The Famine Early Warning Systems Network (FEWS NET) uses satellite rainfall estimates as inputs for monitoring agricultural food production. This information is used to evaluate food security conditions in different parts of the world. By combining high resolution (0.05°) rainfall mean fields with current satellite-derived rainfall estimates, the FEWS NET TRMM IR Pentad (FTIP) precipitation estimates provide pentatdal (5-day) rainfall fields suitable for crop monitoring. The FEWS NET FTIP precipitation product blends 0.25° resolution (c.25 km) Tropical Rainfall Measuring Mission (TRMM) rainfall estimates with higher resolution (4-km) rainfall estimates based on infrared observations. The FTIP product comes in two versions; one the FTIP-RT that uses real time data and is produced every five days, and the FTIP-V6 that uses improved rainfall estimates and is available at least a month after the RT.
The FTIP product takes advantage of a variety of satellite data such as the TRMM rainfall estimate from NASA (Huffman et al. 2007), at 0.25° pixel resolution, and the estimated rainfall from infrared (IR) temperature data from NOAA, at 4-km pixel resolution (Janowiak 2001). FTIP integrates satellite-derived estimates with a long term rainfall climatology developed by FEWS NET (FCLIM, see description below). Pentadal1 (5-day) inputs and outputs are the basis of the FTIP product. One of the inputs to the FTIP precipitation estimate is the TRMM rainfall estimate. The TRMM rainfall estimate combines a variety of satellite measurements from both the TRMM satellite and other low earth orbit platforms. In addition to the three sensors on the TRMM satellite (Precipitation Radar [PR], the TRMM Microwave Imager [TMI], and the Visible Infrared Scanner [VIRS]), the TRMM products also include passive microwave data from the Defense Meteorological Satellite Program satellites, the Aqua satellite, the NOAA satellite series, and infrared data collected by an international constellation of geosynchronous earth orbiting satellites. The TRMM family of products is composed of a variety of datasets based on the satellite data described. The FTIP process uses two of the TRMM products; one is TRMM-RT (3B42) available in 3-hourly increments, and totaled to a daily product with a historical archive 2005- present, used to produce FTIP-RT. Another product is The TRMM-V6 (3B43) which incorporates station data for better accuracy, and is updated monthly, with approximately a one-month lag time and a historical database 1998-2010, used to produce FTIP-V6.
Another input to the FTIP precipitation estimate is IR brightness data from the NOAA National Weather Service, merged from all available geostationary satellites (e.g., GOES-8/10, Meteosat-7/5 and GMS2) , globally-merged between 60°N and 60°S). Essentially, the IR records the emission temperature of either the earth's surface (which is relatively warm) or cloud tops (which are cold). When there are clouds, the observed IR is much colder than when there are no clouds. The colder it is, and more often it's cold (i.e., the cloudier it is), the more likely it is to rain. For many years, earth observing systems have used the amount of time the observed IR is below a threshold to estimate rainfall amounts. The final input to the FTIP precipitation estimate is the FEWS NET climatology (FCLIM) data. The FCLIM captures the historical spatial variability of rainfall by combining monthly mean rainfall measured at stations, elevation parameters, and satellite estimates of precipitation. Building on statistical blending procedures (Funk et al. 2007; Funk & Michaelsen 2004), the FCLIM approach uses a moving window regression to fit local models describing the spatial variations of the mean fields. The FCLIM depicts average monthly rainfall at 0.05° pixel resolution.
The FTIP precipitation estimate combines IR precipitation estimates, the long term average rainfall (FCLIM) and the TRMM products, the TRMM-RT is the imput for the FTIP-RT and the TRMM-V6 is the input for the FTIP-V6. . Both FTIP products use the following two equations.
Where IR%anom is the pentadal IR precipitation estimate divided by the mean of the IR time series (2001-2010) for the same pentad, and TRMM%anom is the pentadal TRMM (RT or V6) precipitation estimate divided by the mean of the TRMM (RT- or V6) time series for the same pentad. The mean TRMM-V6 is just the average of the 10 years time series (2001-2010). The mean TRMM-RT for a given pentad is calculated by averaging the previous pentad, two times the given prentad and the following pentad, to compensate for the short time series (2005-present). An equal weighting has been chosen because comparisons with station data in Guatemala and Colombia suggest that these two data streams have similar levels of accuracy. A small value is added to the denominator and numerator to handle zero values. The blended FTIP anomaly grid (FTIP%anom) (Eq.1) is then multiplied by the corresponding FCLIM grid (Eq.2), producing a rainfall estimate in mm.
The FTIP-RT product is available on the 2nd, 7th, 12th, 17th, 22nd, and 27th of each month. with a historical archive 2005- present. The FTIP-V6 product is available from 2001-2010. The FTIP products could be viewed here. geotif files could be downloaded for different regions from here.
Funk, Chris, Greg Husak, Joel Michaelsen, T. Love & Diego Pedreros. "2007: Third generation rainfall climatologies: satellite rainfall and topography provide a basis for smart interpolation". Crop and Rangeland Monitoring Workshop. Nairobi. March 2007.
Funk, Chris and Joel Michaelsen. "2004: A Simplified Diagnostic Model of Orographic Rainfall for Enhancing Satellite-Based Rainfall Estimates in Data-Poor Regions". Journal of Applied Meteorology 43, 1366-1378.
Huffman, G.J., R.F. Adler, D.T. Bolvin, G. Gu, E.J. Nelkin, K.P. Bowman, Y. Hong, E.F. Stocker, & D.B. Wolff. "2007: The TRMM Multisatellite Precipitation Analysis (TMPA): Quasi-Global, Multiyear, Combined-Sensor Precipitation Estimates at Fine Scales". Journal of Hydrometeorology 8, 38-55.
Janowiak, J.E., R.J. Joyce. & Y. Yarosh. "2001: A Real-Time Global Half-hourly Pixel-Resolution Infrared Dataset and its Applications". Bull. Amer. Meteor. Soc. 82, 205-217.