Surface Rainfall Observations during TRMM-LBA Field Campaign

In support of the NASA Tropical Rainfall Measuring Mission (TRMM) satellite program, a series of field campaigns have being conducted in the rainy tropics to provide the detailed data for algorithm verification and validation of satellite rain estimates. As part of these field campaigns, TRMM-LBA (Large Scale Biosphere-Atmosphere Experiment in Amazonia) was conducted near Ji-Parana, Rondania, Brazil during January and February of 1999. This experiment was designed to acquire a further understanding of the climatological, ecological, biogeochemical and hydrological processes over tropical continent (Amazonia), an objective of the TRMM program. The TRMM is designed to measure tropical precipitation and its variation from a low-inclination orbit satellite. The rainy atmosphere of the TRMM-LBA region provided an excellent data set for the validation of satellite remote sensing estimate of rainfall. This paper focuses primarily on rainfall analysis from surface based disdrometers as a primary instrument for understanding rainfall amount and characteristics at the surface. It also addresses the differences of rainfall measurements within the surface observational network as well as the requirements for validation.

The surface observational network consists of forty (40) tipping bucket rain gauges, ten (10) impact disdrometers, and two (2) video disdrometers. Four different rain gauge networks were constructed within 150 km of the S-band and C-band Doppler radars. Two of the gauge networks had fourteen (14) rain gauges each and were within the dual Doppler radar lobes. The other two networks had five (5) gauges each. Two more gauges were later mounted at a far range for the C-band radar. Out of ten (10) impact disdrometers, a RD-69 disdrometer was located at the airport next to the dual wavelength profiler. Nine (9) APL disdrometers were distributed within the four rain gauge network. The two video disdrometers were at the airport and at the C-band radar site.

During TRMM-LBA, a RD-69 disdrometer was operated for 45 days from January 17 to March 2. The disdrometer recorded 285 mm of rainfall in 79 rainy hours. The mean and maximum rain rates were 0.3 mm h-1 and 120.7 mm h-1, respectively. The disdrometer observations demonstrated that it was raining about 15% for the preceding 15 days, while rain was present only 4 to 5% of the time for the first 30 days. In regard to rainfall intensity, relatively heavier rainfall fell during the first 15 days of the rainfall record. A collocated rain gauge showed a similar trend in percentage of time of rain and rainfall intensity. The RD-69 disdrometer recorded higher rainfall during the first 15 days, while the gauge had higher rainfall during the last 30 days. The differences between gauge and disdrometer rain totals are currently studied by examining rain events with other nearby gauges and disdrometers. More results will be presented during the conference.