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Satellite Services Division
Precipitation Operation

The Satellite Services Division's (SSD) Satellite Analysis Branch (SAB) of the National Environmental Satellite, Data, and Information Service (NESDIS) provides quantitative satellite-derived precipitation estimates (SPE's) and satellite guidance to the National Weather Service (NWS) whenever heavy convective rains threaten to produce, or are producing flash flooding over the lower 48 states and Puerto Rico. Satellite precipitation estimates and guidance are also provided for heavy rain and snow associated with winter storms, and for heavy lake effect snow. The estimates are sent as part of the satellite precipitation estimate message (SPENES) available on the Advanced Weather Interactive Processing System (AWIPS). SPENES is the AWIPS ID, and the WMO header for this message is TXUS20 KNES. In addition to the quantitative satellite precipitation estimates, the SPENES message includes a discussion of satellite analysis and trends. In composing the message, the following data are used: GOES infrared (IR), visible, and water vapor imagery; GOES Sounder data and Derived Product Imagery; GOES satellite-derived winds; various microwave data from the Defense Meteorological Satellite Program (DMSP) Special Sensor Microwave Imager (SSM/I) and the NOAA Advanced Microwave Sounder Unit (AMSU); and surface and upper air data.

Satellite precipitation estimates are produced by the Hydroestimator (H-E), which is the latest version of the Automatic Rainfall Estimator (AE), and the Interactive Flash Flood Analyzer (IFFA). The Auto-Estimator became the primary source of operational satellite rainfall estimates beginning in June 2000. The H-E version became operational in September 2002.

The Auto-Estimator was originally developed by Dr. Gilberto Vicente, Office of Research and Applications (ORA)/NESDIS, to automatically produce rainfall estimates every half hour for the entire continental U.S. (CONUS) using the GOES IR imagery. The current H-E version uses an algorithm based on IR cloud top temperatures to determine an instantaneous rainfall rate. Instantaneous rates now derived every 15 minutes are combined to produce 1 hour, 3 hour, and 6 hour totals. 24 hour estimated totals for the whole U.S. are produced at 12Z. Corrections are applied to the estimates for parallax (satellite viewing angle), available moisture, and orographics. In addition, an automatic equilibrium level (EL) adjustment is applied to warm top convection when the equilibrium level is warmer than -60C. Using autoestimates for operations has enabled SAB Meteorologists to produce more SPENES messages and to cover more heavy rain events when compared to using manual Interactive Flash Flood Analyzer (IFFA) estimates. The timeliness of SAB rainfall estimates and SPENES messages has also improved using autoestimates.

The Hydro-Estimator (H-E) version of the Auto-Estimator (AE) was developed by Clay Davenport, a contractor working for the ORA Hydrology Team under the direction of Dr. Rod Scofield. The Hydro-Estimator algorithm differs from the original AE by using a brightness temperature screening technique. It adjusts the rain rate assigned to each picture element (pixel) according to the surrounding pixel temperatures. This helps separate raining and non-raining pixels and decreases the need for radar screening. It also helps focus rainfall estimate totals into more clearly defined maximums. There is less of a tendency for overestimating for very cold cloud tops using the H-E, and it does a much better job of estimating for large mesoscale convective complexes (MCC's). The H-E also has a different way of handling the moisture corrections, and also produces more frequent products every 15 minutes for all except the 24 hour totals. The 1 hour H-E totals are available on the NWS AWIPS system as a graphic for the whole CONUS every hour.

The Auto-Estimator and Hydro-Estimator work well for most cold top convection. However, they do not work well for winter storms and for some warm top convection. As a result, it is up to the judgment of the SAB Meteorologist, based on his experience, to decide which system (AE, H-E or IFFA) is best to use for SAB Precipitation Operations. For snowfall estimates, only the IFFA is used.

The Interactive Flash Flood Analyzer (IFFA) is a man-machine interactive system where estimates are done manually and then stored and totaled for whatever time period is needed. An orographic correction can be applied to the IFFA estimates for time periods of 9 hours or less in the mountainous areas of the western U.S. and the Appalachians. The quantitative estimating techniques developed by Dr. Rod Scofield of ORA, and modified by SAB meteorologists, are used on the IFFA for convective rains and winter storms for which the Hydro-Estimator/Auto-Estimator are not used. A NESDIS in-house technique developed by SAB is used for lake effect snows. For the IFFA, the estimating techniques use cloud top temperatures and satellite signatures to determine hourly and half hourly estimates from a series of animated IR and visible images.

Web Page: H-E and AE rainfall estimates and IFFA rain and snow estimates are put onto the Internet as a graphical product on the Satellite Services Division (SSD) homepage, at: http://www.ssd.noaa.gov. They can be found under Operational Products and Services. The SPENES messages are posted there as well. The graphics, usually paired up with corresponding SPENES messages, are available in real-time. The autoestimates can be displayed as Instantaneous (rain rate), 1, 3, and 6 hour. IFFA estimates can be displayed as 1 hour and multi-hour totals. (Note.. H-E 1 hour rainfall graphics, as well as IFFA graphics, are also available on AWIPS). A link is provided to ORA's Flash Flood Page which contains H-E graphics for areas such as Alaska, Hawaii, the Caribbean, and Central and South America. The Web address is: http://www.orbit.nesdis.noaa.gov/smcd/emb/ff

The NPPU: The SAB precipitation operations are collocated with the Hydrological Prediction Center (HPC) of the National Center for Environmental Prediction (NCEP) in what is called the National Precipitation Prediction Unit (NPPU) in the NOAA Science Center. The SAB precipitation meteorologists give HPC forecasters 6 regularly scheduled satellite briefings per day as input for their Quantitative Precipitation Forecasts (QPF's), excessive rainfall potential outlooks, and heavy snow forecasts. All of the satellite products that are available for input into the SPENES messages are also used in briefing HPC. Additional unscheduled briefings are given to HPC by the SAB precipitation meteorologists any time there is new satellite data available that would be of value in analyzing or forecasting heavy precipitation. Briefings are also provided to the Basic Weather forecaster, who issues the 1-2 day forecast maps over the continental U.S. SAB and HPC have had this close working relationship since 1993 in the NPPU, and the satellite input has been very important for the HPC operations.