Effect of Natural Variations in Rain Drop Size Distributions on Rain Rate Estimators of 3 cm Wavelength Polarimetric Radar

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概要

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• Statistical errors of rain rate estimators due to natural variations in raindrop size distribution (DSD) are studied for 3-cm wavelength polarimetric radar. Four types of estimators are examined : A classical estimator R(Z_H), and three types of polarimetric radar estimators R(K_<DP>), R(Z_H, Z_<DR>), and R(K_<DP>, Z_<DR>), where R is the rain rate, Z_H is the reflectivity factor at horizontal polarization, K_<DP> is the specific differential phase, and Z_<DR> is the differential reflectivity. The T-matrix method is employed for the scattering calculations, and a total of 7,664 one-minute raindrop size spectra, measured with a Joss-Waldvogel type disdrometer are used. According to simulation results, the normalized errors (NEs) of R(Z_H), R(K_<DP>), R(K<DP>, Z_<DR>), and R(Z_H, Z_<DR>) for all DSD samples are 25%, 14%, 9%, and 10%, respectively. The NEs of all estimators, except R(Z_H), tend to decrease with increasing rain rate. For rain rates larger than 10mmh^<-1>, e.g., the average NEs of R(Z_H), R(K_<DP>), R(K_<DP>, Z_<DR>), and R(Z_H, Z_<DR>) are 25%, 9%, 5%, and 7%, respectively. The simulation results show that the classical estimator R(Z_H) is the most sensitive to variations in DSD and the estimator R(K_<DP>, Z_<DR>) is the least sensitive. The lowest sensitivity of the rain estimator R(K_<DP>, Z_<DR>) to variations in DSD can be explained by the following facts. The difference in the forward-scattering amplitudes at horizontal and vertical polarizations, which contributes K_<DP>, is proportional to the 4.78th power of the drop diameter. On the other hand, the exponent of the backscatter cross section, which contributes to Z_H, is proportional to the 6.38th power of the drop diameter. Because the rain rate R is proportional to the 3.67th power of the drop diameter, K_<DP> is less sensitive to DSD variations than Z_H. However, DSD spectra with unusually large median volume diameter D_0 can increase the estimation error of R(K_<DP>). The differential reflectivity Z_<DR> reduces the effect of unusual D_0 and is useful for further improvement of the estimator R(K_<DP>). This is due to the fact that Z_<DR> itself is a good measure of D_0.

• 社団法人日本気象学会の論文
• 2005-10-25

著者

• Park Sang

  National Research Institute For Earth Science And Disaster Prevention (nied)

• Park Sang-goon

  National Research Institute For Earth Science And Disaster Prevention (nied)

• Bringi V.n.

  Colorado State Univ. Fort Collins

• Maki Masayuki

  National Research Institute For Earth And Disaster Prevention

• Maki Masayuki

  National Res. Inst. For Earth And Disaster Prevention

• BRINGI V.

  Department of Electrical and Computer Engineering, Colorado State University

• Maki Masayuki

  National Inst. For Earth Sci. And Disaster Prevention

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