Wednesday, 12 February 2003
Normalized radar cross-section (NRCS) patterns from QuikSCAT—A new analysis tool over the tropical oceans
This paper examines synoptic-scale surface wind and rain patterns over the tropical oceans as depicted in Normalized Radar Cross-Section (NRCS) images measured by the SeaWinds scatterometer on board the QuikSCAT satellite. The wind and rain field patterns seen in the NRCS images in the vicinity of tropical cyclones (TCs) are examined for their depiction of TC position, structure, and the surrounding environmental wind flow. The NRCS images look similar to the familiar visible and infra-red images of cloud and rain patterns, however, the NRCS product is actually an image of the small-scale roughness of the ocean surface measured at 14 GHz. While the chosen wind vector and multiple (ambiguity) solution data products from QuikSCAT have been available for the past 3 ˝ years, the NRCS product has only been routinely available from the NOAA/NESDIS QuikSCAT Storm web site since September, 2001. Currently only the vertically polarized forward beam NRCS product is displayed. The QuikSCAT NRCS is displayed as shades of gray on a 3km grid for each swath overpass of a TC. Although the NRCS image can be more difficult to interpret than the more standard 25km wind product, synoptic-scale wind and rain patterns are readily discernable. In particular, the lower (darker) sigma-0 signals are easily distinguished from neighboring higher (brighter) sigma-0 signals which generally correspond to a rougher surface caused by higher winds. The directionally-dependent signal also gives the analyst an indication of flow direction. Characteristically, the low wind/rain free center of a TC often shows a sharp contrast to the strong wind and rain surrounding the center. While the depiction of an intense tropical cyclone with a well developed eye tends to be easy to discern in the NRCS images, this study shows that considerable information can be also gleaned for all stages of TC development. Finally this study will show how this product can be a useful analysis tool over any ocean surface for all types of marine weather interests.