Wednesday, 12 February 2003
Scatterometer-based correction of forecast-model coastal winds
Wind stress fields during summer 2000 and 2001 from the operational Eta
mesoscale atmospheric forecast model and the QuikSCAT/SeaWinds
scatterometer are compared for the coastal region west of Oregon and
California, extending offshore to 130 W. A simple correction
is proposed for systematic error in the model fields. The scatterometer
measurements show regions of intensified stress near orographic
features, consistent with previous measurements and results from
high-resolution mesoscale model simulations. In these regions of
orographic intensification, the moderate-resolution (32-km)
Eta model systematically underpredicts the observed wind stress by a
factor reaching 1/2 or less, and places the axis of the maximum wind
farther offshore than do the scatterometer measurements. However,
the temporal correlations between Eta-model and QuikSCAT wind
stresses remain high (0.85-0.9) even in these regions of intensified stress,
except in a narrow region adjacent to the coast, where the correlations
decrease abruptly. These high correlations motivate the use of a
complex linear regression model to reduce the systematic error,
relative to QuikSCAT measurements, in the forecast-model
coastal-zone wind stress.
The regression model is calibrated using a split-sample validation method,
applied for summer 2000 and summer 2001 data.
The regression model produces
a wind stress field on the nominal 25-km QuikSCAT grid from 40-km
Eta output, and substantially reduces the Root Mean Square Error (RMSE)
relative to QuikSCAT measurements. The RMSE of the regression-corrected
Eta model is smaller than that of two
high-resolution, nested, mesoscale dynamical models, implemented
using standard procedures.
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