18 October 2005 Determination of the effect of dew on passive microwave observations from space
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Abstract
Recent field experiments showed that dew has a significant effect on L-band (1.4 GHz) microwave observations. At an experimental grass site in the Netherlands (ELBARA2003), and at an experimental fallow site in France (SMOSREX) several dew events were able to increase the horizontal polarized brightness temperature up to 10 K. The Microwave Polarization Difference Index (MPDI) was shown to be a powerful index to describe the effect of dew. Current satellite missions (i.e. TRMM and SSM/I) but also future missions (i.e. HYDROS and SMOS) observe the Earth surface when dew is likely, between 6-8 AM. These observations are used in soil moisture retrieval methodologies, and ignoring of the dew effect may lead to a significant underestimation of soil moisture. Therefore we started, as a follow up of the previous field studies, an investigation of the effect of dew on microwave observations at satellite scale. Two months of TRMM data were selected to study the diurnal variations of the microwave signal and their relation to morning dew. Between February and March 1998 distinct diurnal MPDI patterns were detected from space. The MPDI values at X band (~10 GHz) were significantly higher in the afternoon, compared to the morning for several agricultural regions in the northern part of the state of Oklahoma in the United States. These diurnal MPDI variations from space were similar as the patterns as detected by the dew affected field observations at L-band, leading us to conclude that TRMM data at X-band is as well affected by dew.
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Richard A. M. De Jeu, Richard A. M. De Jeu, Thomas R. H. Holmes, Thomas R. H. Holmes, Manfred Owe, Manfred Owe, } "Determination of the effect of dew on passive microwave observations from space", Proc. SPIE 5976, Remote Sensing for Agriculture, Ecosystems, and Hydrology VII, 597608 (18 October 2005); doi: 10.1117/12.627919; https://doi.org/10.1117/12.627919
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