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Using Synthetic Aperature Radar (SAR) data as a verification tool for mesoscale modeling

One of the challenges of mesoscale modeling in Alaska is the paucity of observations available for verification purposes. One of the more useful tools in this regard is wind speed derived from synthetic aperature radar (SAR) as part of the Alaska SAR Demonstration Project, supported by NOAA/NESDIS and the Office of Naval Research (ONR)

Below is a SAR image, valid 24-Jan-2002, 03:18:40Z, that shows a westerly jet in Prince William Sound (shading indicates wind speed). The jet, while occupying a good part of the western Sound, is still a sub-gridscale feature to the NOGAPS model at 1 degree by 1 degree resolution (Arrows depict NOGAPS predicted wind speed and direction, produced independently from the SAR processing). Note that SAR is only capable of imaging wind speed. Direction can often be inferred however, as in the image below, from the orientation of wind speed features and knowledge of the local wind climatology.

SAR image valid 24-Jan-2002, 03:18:40Z. The arrows depict wind speed and direction from the NOGAPS model at 1 degree by 1 degree resolution. The shading depicts wind speed for both model and SAR winds. Click here for the full-size version of the image

We conducted a RAMS simulation from the period 23-Jan-2002, 18z to 24-Jan-2002, 6Z on a nested grid configuration described on the "modeling in the Prince William Sound "page. A loop of surface wind vectors and wind speed (m/s) at 1 hour intervals can be seen here.

The model does a fairly good job with the main feature, the jet occupying Prince William Sound. The model does not predict the northerly gap flow originating in the Copper River Delta (to the east of PWS), perhaps because the terrain is not well resolved even on the 4 km grid. It is also possible that the pressure gradient produced in the simulation was not oriented quite correctly to produce the jet. (This area is well-known for its high wind events.) The model also produced a coastal jet emerging from Resurrection Bay (bottom left of simulation figures) that doesn't appear in the SAR image.

Unfortunately, the SAR coverage is such that a time series, similar to that produced by the model, is not possible. We just have this one snap shot for validation purposes. However that one image is still very useful for evaluating the performance of the high-resolution mesoscale model in predicting atmosphere/terrain interactions.