Estimating total attenuation using Rayleigh targets at cloud top: applications in multilayer and mixed-phase clouds observed by ground-based multifrequency radars

Abstract

Abstract. At millimeter wavelengths, attenuation by hydrometeors, such as liquid droplets or large snowflakes, is generally not negligible. When using multifrequency ground-based radar measurements, it is common practice to use the Rayleigh targets at cloud top as a reference in order to derive attenuation-corrected reflectivities and meaningful dual-frequency ratios (DFRs). By capitalizing on this idea, this study describes a new quality-controlled approach that aims at identifying regions of cloud where particle growth is negligible. The core of the method is the identification of a “Rayleigh plateau”, i.e., a large enough region near cloud top where the vertical gradient of DFR remains small. By analyzing co-located Ka–W band radar and microwave radiometer (MWR) observations taken at two European sites under various meteorological conditions, it is shown how the resulting estimates of differential path-integrated attenuation (ΔPIA) can be used to characterize hydrometeor properties. When the ΔPIA is predominantly produced by cloud liquid droplets, this technique alone can provide accurate estimates of the liquid water path. When combined with MWR observations, this methodology paves the way towards profiling the cloud liquid water, quality-flagging the MWR retrieval for rain and drizzle contamination, and/or estimating the snow differential attenuation.

First Light Multi-Frequency Observations with a G-band radar

“…3) The observations presented here reinforce the idea that the sensitivity of all the radar systems involved in future multi-wavelength radar studies should be sufficient to allow the detection of the Rayleigh plateau near the top of ice clouds; that is necessary to ensure that we have a robust estimation of the differential (dual-wavelength) path integrated liquid attenuation (Tridon et al, 2020). For rain studies as well, G-band radar sensitivity should be https://doi.org/10.5194/amt-2020-493 Preprint.…”

“…Finally, observations collected above 4.5 km reveal the G-band's strength in small particle regimes. In this region, absence of Ka-W differential signal (i.e., DWR = 0 dB) suggests the presence of Rayleigh targets (Tridon et al, 2020), which for these frequencies correspond to ice populations with PSDs of mass-weighted mean diameter smaller than ~1 mm (Tridon et al, 2019). While Ka-and Wband signals lack sufficient differential scattering to gain further information about such small ice crystals, our 310 observations suggest that G-band signal can; DWR estimated using G-band show differential signals on the order of a few decibels across most the layer (see Fig 5b-c and Fig.…”

“…4e). Tridon et al (2020) suggest that if DWR reaches a constant value with height (a.k.a. a Rayleigh plateau), the DWR of this plateau can be used to estimate integrated water condensate mass within the 320 layer.…”

“…This signal could be qualified as being the first quantitative hydrometeor mass signal recorded at G-band. Because ice and snow attenuation considerably increase when moving from the W-to the G-band reaching one-way values of 0.9, 2.5, and 325 8.7 dB m 2 kg −1 at 96, 140, and 225 GHz (Tridon et al, 2020;Nemarich et al, 1988), the DWR plateau for the G-band pairs is affected by both the liquid water path and the ice water path. On the other hand, the DWR plateau for the Ka-W pair is mainly driven by the liquid water path.…”

Combination Analysis of Multi-Wavelength, Multi-Parameter Radar Measurements for Snowfall

“…Moreover, attenuation by ice particles might be significant if the large amount of ice were produced in the clouds and the radar beam passed through the ice layers. Tridon et al (2020) proposed a relative path-integrated attenuation (PIA) technique to retrieve liquid-water content using DWR profiles. A key idea of this technique is that the DWR 330 from dual frequency radars near cloud tops, where it is expected that small ice crystals are in the Rayleigh scattering regime for both radar wavelengths, is mainly due to the PIA associated with liquid cloud droplets and ice particles.…”