Improved Precipitation Rate Profile Estimation Method for the Main-lobe Clutter Region in GPM/DPR

In Dual-frequency Precipitation Radar (DPR), the observed radar reflectivity factor cannot be used near the ground surface because of the main lobe clutter. Therefore, the standard algorithm V07 estimates the precipitation rate in the main-lobe clutter region, assuming that the attenuation-corrected radar reflectivity factor does not change in the beam direction. On the other hand, Hirose et al. (2021) produced a database of vertical profiles of precipitation rate (DB21) using observations near the nadir, where the effect of main lobe clutter is relatively small, and applied it to observations outside the nadir to estimate precipitation rate at ground surface level. This estimation method extrapolates precipitation rate profiles estimated by the standard algorithm, and its consistency with other estimates, such as path-integrated attenuation, is not guaranteed.
In this study, we improved the precipitation rate estimation method in the standard algorithm using DB21 and its updated database (DB24). When the standard algorithm estimates using different parameters, the precipitation rate profile in the main lobe clutter region changes in the beam direction according to DB21 or DB24. The precipitation rate estimates obtained in this manner were consistent with those of the other estimates.
Experiments conducted for all orbits in June 2018 showed that the surface altitude precipitation rate of the dual-frequency algorithm increased by 6.6% (10.5%) compared with V07 when using DB21 (DB24). For DB24, the database classification by precipitation rate at the reference altitude (2.25 km or 3.25 km) was added. In addition, the classification of the database by precipitation rate gradient between the reference altitude and 0.5 km higher was subdivided. As a result, the downward increase in precipitation rate, especially in heavy precipitation, can be more easily expressed. The estimated precipitation rate at the clutter-free bottom was 1.2% lower for DB24 than for V07. This is due to the need to compensate for the general increase in precipitation rate in the main lobe clutter region, because the conditions of the surface reference technique remain the same.