terrain, to orography that conserved the maximum barrier height of the Alps (similar to “envelope” terrain currently in use at the European Centre for Medium Range Weather Forecasting). Results showed that even in the 12-hour period preceding cyclogenesis the mountain plays a crucial role in the evolution of the cyclone. Sharp variation in position and strength was evident depending on the terrain employed. Comparison of terrain versus no-terrain cases showed perturbations that compare well with those proposed in a recently published theory. The simplicity of the model and the accuracy of the forecasts further reinforce the idea that lee cyclogenesis is primarily an adiabatic process. Given the failure of coarser mesh models to properly forecast one of the cases, high grid resolution would seem to be another requirement for accurately predicting these storms. The second goal of the paper was to test the sensitivity of forecast solutions to the initial winds. A scheme developed for diagnostic work was used to adjust the input wind fields to enhance the details of the mountain flows. Significant improvement was noted in the accuracy of the resulting forecasts with respect to cyclone intensity and position at both low and high levels of the atmosphere.