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Caveats for the standard products

Caveats of Integrated Water Vapor product

Algorithm development PI: Dr. Yoshiaki Takeuchi (Japan Meteorological Agency)

This document briefly describes some caveats for the AMSR-E Level 2 standard products of integrated water vapor.
Retrieval areas
This product retrieves integrated water vapor over global oceans (except over sea ice). Land areas are masked out by using land-ocean flag information based on the GTOPO30 database. Sea-ice areas are excluded by referring to the AMSR-E Level 3 sea-ice concentration product. In addition, water vapor is not retrieved in regions where the sea-surface wind speed exceeds 20 m/s or the input brightness temperatures are poor quality. Integrated water vapor may be retrieved over large lakes.

Retrieval range and error
Water vapor retrieved ranges from 0 to 70 kg/m2. The retrieval error was estimated to be less than 3.5 kg/m2 using the match-up dataset of radiosonde-derived water vapor and AMSR-E estimates.

Quality flags included in the product indicate the categories of clear, cloudy, heavy rain, and low quality. The retrieval errors are within 3.5 kg/m2 only for the data with the clear category flag. Data qualities of the estimates in other categories are inferior to that of the clear category. Therefore, users must select the data depending on their purpose.

Estimates in heavy rain category
In the version 1.0 product, values of integrated water vapor are set to -999.9 in the heavy rain category because there are not enough match-up datasets with radiosondes for computing retrieval coefficients for the heavy rain category.

Ancillary data used in retrieval
During retrieval, sea-surface wind speeds, 850hPa air temperatures, and sea-surface temperatures (SST) are used as ancillary input data. Sea-surface wind speeds and 850hPa air temperatures are obtained by interpolating (in space and time) the global analysis (GANAL) and forecast data provided by the Japan Meteorological Agency (JMA). The three-day average of AMSR-E Level 3 SST is used as SST ancillary information. Climatological SST values are used as an alternative where the AMSR-E Level 3 SST is not available (e.g., near coast lines).