Analysis of vertical motions in fine scale precipitation bands in winter cyclones using wind profiler doppler spectra

Banded precipitation is a common feature in winter storms, especially in the northwest region of cyclones. A small field project, PlOWS (Profiling of Winter Storms), was carried out to examine the vertical air velocities within these bands using the University of Alabama in Huntsville Mobile Integrated Profiling System (MIPS) 915-MHz Doppler wind profiler. Many studies have used profilers to analyze the structure of warm season convective storms and hurricanes with strong vertical motions, but relatively few studies have been done in cool-season events with weak vertical motions.

The data revealed bands up to six kilometers deep, spaced from 20 – 40 km apart. The Doppler spectra were retrieved from the profiler data during PlOWS, as well as the signal-to-noise ratio (SNR, similar to radar reflectivity) and total vertical velocity. After correcting the total vertical velocity for spectral broadening due to wind shear and the contribution of the horizontal wind speed due to the wide beamwidth, vertical air motion was retrieved above the bright band in several bands from the three cases. Analysis of these Doppler spectra indicated a high correlation between the vertical velocity and the signal-to-noise ratio (SNR) of the bands. High velocities corresponded to higher SNR values and low (even negative) velocities to lower SNR values. The retrieved velocities are believed to be the first measurements of vertical velocities within these bands and were surprisingly large, between 1-2 m s-1. The complete methodology and results will be presented.