Assessing satellite-based precipitation estimates in the Southern Appalachian mountains using rain gauges and TRMM PR

摘要

A study was performed using the first full year of raingauge records from a newly deployed network in the Southern Appalachianmountains. This is a region characterized by complex topography withorographic rainfall enhancement up to 300% over small distances (<8 km).Rain gauge observations were used to assess precipitation estimatesfrom the Precipitation Radar (PR) on board of the TRMM satellite,specifically the TRMM PR 2A25 precipitation product. Results showsubstantial differences between annual records and isolated events (e.g. tropicalstorm Fay). An overall bias of ?27% was found between TRMM PR2A25 rain rate and rain gauge rain rates for the complete one year of study(?59% for tropical storm Fay). Besides differences observed forconcurrent observations by the satellite and the rain gauges, a large numberof rainfall events is detected independently by either one of the observingsystems alone (rain gauges: 50% of events are missed by TRMM PR; TRMM PR:20% of events are not detected by the rain gauges), especially for lightrainfall conditions (0.1–2mm/h) that account for more than 80% of all themissed satellite events. An exploratory investigation using a microphysicalmodel along with TRMM reflectivity factors at selected heights was conductedto determine the shape of the drop size distribution (DSD) that can beapplied to reduce the difference between TRMM estimates and rain gaugeobservations. The results suggest that the critical DSD parameter is thenumber concentration of very small drops. For tropical storm Fay an increaseof one order of magnitude in the number of small drops is apparently neededto capture the observed rainfall rate regardless of the value of themeasured reflectivity. This is consistent with DSD observations that reporthigh concentrations of small and/or midsize drops in the case of tropicalstorms.