2009 年 87A 巻 p. 31-52
This paper presents the statistical properties of bright band (BB) measured by the Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR). Since BB is detected by PR standard version 6 (V6) algorithm 2A23 V6, this paper first outlines 2A23 V6, which classifies rain into three main categories: stratiform, convective, and other.
This paper studies BB from the viewpoint of algorithm development. It demonstrates that the detected BB count strongly depends on the antenna scan angle. The BB height (HBB) detected by the PR is compared with the 0°C height computed using the National Centers for Environmental Prediction-Department of Energy Atmospheric Model Intercomparison Project-II reanalysis (NCEP2) data. On average, the BB height is about 500 m below the 0°C height, which is computed from the NCEP2 data. However, a large difference between the above two heights is sometimes observed, mainly due to the false detection of BB. The reliability of BB detection by the current PR algorithm 2A23 V6 could be increased by introducing a simple filter that rejects BBs by regarding them as false when the number of BBs detected in one antenna scan (consisting of 49 antenna beam directions) is less than or equal to 5. An improved version of the filter is planned to be installed in the coming 2A23 V7. Statistics of 10 years of PR data indicate a small but discernible effect of the August 2001 boost of the altitude of the TRMM satellite on the rain type statistics and on the BB statistics. The zonal mean of the maximum value of reflectivity factor in the BB peak (ZmaxBB) over water is almost constant in latitude, but the zonal mean of ZmaxBB over land is less at latitudes above 20 degrees in the northern hemisphere. The width of BB increases as ZmaxBB increases. The zonal mean of the BB width in mid latitude exhibits a small but smooth decrease as the latitude increases.