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Title: Analysis of windsat 3rd and 4th stokes components over Arct Sea ice

Author
item NARVEKAR, P - University Of Bremen
item HEYGSTER, G - University Of Bremen
item TONBOE, R - Collaborator
item Jackson, Thomas

Submitted to: IEEE Transactions on Geoscience and Remote Sensing
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/1/2011
Publication Date: 5/31/2011
Citation: Narvekar, P., Heygster, G., Tonboe, R., Jackson, T.J. 2011. Analysis of windsat 3rd and 4th stokes components over Arct Sea ice. IEEE Transactions on Geoscience and Remote Sensing. 49:1627-1636.

Interpretive Summary: The effects of geophysical features of the Arctic on passive microwave emission were analyzed to determine if geometric feature information could be extracted from higher order components of the measurements, which could contribute to improved soil moisture estimation in other regions. Satellite-based passive microwave radiometers provide valuable information on a variety of the Earth’s land surface phenomena, including soil moisture for hydrology and climate. One of the current resources is a satellite instrument called WindSat, which includes capabilities beyond those of previous satellites sensors. This additional information is hypothesized to reflect geometric or other features, which could be used to improve into current retrieval techniques. In this study these effects were analyzed for arctic surfaces. Analysis of a surface without the effects of varying soil moisture allows us to identify what contribution the geometric, or other, features have on the sensor observations. From the quantitative analyses conducted, it was found that varying melt conditions of sea ice play a dominant role in the response of the new sensor measurements. This additional information would improve sea ice algorithms and indicates that it may be useful for identifying freezing and thawing conditions over land. WindSat is a predecessor to the future operational weather and climate satellites that will be employed by forecasters in the future. Improvements in the WindSat products, including soil moisture, will be utilized in numerical weather prediction and climate models that provide important decision information to agricultural hydrology.

Technical Abstract: WindSat has provided an opportunity to investigate the first spaceborne passive fully polarimetric observations of the Earth’s surface. In the present study, we investigated the Arctic sea ice. The passive polarimetric data is provided in the form of the modified Stokes vector consisting of four parameters. The first two components of the modified Stokes vector are the vertically and horizontally polarized brightness temperatures, which have been continuously measured by various radiometers over last three decades. The 3rd and 4th Stokes components provide information on the degree of polarization of the emission. In the present study, three types of analysis are carried out: spatial (maps considering different azimuth angle intervals), temporal (time series of daily averaged Stokes components over small selected azimuth angle range) and azimuthal (variations with respect to azimuth angle over selected study areas). Analysis showed that the highest brightness temperature variations for the 37 GHz 3rd Stokes component (>2 K) during summer. The next highest signals were observed for the 10.7 GHz 3rd and 4th Stokes components (>1 K) also during summer. The 37 GHz 4th Stokes component showed the least variability (>1 K). Spikes of up to 2 K were identified in the time series of the 37 GHz 3rd Stokes component during mid January 2004 (winter) over first-year ice regions. The near surface air temperature of the European Center for Medium-Range Weather Forecasts (ECMWF) model data and another satellite product derived from the Scanning Multichannel Microwave/ Imager (SSM/I, revealed that during these events the surface temperatures reached near melting and the retrieved ice concentrations were reduced to about 80%. Moreover, observations made during these events showed a clear first harmonic azimuthal dependency. Geophysical parameters, such as temperature and ice leads, are possible causes. Ice surface temperatures near melting were identified as the geophysical processes that caused the larger signals during summer.