Computing Deep-Tropospheric Vertical Wind Shear Analyses for Tropical Cyclone Applications: The Methodology Matters!

Deep-layer Vertical Wind Shear (VWS) is well known in the TC forecasting community as an important environmental influence on storm structure and intensity change. From the research side, there have been many studies on VWS, in particular, model analysis depictions and impacts on intensity prediction. The traditional way to parameterize deep-tropospheric VWS in most of these studies, and in operational forecast applications, is to use the vector difference of the 200 and 850hPa wind fields based on global model analyses. For example, this is how the operational SHIPS model employs VWS as one of its principle predictors of future TC intensity. However, is this approach to depict VWS optimal? In this study, we compare a different methodology to generate fields of VWS as produced by the University of Wisconsin-CIMSS. The CIMSS analyses use a 3-dimensional Recursive Filter at high spatial resolution to put heavy weight on available high-density satellite-derived winds. Global model wind fields are only used as background analyses for data-void regions. The resultant isobaric analyses are then used to create two layer-mean wind analyses; one upper tropospheric and one lower tropospheric. This approach differs from using just the two discreet levels as in the traditional methodology, and it will be shown how the resultant VWS fields can diverge significantly in certain situations. This could impact the forecaster interpretation of the VWS significance in certain cases, and also influence objective method forecasts of intensity.