Satellite images that illustrate the clearing of cumulus clouds downwind from snow-covered areas are presented. The cloud clearing resembles that occasionally observed with lakes during warm advection, supporting the suggestion that the thermal forcing associated with a uniform snow-covered area is comparable to that of a cold-water lake of similar size. Analysis of snow cover patterns in the central United States suggests that the climatological probability for situations conducive to the cloud clearing is at most once per month.
Abstract Estimation of stream channel heads is an important task since ephemeral channels play a significant role in the transport of sediment and materials to perennial streams. The slope‐area method utilizes digital elevation model ( DEM ) and related information to develop slope‐area threshold relationships used to estimate the position of channel heads in the watershed. A total of 162 stream channel heads were mapped across the three physiographic regions of Alabama, including the Southwestern Appalachians (51), Piedmont/Ridge and Valley (61), and Coastal Plains (51). Using Geographic Information System and DEM , the local slope and drainage area for each mapped channel head was calculated and region‐specific models were developed and evaluated. Results demonstrated the local slope and drainage area had an inverse and strong correlation in the Piedmont/Ridge and Valley region ( r 2 = 0.71) and the Southwestern Appalachian region ( r 2 = 0.61). Among three physiographic regions, the weakest correlation was observed in the Coastal Plain region ( r 2 = 0.45). By comparing the locations of modeled channel heads to those located in the field, calculated reliability and sensitivity indices indicated model accuracy and reliance were weak to moderate. However, the slope‐area method helped define the upstream boundaries of a more detailed channel network than that derived from the 1:24,000‐scale National Hydrography Dataset, which is commonly used for planning and regulatory purposes.
We used 20th century simulations by nine global climate models (GCMs) to provide input for a streamflow model to simulate baseline hydrologic conditions in the Upper Mississippi River Basin (UMRB). Statistical tests revealed that streamflow data produced by members of the GCM multi‐model ensemble were serially uncorrelated at all lags and formed unimodal distributions and that GCM multi‐model results may be used to assess annual streamflow in the UMRB. Although all low‐resolution GCMs produced large differences from observations of streamflow and hydrological components simulated by the streamflow model, the nine‐member ensemble performed quite well. Results of statistical tests indicate that, of all models used, the high‐resolution GCM – the only high‐resolution model tested – gives simulated streamflows much closer to observed values, despite the fact that its low‐resolution sister model has no advantage over the other seven low‐resolution models.
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