Rapid Refresh (RAP) and High-Resolution Rapid Refresh (HRRR) Forum

Using Maximum Wind Forecast to Compute Shear

Posted By: Neil Thompson
Date: 17-February-14 1809Z

I am attempting to compute vertical wind shear at the altitude of the maximum wind and the altitude of the tropopause using RAPS forecast data. Consistent results are proving to be elusive. Something is either wrong with my method or my understanding of the data. For example, when I compute shear near the height of the maximum wind as the magnitude of the vector difference between the forecasted maximum wind and the forecasted wind at an adjacent pressure level divided by their geopotential altitude difference then the computed values are sometimes very high - much higher than the shear computed between the winds at the nominal pressure levels above and below the maximum wind. Upon closer examination, the forecasted maximum wind and to a lesser extent the forecasted wind at the tropopause often appear to be slightly inconsistent with the forecasted winds at the surrounding 25 hPa-spaced pressure levels. It seems that (1) either the pressure marking the height of the maximum wind, or (2) the wind speed/direction of the maximum wind are sufficiently out-of-synch with the wind at the neighbouring pressure levels to render the vertical winds shear calculation to be nonsensical. Furthermore, the maximum wind often defies understanding. In one example observed this past week, the maximum wind over a grid point was forecast to be 80 KT at the 216 hPa pressure level. However, the maximum wind was bounded below by a forecasted 120 KT wind at the 225 hPa level and above by a 120 KT wind at the 200 hPa level. (1) Can anyone offer some insight into relationship of the maximum wind and tropopause wind forecasts to the wind forecasts at the nominal 25 hPa pressure levels? How can the maximum wind be less in magnitude then the neighbouring pressure level winds? (2) Should it be possible to compute wind shear between the maximum wind, tropopause wind and the nominal pressure level winds and get reasonable results as can be done when computing shear between the nominal pressure level winds? (3) I understand that pressure is not the RAPS model’s native vertical coordinate. Is it possible that something unintended happens to the relationship between the maximum wind and pressure level winds when the conversion is made from the native vertical coordinate to the pressure vertical coordinate?

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