The Challenge of Surface Type Changes Over the Aral Sea for Satellite Remote Sensing of Precipitation

Abstract

Frequent false signals of precipitation in satellite passive microwave retrievals over the Aral Sea have been identified as being caused by an outdated surface database. The database includes the surface type, elevation, and the percentage of the primary surface type in each grid. It was also found that the grid resolution of 1/6 degree (∼18 km at the equator) of the outdated surface data was too coarse to process Global Precipitation Measurement (GPM) Mission microwave (GMI) imager data, which have a field of view (FOV) resolution of 31.68 km <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> for channels at 89 GHz, 166.5 GHz and 183.31 GHz). In this study, we generated a new surface database at a resolution of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$0.05^\circ \times 0.05^\circ $</tex-math></inline-formula> (∼5.6 km at the equator). Using the new surface data, the false precipitation problem above is addressed. At the same time, the retrieval accuracy for other parameters such as total precipitable water over the Aral Sea is significantly improved as well. The resolution of the database is suitable for most microwave and infrared sounding measurements of atmospheric temperature and moisture profiles as well as precipitation. By comparing this new surface data against the outdated surface data, we also see the loss of permanent ice in Antarctica and a dramatic reduction of water surface over the Aral Sea. Using a 40-year record of remote sensing data, we can observe the steady decrease in size of the Aral Sea, as a result of regional water use policies and natural climate change.