[1] A method for retrieving precipitation over the ocean using spaceborne W-band (94 GHz) radar is introduced and applied to the CloudSat Cloud Profiling Radar. The method is most applicable to stratiform-type precipitation. Measurements of radar backscatter from the ocean surface are combined with information about surface wind speed and sea surface temperature to derive the path-integrated attenuation through precipitating cloud systems. The scattering and extinction characteristics of raindrops are modeled using a combination of Mie theory (for raindrops) and the discrete dipole approximation (for ice crystals and melting snow), and a model of the melting layer is implemented to represent the transition between ice and liquid water. Backward Monte Carlo modeling is used to model multiple scattering from precipitating hydrometeors between the radar and ocean surface, which is shown to be significant for precipitation rates exceeding 3–5 mm h−1, particularly when precipitating ice is present. An uncertainty analysis is presented and the algorithm is applied to near-global CloudSat observations and compared with other near-global precipitation sources. In the tropics, CloudSat tends to underestimate the heaviest precipitation. It is found that in the middle latitudes, however, CloudSat observes precipitation more often and with greater resulting accumulation than other spaceborne sensors.

Rainfall retrieval over the ocean with spaceborne W‐band radar

Measurements of rain were obtained with a vertically pointing micro radar (MRR) with 1 min time resolution and 50(100) m height resolution at the German Baltic coast on the Zingst peninsula (54.43°N, 12.67°E). The comparison with a conventional rain gauge (30 min integration time) for a five months summer period show a correlation coefficient of p = 0.87 for the rainrate and agreement within 5% for the total rainfall integrated over the whole period. Single measurements with 30 min integration time showed deviations up to a factor of 2 between MRR and rain gauge. Classification of the measurements into rainrate intervals shows that rainrates around 0.2 mm h -1 provide the highest contribution per rainrate interval to the total rainfall. Typical distributions of number-concentration, liquid-water- concentration and rainrate versus drop size, retrieved with the MRR, are presented. Simultaneous estimates of rainrate and reflectivity factor with data of a C-band (frequency 6 GHz) weather radar suggest that the MRR may be used to support quantitative rainrate estimates with weather radars. The weather radar used for comparison is operated by the German Weather Service and is situated 51 km from the MRR.

Rain observations with a vertically looking Micro Rain Radar (MRR)

Polarimetric radar measurements are used to retrieve properties of raindrop distributions. The procedure assumes that drops are represented by a gamma distribution and retrieves the governing parameters from an empirical relation between the distribution shape and slope parameters and measurements of radar reflectivity and differential reflectivity. Retrieved physical characteristics of the drop size distribution (DSD) were generally well matched with disdrometer observations. The method is applied to select storms to demonstrate utility. Broad DSDs were determined for the core (high reflectivity) regions of thunderstorms. Largest drop median volume diameters were at the leading edge of the storm core and were displaced slightly downwind from updrafts. Rainy downdrafts exhibited what are believed to be equilibrium DSDs in which breakup and accretion are roughly in balance. DSDs for stratiform precipitation were dominated by relatively large drops. Median volume diameters at the ground were closely related to the intensity of an overlying bright band. The radar measurements suggest that, although DSDs in stratiform rain were also broad and nearly constant in the rain layer, they were not at equilibrium but were merely steady. DSD invariance is attributed to small total drop numbers, which result in few collisions.

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Drop Size Distribution Retrieval with Polarimetric Radar: Model and Application

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GPM/DPR Level-2 Algorithm Theoretical Basis Document

This paper presents the statistical properties of bright band (BB) measured by the Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR). Since BB is detected by PR standard version 6 (V6) algorithm 2A23 V6, this paper first outlines 2A23 V6, which classifies rain into three main categories: stratiform, convective, and other.  This paper studies BB from the viewpoint of algorithm development. It demonstrates that the detected BB count strongly depends on the antenna scan angle. The BB height (HBB) detected by the PR is compared with the 0°C height computed using the National Centers for Environmental Prediction-Department of Energy Atmospheric Model Intercomparison Project-II reanalysis (NCEP2) data. On average, the BB height is about 500 m below the 0°C height, which is computed from the NCEP2 data. However, a large difference between the above two heights is sometimes observed, mainly due to the false detection of BB. The reliability of BB detection by the current PR algorithm 2A23 V6 could be increased by introducing a simple filter that rejects BBs by regarding them as false when the number of BBs detected in one antenna scan (consisting of 49 antenna beam directions) is less than or equal to 5. An improved version of the filter is planned to be installed in the coming 2A23 V7. Statistics of 10 years of PR data indicate a small but discernible effect of the August 2001 boost of the altitude of the TRMM satellite on the rain type statistics and on the BB statistics. The zonal mean of the maximum value of reflectivity factor in the BB peak (ZmaxBB) over water is almost constant in latitude, but the zonal mean of ZmaxBB over land is less at latitudes above 20 degrees in the northern hemisphere. The width of BB increases as ZmaxBB increases. The zonal mean of the BB width in mid latitude exhibits a small but smooth decrease as the latitude increases.

https://www.jstage.jst.go.jp/article/jmsj/87A/0/87A_0_31/_pdf

TRMM PR Standard Algorithm 2A23 and its Performance on Bright Band Detection

The melting layer in precipitation is physically modeled and compared with high resolution Doppler radar data. The model includes a new formulation of the dielectric properties and can handle all ice particles with densities ranging from pure snow to hail. The air temperature is calculated from the vertical air velocity. The model can simulate the aggregation and breakup of the melting particles. The melting layer is often observed as a bright band; the prominence of the bright band is related to the density of particles before melting. The width of the band increases with the rain intensity; according to the model this is mainly caused by an increase in the almost isothermal layer at the upper part of the melting layer. The results of the model are in good agreement with Doppler radar observations. According to the model, the reflectivity is very sensitive to the dielectric properties and density of the melting particles and the influence of aggregation is restricted. Because of the assumption o...

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Radar Observations and Simulation of the Melting Layer of Precipitation

Attenuation and reflection of a melting layer are calculated using a meteorological model. The model employs a new scheme for the calculation of the dielectric properties of melting ice particles with densities ranging from those of loose snow to hail, and a new scheme for calculating the melting rate is employed. The input parameters are derived from high resolution Doppler radar data and provide a data set for statistical analysis. Statistical relations were derived for attenuation in the melting layer based on measurements made with a surface rain gauge, a radar (with and without a polarisation facility), and a lower frequency satellite link. It was found that the specific attenuation of melting snow surpasses the value of rain because of the larger size and particle number density during melting, the attenuation within the melting layer increasing with its reflection. In stratiform precipitation, the attenuation in the melting layer is found to increase only slightly with frequency. The reflection in the melting layer decreases with the frequency of the radio waves in such a way that a bright band is only observed for frequencies below 20 GHz.

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Attenuation and reflection of radio waves by a melting layer of precipitation

A model of the melting layer is employed on radar measurements to simulate the attenuation of radio waves at 12, 20 and 30 GHz. The attenuation in the melting layer is simulated to be slightly larger than that of rain with the same path length and precipitation intensity. The result appears to depend on the maximum reflectivity in the melting layer.

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Wim Klaassen

Papers

Radar Observations and Simulation of the Melting Layer of Precipitation

Attenuation and reflection of radio waves by a melting layer of precipitation

Attenuation in melting layer of precipitation