Numerical and Observational Study of Secondary Potential Vorticity (PV) Banners and Wakes in the Alps

Nevada System Of Higher Education, Desert Research Institute, Reno NV

Investigators

Abstract

This research focuses on the observational analysis and numerical simulations of multiple shearlines and wakes and the associated potential vorticity (PV) anomalies (banners) downstream of a highly irregular mountain topography. The goal of the study is to quantify different scales and regimes of orographic forcing and to determine effects such forcing has on observed mesoscale weather phenomena in the proximity of a major mountain range. The Principal Investigator will take advantage of the observational data collected during the recently completed Mesoscale Alpine Program (MAP). The extensive data sets collected during the MAP provide the most detailed documentation to date of the spatial and temporal structure of airflow in the vicinity of a major mountain range. In this study, the observational data from selected MAP Intensive Observation Periods (IOPs) will be used to perform analysis of the wind and thermodynamic fields and to reconstruct corresponding PV banners downwind of the Alps. Each analyzed case will be numerically simulated in order to scrutinize mesoscale model's ability to reproduce observations at high temporal and spatial accuracy. The objectives of the modeling work are to (1) reconstruct the observed PV anomalies during the selected MAP IOPs; (2) evaluate existing theories of orographic PV generation and dynamic evolution; and (3) investigate sensitivities of the model simulated PV field to different parameterizations (especially, boundary-layer representation and the subgrid scale orography) in order to recommend the optimal parameterizations. Improved knowledge of climatological positions and strengths of major jets, orographic vortices and downslope windstorms, and the range of conditions under which they develop, will be beneficial in decreasing the negative economic impact of severe weather phenomena. Such knowledge has practical application to problems such as the determination of bridge locations, prevention of road accidents in severe wind zones, avoidance of aviation hazards related to strong cross-runway winds and forecasting of the onset and termination of pollution episodes and convection.