GPM-Derived Climatology of Attenuation Due to Clouds and Precipitation at Ka-Band

Abstract

Attenuation from clouds and precipitation hinders the use of Ka-band in SARs, radar altimeters and in satellite link communications. The NASA-JAXA Global Precipitation Measurement (GPM) mission, with its core satellite payload including a dual-frequency (13.6 and 35.5 GHz) radar and a multifrequency passive microwave radiometer, offers an unprecedented opportunity for better quantifying such attenuation effects. Based on four years of GPM products, this article presents a global climatology of Ka-band attenuation caused by clouds and precipitation and analyses the impact of the precipitation diurnal cycle. As expected, regions of high attenuation mirror precipitation patterns. Clouds and precipitation cause two-way attenuation at 35.5 GHz in excess of 3 dB about 1.5% of the time in the regions below 65 • , peaking at as much as 10% in the tropical rain belt and the South Pacific Convergence Zone and at circa 5% along the storm tracks of the North Atlantic and Pacific Oceans. Confirming previous findings, the diurnal cycle is particularly strong over the land and during the summer period; while over the ocean, the diurnal cycle is generally weaker some coherent features emerge in the tropical oceans and in the northern hemisphere. Results are useful for estimating data loss from (sun-synchronous) satellite adopting active instruments/links at a frequency close to 35 GHz.

Mind the gap – Part 2: Improving quantitative estimates of cloud and rain water path in oceanic warm rain using spaceborne radars

“…The adoption of aforementioned sampling strategies could lead to a substantial improvement in our ability to map the vertical structure of warm rain. However, quantitative retrievals using single-frequency radar measurements remain challenging (Lebsock and Su, 2014;Battaglia et al, 2020b). Retrievals of the profile of liquid water content (Lebsock and L'Ecuyer, 2011;Leinonen et al, 2016;Awaka et al, 2016) mainly rely on radar reflectivity profiles.…”

“…For instance, in the majority of retrievals, a constant-with-height intercept parameter and an exponential drop size distribution (DSD) are assumed. The prospect of using multiwavelength radar measurements can provide additional information about the vertical structure of cloud and precipitation and thus relax such assumptions (Battaglia et al, 2020c, a). Using CloudSat observations, Rapp et al (2013) and Wang et al (2017) suggest that the onset of precipitation occurs for the cloud liquid water path exceeding values ranging from 150 to above 300 g m −2 ; the c-LWP is then expected to grow with the r-LWP (see Fig.…”

“…All the aforementioned integral constraints can substantially improve the estimates of c-LWPs and r-LWPs; however, they also have drawbacks. Visible optical thicknesses have collocation issues and may be affected by 3D effects; PIA estimates and T B measurements are often too noisy at small liquid water paths (LWPs) and strongly affected by nonuniform beam filling (NUBF; Kozu and Iguchi, 1999;Battaglia et al, 2020b;Wentz and Spencer, 1998;Lebsock and Suzuki, 2016) associated with the considerable spatial inhomogeneity of warm rain .…”

The Summertime Diurnal Cycle of Precipitation Derived from IMERG

“…The diurnal cycles of occurrence, intensity and accumulation are determined at each grid box by conducting a harmonic analysis [4,14,17,28]. As the diurnal cycle of precipitation can be affected by sub-daily processes, it may not be fully represented by a 24 h harmonic, and may be influenced by higher-order ( 24 n h) harmonics.…”

“…Use of three-dimensional measurements from the GPM dual-frequency precipitation radar in localised studies can aid in better characterising the differences between the diurnal cycles of accumulation, occurrence and intensity. Finally, these results offer the opportunity to optimise the orbit selection for satellites with instruments that are detrimentally affected by precipitation [28].…”