Horizontal Divergence and Vertical Velocity Retrievals from Doppler Radar and Wind Profiler Observations

Abstract

Vertical motion profiles can be diagnosed with the mass continuity equation using horizontal divergence fields derived from various single-Doppler radar techniques such as EVAD (extended velocity-azimuth display), CEVAD (concurrent extended velocity-azimuth display), and VVP (volume velocity processing). These methods allow for the retrieval of mesoscale air motions in precipitating regions when the wind field is relatively homogeneous. In contrast, VHF wind profiler data can provide a direct measurement of vertical motion, albeit across a much smaller domain compared to the single-Doppler radar techniques. In this study, we compare horizontal divergence and vertical motion patterns derived from the various single-Doppler methods with those obtained from VHF profiler data. The diagnosed profiles of horizontal divergence and vertical velocity from the single-Doppler (scanning radar) techniques are in qualitative agreement in the lower troposphere but often exhibit large variability at higher levels. Because of less stringent radar echo requirements, the VVP technique often analyzed data above the top of the EVAD-CEVAD analysis domain, resulting in a deeper layer of upper-level divergence. The CEVAD technique often produced a deeper and larger region of upward motion despite similar profiles of divergence, probably due to the CEVAD top boundary condition specification of particle terminal fall speed as opposed to the vertical air motion, as well as to the adjustment procedure employed during the regression solution. The wind profiler data showed much larger vertical gradients and magnitudes of divergence and vertical velocity when averaged over the same time interval required to collect data for a single-Doppler retrieval. However, when all the available data were composited, the high-frequency variability in the wind profiler retrievals was reduced resulting in relatively good agreement between all analysis methods. The wind profiler usually sampled vertical motion (divergence) several kilometers above the single-Doppler retrievals, which the authors attribute to the stringent precipitation echo coverage requirements imposed by the scanning radar analysis techniques, thus limiting their vertical extent new echo top.