Dual‐frequency radar Doppler spectral retrieval of rain drop size distributions and entangled dynamics variables

Abstract

A novel technique based on K a -W band dual-wavelength Doppler spectra has been developed for the simultaneous retrieval of binned rain drop size distributions (DSD) and air state parameters like vertical wind and air broadening caused by turbulence and wind shear. The rationale underpinning the method consists in exploiting the peculiar features observed in Doppler spectra caused by the wavelength dependence of scattering and absorption properties. A notional study based on a large data set of DSDs measured by a two-dimensional video disdrometer demonstrates that the retrieval performs best for small/moderate air broadening spectral width and when mean volume diameters exceed at least 1 mm. The retrieval is also limited to ranges below cloud base and where the signal-to-noise ratio of both radars exceed 10 dB, which rules out regions affected by strong attenuation. Broadly speaking, it is applicable to rain rates comprised between roughly 1 and 30 mm h −1 . Preliminary retrieval for observations at the Atmospheric Radiation Measurement Southern Great Plains site shows very good agreement with independent reflectivity measurements from a 0.915 GHz wind profiler. The proposed methodology shows great potential in linking microphysics to dynamics in rainfall studies.

Evaporation in action sensed by multiwavelength Doppler radars

“…This is confirmed by the air broadening (Figure b) which reaches maximum values at those levels. 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 , ].…”

“…By using the raindrops fall speed where v t is the terminal fall speed corrected for air density (see equation (3) in Tridon and Battaglia [] which is based on Atlas et al [] and Frisch et al []) and w is the vertical wind speed (defined positive upward), the differential equation with respect to time can be converted into a differential equation with respect to height, z , measured downward from a given level or equivalently which provides the mass evaporation rate of the drop ( ρ L =10 3 kg m −3 is the density of water).…”

4-D-VAR assimilation of disdrometer data and radar spectral reflectivities for raindrop size distribution and vertical wind retrievals

“…For this purpose, we plan to use dual-frequency wind profilers (see for instance Williams, 2012, which retrieves vertical wind in rain from two profilers). Another possibility would be to use the methods developed by Giangrande et al (2010) or Tridon and Battaglia (2015), which retrieve vertical winds and DSD from K band and/or W band Doppler spectra. We could also study the impact on the quality of the retrievals of the addition of other frequencies directly in the assimilation process.…”

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

“…Analysis of the cases used in this study shows that the DSR plateau extends up to velocities of 2.5–3.5 m/s and 4–5 m/s for Ka/W and X/Ka spectral ratios, respectively; see Figures d and e for an example. 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, ).…”

“…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, ).…”