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POLSCAT Observes High Spatial and Temporal Variability of Coastal Ocean Wind

Simon H. Yueh
tel, 818-354-3012; fax, 818-393-3077; email:

The Jet Propulsion Laboratory (JPL) polarimetric scatterometer (POLSCAT) is an airborne microwave radar operating at 13.9 GHz, designed to measure VV, HH, VH, and HV polarized radar response of ocean surfaces. It was developed to demonstrate the polarimetric radar technique for ocean wind measurements with unique wind direction determination capability. POLSCAT was deployed on the National Center for Atmospheric Research (NCAR) C-130 aircraft for proof-of-concept flights in August 2000. Several flights over the Monterey Bay Aquarium Research Institute buoys were completed with the successful demonstration of polarimetric wind scatterometer concept (Yueh et al, in press).

POLSCAT was flown on the NASA DC-8 together with the JPL airborne synthetic aperture radar (AIRSAR) on 17 February 2002 to acquire ocean surface wind off the coast of Santa Monica and Santa Barbara, California. The POLSCAT surface wind data will facilitate the interpretation of ocean current inferred from the AIRSAR along track interferometry. Two sets of flight lines were performed with a time separation of about 4 hours. The surface wind speed was retrieved from the POLSCAT radar data using the QuikSCAT geophysical model function together with the wind direction observations made by nearby ocean buoys. The wind was from the west during the DC-8 flights. The retrieved wind speeds are illustrated in Figures 1 and 2 with the start time labeled against each flight line.

The shadowing effects of the Santa Cruz and Catalina islands on the surface wind were apparent in Figure 1 (16:38 UT and 16:58 UT lines). The POLSCAT wind speed on the leeward side of the islands appeared weaker than the wind speed over other portions of the flight lines. The shadowing effects were most obvious near the Santa Cruz Island with the wind speed approaching as low as 1 m/s to the east of island in comparison with 4-6 m/s in adjacent areas (mid- and lower-right panels in Figure 1). Additional, there was a strong wind gradient near Santa Barbara (17:53 UT flight line, upper-right panel in Figure 1), which could also be a result of land surface topography (mountain range from Point Conception to Santa Barbara).

The high temporal variability of surface wind is indicated by the data from a set of repeated flight lines near Santa Monica, performed about 4 hours after the first set. Comparing the wind speed data plotted in Figures 1 and 2 shows a dramatic increase of wind by about 3-5 m/s from the morning to the afternoon off the coast of Santa Monica. This dramatic change could be related to the weather system passing through the southern California in the morning of February 17.

Figure 1:
POLSCAT wind from 7 flight lines off the coast of santa barbara and Santa Monica, California on 17 February 2002 indicates the high spatial variability of winds in coastal areas.

Figure 2:
POLSCAT wind from 4 flight lines off the coast of Santa Monica, California on 17 February 2002.

Yueh, Simon H., W. J. Wilson, and S. Dinardo, Polarimetric Radar Remote Sensing of Ocean Wind, IEEE Trans. Geoscience and Remote sensing, in press.

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