A Methodology for Estimating the Parameters of a Gamma Raindrop Size Distribution Model from Polarimetric Radar Data: Application to a Squall-Line Event from the TRMM/Brazil Campaign

Abstract

A methodology is proposed for estimating the parameters of a gamma raindrop size distribution model from radar measurements of Z h , Z dr , and K dp at S band. Previously developed algorithms by Gorgucci et al. are extended to cover low rain-rate events where both Z dr and K dp are noisy. Polarimetric data from the S-band Dual-Polarization Doppler Radar (S-Pol) during the Tropical Rainfall Measuring Mission (TRMM)/ Brazil campaign are analyzed; specifically, the gamma parameters are retrieved for samples of convective and trailing stratiform rain during the 15 February 1999 squall-line event. Histograms of N w and D o are retrieved from radar for each rain type and compared with related statistics reported in the literature. The functional behavior of N w and D o versus rain rate retrieved from radar is compared against samples of 2D-video and RD-69 disdrometer data obtained during the campaign. The time variation of N w , D o , and averaged over a 5 km 5 km area (within which a network of gauges and a profiler were situated) is shown to illustrate temporal changes associated with the gamma parameters as the squall line passed over the network. The gauge-derived areal rainfall over the network is compared against radar using the areal dp method, and the concept of an effective slope of a linear axis ratio versus diameter model is shown to significantly reduce the bias in radar-derived rainfall accumulation.

Overview: Precipitation characteristics and sensitivities to environmental conditions during GoAmazon2014/5 and ACRIDICON-CHUVA

“…Using the disdrometer or the CDP, the mass-weighted mean diameter (D m ) was computed as the ratio between the fourth and third moments (liquid water content) of the DSD (see Bringi et al, 2002, for a detailed description). For every 5 min of a continuous rainfall event (defined as RR ≥ 0.5 mm h −1 ), the moments were computed using a parameterization derived by Tokay and Short (1996).…”

A method for using weather radar data to test cloud resolving models

“…These capabilities have been discussed at length in many papers (e.g. May et al, 1999;Zrnic and Ryzkhov, 1999;Bringi et al, 2002). These additional observations lead to at least two significant capabilities compared with conventional radar.…”

Retrieval of the raindrop size distribution from polarimetric radar data using double-moment normalisation

“…Bringi et al, 2002;Schneebeli et al, 2014). We propose here a method to deal with this noise for the current application of DSD retrieval.…”

“…A threshold value on Z H selects values of Z H for which K dp and Z DR showed large variation around their expected values in the three radar data sets used here. Threshold values on Z DR and K dp are those of Bringi et al (2002). Although these were established for S band, their influence is rather limited compared to the threshold on Z H .…”

“…Retrieval of the gamma model shape parameter with the beta method is subject to high uncertainty Anagnostou et al, 2008). To deal with noisy Z DR and K dp data at low rain rates, Bringi et al (2002Bringi et al ( , 2003 used the beta method for heavy rain and disdrometer-based regressions on Z H and Z DR for light rain. Brandes et al (2004b) found that the constrained gamma method was in better agreement with disdrometer data than the beta method, while Anagnostou et al (2008) reported similar performance from the two techniques, and both studies noted that the beta method is sensitive to errors in K dp .…”