Blended TPW Products Overview

1. Introduction

The TPW products are generally available from various sensors/satellites. However, these products carry different biases and sampling errors due to the difference in algorithms, instruments, and also temporal and spatial sampling resolutions. What would be helpful, especially to forecasters, is a unified, meteorologically significant TPW field, which merges all available TPW products, with a non-gap global coverage.

The blended TPW products, developed at CIRA with forecasters in mind, merges the TPW retrievals from various satellites/sensors/algorithms, currently including NOAA-19/-20, MetOP-B, S-NPP, GCOM-W1 and GPM Microwave Imager (GMI) from Global Precipitation Mission (GPM), and also Met-GPS, to provide a unified global TPW map in supporting of the NWS users’ need.

The blended TPW product, together with its counterpart Percentage of TPW Normal, have being used by forecasters and satellite analysts for years, and approved their value on helping forecasters to analysis and forecast heavy rain and flooding, and also the transfer of moisture from ocean to land. The products are tailored to fit the requests of different end users, such as, AWIPS format over desired AWIPS regions for NWS users, McIDAS format for NESDIS/SAB, and PNG/GIF images for public internet users.

2. Total Precipitable Water

Total Precipitable Water (TPW, also called Integrated Water Vapor or IWV) is the integrated amount of water vapor in a column from the surface of the earth to space in kilograms per square meter or, equivalently, in millimeters of condensate. The blended TPW product is a global composite derived from several data sources, which include the Advanced Microwave Sounding Unit (AMSU) from Polar Operational Environmental Satellites (POES) and the Meteorological Operational satellites (MetOp), Advanced Technology Microwave Sounder(ATMS) form Suomi-NPP and NOAA-20, Advanced Microwave Scanning Radiometer 2(AMSR2) from GCOM-W1, and also TPW retrievals from Global Positional Satellite (GPS) meteorological data. By combining all of these observations, the blended TPW eliminates the bias between data sets and provides a global TPW map for weather forecasters and satellite data analysts.

3. Percentage of TPW Normal

The Percentage of TPW normal, or TPW anomaly, shows the percentage departures from the climatology. The normal value is based on a weekly climatology of total precipitable water that comes from NASA's Water Vapor Project (NVAP). The data used to derive the climatology is from 1988-1999. The TPW anomaly assists forecasters to answer the question, "How unusual is the moisture field?" Very high percentage values (200% or more) indicate a strong flooding potential or a possible severe weather indicator, while low values indicate potential fire hazards.

4. Global Map of TPW and TPW anomaly

The TPW and TPW anomaly are mapped with a map projection that is useful to forecasters and then composited, by either averaging or latest value, with the latest 12 hour worth of data to make a global map. These maps are produced hourly within the framework of Data Processing and Error Analysis System (DPEAS). The products are output with a Mercator projection with 16-km resolution at the equator. The map is centered at the equator and 160°W. It has 1251 lines and 2501 elements, which covers the earth from about 71°N to 71°S. The cut line is at 20°E, which was chosen to emphasize ocean areas. A new Cylindrical map projection data file in netCDF4 format is also made available when the product is updated to include NOAA-20 TPW retrievals. The gridded products are converted into McIDAS format for NESDIS Satellite analysts and are also sectorized into 4 AWIPS regions (Super National, Alaska, Hawaii, Puerto Rico) for NWS forecasters.

For more information, or to receive the operational products, please contact NESDIS Precipitation Products Area Lead.