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ASCAT NWP Ocean Calibration

JA Verspeek, A Stoffelen

The objective of Ocean Calibration (OC) is to find corrections of the normalized radar cross section, per antenna and Wind Vector Cell (WVC) that improve the ASCAT wind retrieval. The Numerical Weather Prediction (NWP) Satellite Application Facility (SAF) ASCAT Wind Data Processor (AWDP) is used for the ASCAT wind retrieval [NWPSAF site; ASCAT user manual].

Currently, AWDP uses corrections based on a visual correction method for OC (VOC) [Verspeek et al. 2008]. In this method the Geophysical Model Function (GMF) is evaluated in the measurement space for its consistency with the distribution of measurement points. The measurement space is defined for each WVC as the three-dimensional or 3D (x,y,z) space spun up by the measured values of respectively the fore, aft and mid beams [Stoffelen and Anderson, 1997]. The two-dimensional (2D) GMF surface is a
conical surface and the (fore, aft, mid) measurement triplets should generally lie in the proximity of this surface. The visual correction method scales the three axes of the measurement space, i.e., the fore, aft and mid beam backscatter values, such that the distribution of measurement triplets is shifted towards the conical GMF surface.

Another method for Ocean Calibration (OC) resides in direct comparison of measured backscatter values with simulated values from NWP winds using the GMF [Stoffelen, 1998; Freilich, 1999; Verspeek, 2006]. For the ASCAT and ERS scatterometers this NWP-based OC (NOC) estimates , the mean transformed backscatter over the ocean for a uniform wind direction distribution and compares this with the mean measured backscatter over the ocean for a given wind distribution, as further explained in section 2.

The VOC and NOC methods each have their pros and cons. In this report we will evaluate these and verify how both VOC and NOC antenna and WVC dependent correction tables improve the wind retrieval. The aspects of the wind retrieval that are most affected by the correction tables are the Quality Control (QC), retrieved wind direction distribution and thus wind direction error and the distribution of normalized distance between measurement triplet and GMF in measurement space. Metrics based on the statistical distribution of these variables will be discussed and evaluated in section 6.

Bibliografische gegevens

JA Verspeek, A Stoffelen. ASCAT NWP Ocean Calibration
2010, OSISAF, 2010

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