A Combined Wind Profiler and Polarimetric Weather Radar Method for the Investigation of Precipitation and Vertical Velocities

Abstract

A method is presented by which combined S-band polarimetric weather radar and UHF wind profiler observations of precipitation can be used to extract the properties of liquid phase hydrometeors and the vertical velocity of the air through which they are falling. Doppler spectra, which contain the air motion and/or fall speed of hydrometeors, are estimated using the vertically pointing wind profiler. Complementary to these observations, spectra of rain drop size distribution (DSD) are simulated by several parameters as related to the DSD, which are estimated through the two polarimetric parameters of radar reflectivity (Z H ) and differential reflectivity (Z DR ) from the scanning weather radar. These DSDs are then mapped into equivalent Doppler spectra (fall speeds) using an assumed relationship between the equivolume drop diameter and the drop's terminal velocity. The method is applied to a set of observations collected on 11 March 2007 in central Oklahoma. In areas of stratiform precipitation, where the vertical wind motion is expected to be small, it was found that the fall speeds obtained from the spectra of the rain DSD agree well with those of the Doppler velocity estimated with the profiler. For those cases when the shapes of the Doppler spectra are found to be similar in shape but shifted in velocity, the velocity offset is attributed to vertical air motion. In convective rainfall, the Doppler spectra of the rain DSD and the Doppler velocity can exhibit significant differences owing to vertical air motions together with atmospheric turbulence. Overall, it was found that the height dependencies of Doppler spectra measured by the profiler combined with vertical profiles of Z, Z DR , and the cross correlation (r HV ) as well as the estimated spectra of raindrop physical terminal fall speeds from the polarimetric radar provide unique insight into the microphysics of precipitation. Vertical air motions (updrafts/downdrafts) can be estimated using such combined measurements.

Microphysical retrievals from simultaneous polarimetric and profiling radar observations

“…It is found that DSDs measured using the distrometer at the surface and estimated using the wind profiler and polarimetric weather radar generally showed good agreement. The DSD retrievals using the wind profiler were improved when the estimates of the vertical wind were included into the analysis, thus supporting the method of Teshiba et al (2009). Furthermore, the the study presents a method of investigating the time and height structure of DSDs.…”

“…Vertical profiles of the polarimetric and DSD data from KOUN are extracted from the RHI or PPI scans and interpolated onto a grid that matches the range gate spacing of the wind profiler (Teshiba et al, 2009). For the data presented in this study, wind profiler and KOUN data from the third range gate above the profiler (600 m a.g.l.)…”

“…The DSD may be derived from the Doppler spectrum, which is a powerweighted distribution of radial velocities in the resolution volume (Doviak and Zrnic, 1993), by applying an appropriate fallspeed relationship, but this process is complicated by the presence of vertical air velocity and turbulence. A recently proposed analysis (Teshiba et al, 2009) is implemented to alleviate this issue.…”

“…A brief introduction of the radar systems is first presented along with an outline of the method described in (Teshiba et al, 2009) for combining these data sets in order to retrieve information pertaining to the properties of the precipitation and the environment in which it is observed. The present study extends the work of Teshiba et al (2009) and places a greater emphasis on the investigation of precipitation microphysics. Next data collected during two MCS events occurring in Oklahoma are analyzed.…”

“…Ground truth is provided by the disdrometer. A complicating factor in the retrieval from UHF profiler spectra is the presence of ambient air motion, which can be corrected using the method proposed by Teshiba et al (2009), in which a comparison between idealized Doppler spectra calculated from the DSDs retrieved from KOUN and those retrieved from contaminated wind profiler spectra is performed. It is found that DSDs measured using the distrometer at the surface and estimated using the wind profiler and polarimetric weather radar generally showed good agreement.…”