DSD参数对双频雷达估测降水的影响研究

Study on the Impacts of DSD Parameters on Precipitation Estimation Using Dual-Frequency Radar

双频雷达通过估计降水的雨滴谱分布(Rain Drop Size Distribution,DSD)参数来估测降水,相比多普勒天气雷达利用Z-I关系估测降水的方法,估测结果更加准确。真实的雨滴谱分布随时空不断变化,难以准确描述。本研究的目的是评估将其描述成Gamma分布模型对双频雷达估测降水造成的误差。研究中利用架设在广东龙门和西藏那曲的雨滴谱仪观测资料,计算Ku/Ka双频雷达的反射率因子,利用双频雷达算法反演降水率R和衰减系数k,并将其与雨滴谱仪直接计算的结果进行比较,探讨双频雷达反演算法中DSD模型对反演结果造成的影响,并对比反演误差的地区差异性。研究结果表明:DSD参数对双频雷达反演降水的影响在那曲和龙门地区有一定的差异。从双频技术反演的质量加权的粒子直径(Dm)、Ka波长衰减系数(kKa)、Ku波长衰减系数(kKu)和降水率(R)来看,当Gamma分布的形状因子μ分别取2、3和4时,龙门地区对四个参数的反演都能得到较为稳定准确的结果,相对误差基本<10%。而就kKa和5~40 mm·h^-1的降水率而言,那曲和龙门有很大的不同,在龙门当μ取3时相对误差最小,都在0值附近波动,而那曲则要需要μ值在4和6附近才能使得相对误差达到最小。

Compared with Doppler radar,which uses radar reflectivity-rainfall rate(Z-R)relationship to esti‐mate precipitation rates,the dual-frequency radar can provide more accurate estimates of precipitation rates based on retrieving the Rain Drop Size Distribution(DSD)parameters.However,actual DSD spectra varies with time and space and is difficult to be described accurately.The purpose of this study is to evaluate the retriev‐al errors caused by DSD models adopted in Ku/Ka-band dual-frequency radar retrieval.Based on continuous sum‐mer DSD data from PARSIVEL disdrometer at Longmen and Naqu observatories,the radar effective reflectivity factors at the Ka and Ku bands are calculated.Then,the median mass-weighted diameter Dm,attenuation coeffi‐cients k and rain rates R obtained from the DSD estimated by dual-frequency techniques are compared with those directly computed from DSD spectra measured by the disdrometer to investigate the impact of DSD model on the dual-frequency retrieval techniques and compare the differences between different regions.Overall,a gamma DSD model with fixedμvalues of 2,3 or 4 could yield consistent and fairly accurate retrievals,although the im‐pacts of DSD parameters on the dual-frequency radar retrieval technique in Naqu and Longmen regions are differ‐ent.In Longmen region,the relative biases of the retrieved Dm,the Ka-band attenuation coefficient kKa,the Kuband attenuation coefficient kKu and R are less than±10%bias when the shape factorμof the gamma distribution ranges from 2 to 4.In terms of kKa and rain rate of 5~40 mm·h^-1,the Gamma DSD model withμ=3 in Longmen region could yield the best accuracy and the relative bias fluctuates around 0,while the Gamma DSD model withμ=4,6 in the Naqu region could produce the smallest errors.