Previous studies have suggest a dual-wavelength radar system comprised of X- and K_a-bands is well-suited for ground-based remote sensing of single and mixed-phase (ice and liquid) clouds. Transmitted radiation at K_a-band is measurably attenuated by liquid water; the range-differentiated difference between the reflectivities is proportional to the amount of liquid. In practice, factors such as non-Rayleigh scattering, the presence of ice crystals, measurement errors and sensitivity of the instruments have confounded evaluations of liquid water content (LWC) retrievals during field trials.

The analyses confirm that retrieval of LWC and median volume diameter from radar reflectivity alone is not feasible. An earlier study defined a radar estimated size (RES), based on reflectivity and attenuation measurements available from the dual-wavelength system. Analysis of the simulation results suggests in the case of cloud and drizzle conditions, dual-wavelength radar observations are capable of retrieving LWC and RES. Mixed-phase (ice and liquid) radar measurements were detected using a dual-wavelength ratio when ice particle size is comparable to radar wavelength. Small ice crystals (<~1 mm diameter) do not affect the LWC retrieval but the RES estimate is biased upward by their contribution to total reflectivity. In the case of non-Rayleigh scattering from larger ice crystals or raindrops, the difference in reflectivity between the two wavelengths is no longer monotonically increasing as it is in the case of pure Rayleigh scattering. In these cases, the local minimum reflectivity differences were used to estimate attenuation. As a result, spatial resolution of the LWC estimate is compromised in the mixed-phase regions. The effect of radar measurement error on attenuation estimation was also investigated using various range-averaging lengths. Based on these analyses, an optimum design and data processing scheme for a dual-wavelength system is presented.

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