17:15 〜 18:45
[AAS01-P05] Estimating terminal velocity of raindrops in stratiform precipitation using vertical observation data from ground-based X-band radar
キーワード：雨滴の終端落下速度、層状性降水、地上レーダ、EarthCARE
Earth Cloud Aerosol and Radiation Explorer (EarthCARE) is equipped with a cloud radar called CPR (94 GHz) that can measure Doppler velocity and is expected to obtain observational vertical velocities within clouds over the globe. However, vertical Doppler velocity (Vd) observed from the satellite is the sum of the terminal velocity of precipitation particles weighted by radar reflectivity factor (Vtz) and vertical air motion (Va). To investigate the velocity of falling precipitation particles, Vtz and Va need to be separated from Vd. The aim of this study is to estimate the terminal velocity of raindrops using a method for estimating vertical air motion from radar alone, prior to EarthCARE observations, and to estimate the variation in the power law of the radar reflectivity factor required for this estimation.
A ground-based X-band radar (9 GHz) of Nagoya University is conducting vertical pointing observations at the Shigaraki MU Observatory (34.85°N, 136.11°E). Radar reflectivity factor (Ze) [mm6/m3] and vertical Doppler velocity (Vd) [m/s] observed by ground-based X-band radar are used. The effective radius of raindrops is re, and from Ze∝re6 and |Vtz|∝reη, we can express |Vtz|∝ Zeη/6=Zeα. Vax is the vertical air motion estimated from X-band radar, since Vax=Vd-Vtz=Vd-Zeα, we can estimate the vertical air motion from Vd and Ze. The values of 10-minute vertical air motion (Vamu) from the MU radar is assumed as true, and the optimal value of α is set when the difference between Vax and Vamu is the smallest. To restrict the analysis to the rainfall region, only grid points less than 1500 m from the melting layer are used. ERA5 is used for detection of altitude in melting layer. In order to assume Vamu (10-minute mean) to be true, only periods of stratiform precipitation which are considered to have a small temporal variation, are extracted and analyzed. The Bright Band Fraction method is used to extract periods of stratiform precipitation.
The analysis for June 2, 2023, 9:40 am to 4:59 pm (JST, case (a)), and June 30, 2023, 5:30 pm to July 1, 2023, 10:40 am (JST, case (b)), resulted in the optimal mean values of α are 0.24 for case (a) and 0.39 for case (b). However, the differences of the values in α with respect to dBZe (=10log10Ze) are small between the cases. The value of α tends to increase when dBZe is low (roughly less than 10 dBZ). In the case (b), low dBZe is observed, which resulted in a larger mean value of α compared to the case (a). For stratiform rainfall which is considered to be less influenced by vertical air motion, we suggest that the optimal α can be given as a function of dBZe, which may facilitate the estimation of |Vtz|. Since this characteristic is expected to appear in the Rayleigh regime for stratiform rainfall, the α is given as a function of dBZe, also in observations of CPR.
A ground-based X-band radar (9 GHz) of Nagoya University is conducting vertical pointing observations at the Shigaraki MU Observatory (34.85°N, 136.11°E). Radar reflectivity factor (Ze) [mm6/m3] and vertical Doppler velocity (Vd) [m/s] observed by ground-based X-band radar are used. The effective radius of raindrops is re, and from Ze∝re6 and |Vtz|∝reη, we can express |Vtz|∝ Zeη/6=Zeα. Vax is the vertical air motion estimated from X-band radar, since Vax=Vd-Vtz=Vd-Zeα, we can estimate the vertical air motion from Vd and Ze. The values of 10-minute vertical air motion (Vamu) from the MU radar is assumed as true, and the optimal value of α is set when the difference between Vax and Vamu is the smallest. To restrict the analysis to the rainfall region, only grid points less than 1500 m from the melting layer are used. ERA5 is used for detection of altitude in melting layer. In order to assume Vamu (10-minute mean) to be true, only periods of stratiform precipitation which are considered to have a small temporal variation, are extracted and analyzed. The Bright Band Fraction method is used to extract periods of stratiform precipitation.
The analysis for June 2, 2023, 9:40 am to 4:59 pm (JST, case (a)), and June 30, 2023, 5:30 pm to July 1, 2023, 10:40 am (JST, case (b)), resulted in the optimal mean values of α are 0.24 for case (a) and 0.39 for case (b). However, the differences of the values in α with respect to dBZe (=10log10Ze) are small between the cases. The value of α tends to increase when dBZe is low (roughly less than 10 dBZ). In the case (b), low dBZe is observed, which resulted in a larger mean value of α compared to the case (a). For stratiform rainfall which is considered to be less influenced by vertical air motion, we suggest that the optimal α can be given as a function of dBZe, which may facilitate the estimation of |Vtz|. Since this characteristic is expected to appear in the Rayleigh regime for stratiform rainfall, the α is given as a function of dBZe, also in observations of CPR.