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The Moderate Resolution Imaging Spectroradiometer (MODIS) sensor aboard the Terra and Aqua satellites allows roughly for the same region of Earth to be sampled twice in a nowcasting time frame. Using the MODIS cloud mask at 5-km resolution and radiosonde data, cloud nowcasting methods were evaluated from December 2008 through November 2009 over Utah and southwestern Wyoming (207 cases), a region with a variety of terrain. The hypothesis that a combination of persistence and displacement techniques (i.e., a hybrid technique) in regions of complex terrain should yield a better forecast than either method alone is tested; clouds forced by complex terrain often appear to persist along topographic barriers. A new forecast skill evaluation method is introduced, designed to equate correct cloudy and correct clear areas. The persistence nowcasting method demonstrated the highest skill [as much as 10% critical success index improvement (CSI) over other methods]. However, all nowcasting techniques performed similarly during the summer months (~65% CSI). It is concluded that cloud nowcasts at 5-km resolution over complex terrain in the forecast area, using no model wind or moisture data, do not improve upon a persistence nowcast. However, because these basic nowcasting methods are computationally fast, educated decisions on minute to hour time scales can be made nearly instantaneously.

Evaluating Satellite-Based Cloud Persistence and Displacement Nowcasting Techniques over Complex Terrain