Rain retrieval from dual‐frequency radar Doppler spectra: validation and potential for a midlatitude precipitating case‐study

Abstract

A recently developed technique retrieving the binned raindrop size distributions (DSDs) and air state parameters from ground-based K a and W-band radars Doppler spectra profiles is improved and applied to a typical midlatitude rain event. The retrievals are thoroughly validated against DSD observations of a 2D video disdrometer and independent X-band observations. For this case-study, profiles of rain rate, R, mean volume diameter and concentration parameter are retrieved, with low bias and standard deviations. In light rain (0.1 < R < 1 mm h −1 ), the radar reflectivities must be calibrated with a collocated disdrometer which introduces random errors due to sampling mismatch between the two instruments. The best performances are obtained in moderate rain (1 < R < 20 mm h −1 ) where the retrieval is providing self-consistent estimates of the absolute calibration and of the attenuation caused by antenna or radome wetness for both radars.

Evaporation in action sensed by multiwavelength Doppler radars

“…This study demonstrates that, for a stratiform case study characterized by strong evaporation, the profile of the raindrop size distributions (DSDs) retrieved by dual‐frequency K a ‐W band Doppler radars is consistent with independent observations of relative humidity. This result and the good performances in retrieving the DSDs at the ground [ Tridon et al , ] strongly suggest that such a technique allows for accurately profiling the evolution of the DSDs. In case of dry conditions, the knowledge of the vertical evolution of the DSD can be exploited in a twofold way.…”

“…To overcome this issue, Tridon et al [] and Tridon and Battaglia [] have proposed a variational method (Optimal Estimation, OE) exploiting dual‐Doppler spectra at K a and W band to retrieve binned rain DSDs, vertical wind speed, and air broadening. The technique has been recently refined and extensively validated using a light precipitation event which occurred in Finland [ Tridon et al , ], the characteristics of which are very similar to the case study under examination. The retrieval is applied to KAZR data [ Matthews et al , ] and WSCAR data [ Matthews et al , ] in the region delimited by the black rectangle in Figure , between 19:32 and 19:57 UTC starting from the lowest available observations at W band (700 m above ground level, agl) up to 3.3 km asl in order to ensure that all particles are in the liquid phase.…”

“…Air broadening is minimal in the other regions, thus signaling low turbulence conditions which are ideal for the applicability of the retrieval [ Tridon and Battaglia , ]. Similarly, integrated two‐way attenuations (Figure c) always exceed 4–5 dB, i.e., which has been found to be the threshold value for the applicability of the methodology [ Tridon et al , ]. Corresponding rain parameters, mean mass diameters D m (Figure d), rain rate RR (Figure e), and concentration parameters (Figure f) are derived from the retrieved DSDs and show the spatiotemporal variability of precipitation with the clear presence of several fall streaks.…”

“…Overall, the retrieved DSDs seem to provide a sensible description of the evaporation process and the shrinking of raindrops diameter when falling through this dry environment can be exploited as a signature of the ambient humidity. The good agreement with the Raman lidar RH complements the validation of the DSD retrieval made in Tridon et al [] with disdrometer observations. Furthermore, this suggests that the combination of the radar‐based DSD retrieval with the Raman lidar‐based RH retrieval will enable the opportunity to study other rain microphysical processes such as collision and breakup by independently taking evaporation into account.…”

Melting Layer Attenuation at Ka‐ and W‐Bands as Derived From Multifrequency Radar Doppler Spectra Observations

“…Tridon and Battaglia () have shown that the spectral broadening, due to air motion, could affect the Rayleigh plateau and bias the DSR measurements. To mitigate this, the optimal estimation method has been utilized (Tridon et al, ; Tridon et al, ; Tridon & Battaglia, ). After inspecting the cases analyzed in this paper (see the list in the supporting information), we found that the DSR in the Rayleigh region is rather flat and does not seem to be affected by the spectral broadening, which would skew the plateau (Tridon et al, ).…”