CloudSat Level 2-C Precipitation Column Algorithm
[Source: CloudSat Level 2-C Precipitation Column Algorithm Product Process Description and Interface Control Document, http://www.cloudsat.cira.colostate.edu
The basis of the work is outlined in Haynes et al. , and all quantitative and mathematical details may be obtained from this source. An overview of the physical basis of the retrieval follows. ... The algorithm makes use of path integrated attenuation (PIA) due to hydrometeors as a geophysical measurement. The method depends on the well-behaved relationship between the backscatter cross section of the ocean surface, σ0 , and the wind speed, V at the ocean surface. Higher wind speeds cause greater roughening of the ocean surface, resulting in increased scattering of microwave radiation away from the radar receiver and a lower resulting surface backscatter cross section. The sea surface temperature (SST) of the ocean surface also inﬂuences the backscatter cross section through variation of the index of refraction. A database of observations of the surface backscatter cross section under clear-sky conditions, σclr , provides a background reference for the state of the surface when hydrometeors are absent. When cloud or rain is present, the observed backscatter cross section is reduced by hydrometeor attenuation. This reduction allows calculation of PIA given knowledge of the wind speed at the ocean surface (derived from a numerical model) and, to a lesser extent, the SST .
The unattenuated radar reﬂectivity, Zu , near the surface is closely related to the presence of rain; the higher Zu the more likely precipitation is occurring. Zu is the sum of the measured reﬂectivity, the PIA, and a component due to gaseous attenuation, G (determined from the ECMWF-AUX temperature and moisture proﬁle).
Multiple scattering within the precipitating column can be signiﬁcant for rainfall exceeding a few millimeter per hour , so Monte Carlo modeling is used to simulate the relationship between rainfall and observed PIA for various vertical proﬁles of precipitation. A model of the melting layer is also incorporated into the Monte Carlo calculations to better represent the transition from snow to rain. This melting layer model aims to treat the attenuating characteristics of melting snowﬂakes. The model follows snow (modeled through the discrete dipole approximation) falling through a melting layer and melting into rain, assuming a constant lapse rate, Γe , of 6 ◦ C km−1 .
Liquid or mixed precipitation layers are considered to extend to the height of the lowest continuous cloud layer, HCT L , as determined from the 2B-GEOPROF cloud mask, capped by the height of the freezing level, Hf , from ECMWF-AUX. The effects of purely frozen precipitation on PIA are only considered when a core of 10 dBZ of greater reﬂectivity extends through the freezing level, Hsig . When such a core is absent, melting is considered to start at the freezing level itself. The combination of HCT L , Hf , and Hsig allow determination of the total depth of all precipitation, Dtot , and liquid precipitation, Dliq .
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