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.
Highlights
Clouds and precipitation play a crucial role in weather prediction and for climate projections, as they have manifold impacts on the radiation and energy budget (IPCC, 2013; Wild et al, 2013; Zelinka et al, 2017), water cycle (Stephens et al, 2012; L’Ecuyer et al, 2015), and large-scale circulation (Houze, 2014)
While the obtained slight positive biases are similar for both methods and can be explained by the unaccounted attenuation produced by the thick ice cloud, the Rayleigh plateau method seems to outperform the reflectivity threshold approach in terms of standard deviation. These results suggest that liquid water path (LWP) larger than roughly 100 g m−2 can safely be retrieved with the path integrated attenuation (PIA) method provided that the PIA is due to liquid cloud water only
Multifrequency radar retrievals often require as important integral constraint a reliable estimate of the differential pathintegrated attenuation ( PIA) caused by gases, rain, melting hydrometeors, cloud liquid water, and snow
Summary
Clouds and precipitation play a crucial role in weather prediction and for climate projections, as they have manifold impacts on the radiation and energy budget (IPCC, 2013; Wild et al, 2013; Zelinka et al, 2017), water cycle (Stephens et al, 2012; L’Ecuyer et al, 2015), and large-scale circulation (Houze, 2014). Tridon et al (2017a) used this principle to retrieve PSD and turbulence during rainfall and Li and Moisseev (2019) derived the attenuation characteristics due to the melting layer This technique requires a very high quality Doppler spectra, including a highly accurate radar beam alignment and low turbulence conditions. This study focuses on the third method, i.e., the exploitation of the small targets that backscatter according to Rayleigh law (Bohren and Huffman, 1983) at all radarobserving frequencies as tracers of the differential pathintegrated attenuation PIA This method exploits standard radar moments, does not require to record highquality Doppler spectra and can, in principle, be applied to scanning ground-based multifrequency radars.
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