Continuity of 3B42 and 3B42RT (G. J. Huffman, 9/5/2009)
The TRMM Multi-Satellite Precipitation Analysis (TMPA) produces three-hourly precipitation rates on a 0.25°x0.25° latitude/longitude grid covering the latitude band 50°N-50°S. The TMPA is computed twice by TRMM, first as an experimental best-effort real-time (RT) monitoring product (3B42RT), and then as a post-real-time research-quality product (3B42). In both cases, precipitation estimates from the available microwave sensors, including TMI, AMSR-E, SSM/I, AMSU, and MHS, are intercalibrated and combined (respectively available as product 3B40RT and a field in the upcoming Version-7 3B42). The microwave precipitation and brightness temperatures from the international constellation of geosynchronous-orbit infrared (IR) sensors are used to create a calibrated IR estimate (respectively 3B41RT and a field in the upcoming Version-7 3B42). A multi-satellite estimate is created by selecting the combined microwave precipitation where available and the IR elsewhere. The final output is then computed by calibrating the multi-satellite estimate to a monthly satellite-gauge combination (respectively 3B42RT and 3B42). The RT satellite-gauge calibrator is climatological, whereas for the research product (product 3B43) the calibrator is computed month-by-month. The selection of inputs used may change and/or production may be suspended for either data set as needed to accommodate anomalies in the input data.
It is planned that the TMPA products will be computed continuously until successor products are available from the Global Precipitation Measurement (GPM) Mission (see below). If TRMM should fail before GPM is available, the impact would be somewhat modest. TRMM’s actual contribution of data is modest, around 10% of the gridbox values. More important is TRMM’s role as the calibration standard. For the RT product, a transition was made in 2005 to a climatological intercalibration to adjust the various microwave sensors to the TMI, based on studies showing relatively modest differences between using climatological and month-to-month calibrations. On the other hand, these differences are more important for the research product, where the TMI-PR combined-sensor estimate is used as the calibrator. So, in the event of a TRMM failure, the research version would have to make do with a climatological calibration. The quantitative impact of losing TRMM has not been assessed for the TMPA, but it is expected to result in additional uncertainty in the finest time and space scales.
Global Precipitation Measurement (GPM) Mission
GPM is a cooperative mission between NASA, the Japan Aerospace Exploration Agency (JAXA), and other international partners. Building on the success of TRMM, GPM will solidify the international constellation of satellites used to estimate global precipitation, a key climate factor. GPM science objectives are to (1) improve knowledge of the global water cycle variability and its links to climate change; (2) gain new insights into storm structures, cloud microphysics, and mesoscale dynamics error; and (3) improve understanding of climate processes and prediction of future climate.
For more information:
+ GPM mission site
+ JAXA GPM site
+ GPM Brochure