At the left-hand side in Fig. 5 is schematically illustrated the hydrometer type as follows: (starting from the top) a dry snowflake, two melting snowflake models in which water coatings are accumulating on the ice crystal branches and interbranch cavities of the shrinking particles, an irregular water-enclosed severely melted snowflake, a near-spherical mixed-phase drop, a drop with most of the ice melted, and, finally, a homogeneous raindrop. The relative size of the particles is based roughly on a 10:1 ice-to-water particle density ratio. These images are more or less what are actually sensed by visible wavelength lidar, where backscattering responds to the exact details of particle shape. In contrast, further to the right is a characterization of the radar cross sections that an S-band radar would sense, where simple particle models can be employed and the difference in particle phase (i.e., dielectric constant) is of significance to backscattering. Such Rayleigh scattering particle models are apparently always violated by the larger particles present in rain showers with W-band radars, such that a mixture of the optical and microwave models is, in effect, sensed. Accordingly, K-band radars, with their intermediate wavelengths, may sense conditions somewhere between the S and W bands, depending on the sizes of the hydrometeors in each case. The remainder of Fig. 5 shows idealized lidar backscattering (with the generic effects of attenuation on returned power above the dark band, included as the dashed line), and W- and S-band Ze and V profiles.
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