ice and water as a function of temperature and frequency are given in Table 1. These values closely agree with values reported by other authors (22,23). III. MICROPHYSICAL PARAMETERS A number of droplet size distribution and rainfall rate models are used to examine the attenuation of microwave radiation by both scattering and absorption processes. The Rain L and Rain M size distribution functions for light and moderate rainfall conditions have been proposed by Deirmendjian (24). Rain L has a liquid water (wj content of 0.117 g/m3, with drop mode radius (rm) of 0.07 mm, while Rain M has wL = 0.495 g/m3 and rm = 0.05 mm. For moderate and heavy rainfall conditions, Deirmendjian has added the Rain 10 and Rain 50 rain drop distribution functions, respectively. Rain 10 represents drop spectra below the cloud base for a precipitation rate of 10 mm/h at the ground with rm = 0.33 mm and wL = 0.509 g/m3. Rain 50 represents heavy rain during rainfall rates of 50 mm/h at the ground, with rm = 0.60 mm and wL = 2.11 g/m3. Table 2 shows the volume extinction (/3e) and absorption (/3„) coefficients (km-1) for selected microwave frequencies as a function of temperature for the four Deirmendjian drop distribution functions mentioned above. Rain 50 represents rainfall rates of 50 mm/h, while measurements indicate that rates as high as 150-250 mm/h are not uncommon (25). The Marshall-Palmer (26) drop size distribution n(r) has been found to provide realistic fits for most continental mid-latitude situations, expressed with a negative exponential form: n(r) = noe~kr , (3) where n0 = 1.6 x 104 m-3 mm-1 for all rain (27) and r is the drop radius. The slope parameter X depends only upon rainfall rate according to X(R) = 8.2R-0-21, (4) with X in mm-1 and R in mm/h. These values are appropriate for moderate widespread rain. However, n0 is related to the intensity of convective activity and the process responsible for precipitation; therefore, n0 is not a constant (28). Applying the Marshall-Palmer (MP) distribution function for the values given above, Table 3 shows attenuation rates as a function of temperature for rainfall rates between 25 and 150 mm/h. The MP distribution is not appropriate for heavy rainfall. Nevertheless, it is utilized here in order to provide at least qualitative estimates. Underestimation of the number density of large drops using the MP distribution leads to an underestimation of the scattering characteristics. Comparison of Tables 2 and 3 shows that for a rainfall rate of 50 mm/h there are noticeable differences between the attenuation rates for the assumed drop size distributions. In general, the extinction and absorption coefficients are larger for the MP distribution than for the Deirmendjian modified gamma size distribution. While the liquid water content is slightly larger for the Marshall-Palmer Rain 50 (MP Rain 50) distribution than for the Deirmendjian Rain 50 (D Rain 50) distribution, this difference cannot account for the large variation in attenuation coefficients. The MP distribution for Rain 50 has a larger drop mode radius and a larger concentration of large drops. Attenuation parameters, in general, increase in value with decreasing tempera-
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