Estimation of Cloud Content by W-Band Radar

Kenneth Sassen Department of Meteorology, University of Utah, Salt Lake City, Utah

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Liang Liao Department of Meteorology, University of Utah, Salt Lake City, Utah

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Abstract

W-band (3.2-mm) radars are seeing increasing utilization as a result of improving microwave technologies and the increased research emphasis being given to nonprecipitating clouds. This niche is exemplified by the study of the radiatively important stratus and cirrus clouds, which essentially require the application of Rayleigh and nonspherical scattering solutions, respectively. To increase the utility of such studies, the authors provide the following relations derived from empirical and model-derived particle size distributions that rely on a combination of Rayleigh and conjugate gradient-fast Fourier transform scattering theory approaches to relate (equivalent) radar reflectivity factors (Ze) Z (mm6 m−3) to liquid water content (LWC, g m−3) and ice water content (IWC, mg m−3): Z = (3.6/Nd)LWC1.8 for stratus clouds, where Nd (cm−3) is the droplet concentration, and IWC = 21.7 Ze0.83 for cirrus clouds using the dielectric constant appropriate for ice, which is valid over a IWC range of 3–100 mg m−3. Sources of 95-GHz attenuation are also discussed.

In addition, radar estimates of the lidar volume extinction coefficient σl (m−1) are derived using the exponential ice particle size distributions, yielding σl = 6.5 × 10−4Ze0.86 for solid ice particles, or 9.65 × 10−4Ze0.81 if an ice density of 0.5 g cm−3 is used to approximate the effects of hollow ice crystals in cirrus clouds.

Abstract

W-band (3.2-mm) radars are seeing increasing utilization as a result of improving microwave technologies and the increased research emphasis being given to nonprecipitating clouds. This niche is exemplified by the study of the radiatively important stratus and cirrus clouds, which essentially require the application of Rayleigh and nonspherical scattering solutions, respectively. To increase the utility of such studies, the authors provide the following relations derived from empirical and model-derived particle size distributions that rely on a combination of Rayleigh and conjugate gradient-fast Fourier transform scattering theory approaches to relate (equivalent) radar reflectivity factors (Ze) Z (mm6 m−3) to liquid water content (LWC, g m−3) and ice water content (IWC, mg m−3): Z = (3.6/Nd)LWC1.8 for stratus clouds, where Nd (cm−3) is the droplet concentration, and IWC = 21.7 Ze0.83 for cirrus clouds using the dielectric constant appropriate for ice, which is valid over a IWC range of 3–100 mg m−3. Sources of 95-GHz attenuation are also discussed.

In addition, radar estimates of the lidar volume extinction coefficient σl (m−1) are derived using the exponential ice particle size distributions, yielding σl = 6.5 × 10−4Ze0.86 for solid ice particles, or 9.65 × 10−4Ze0.81 if an ice density of 0.5 g cm−3 is used to approximate the effects of hollow ice crystals in cirrus clouds.

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