Local versus remote grid points in climate downscaling

Abstract

The association between daily as well as monthly winter precipitation and concurrent 700 hPa heights for 3 different North American precipitation regimes was examined using correlation fields and a typing of daily 700 hPa height patterns. Wet days, regardless of circulation type, were found to be associated with negative height anomalies immediately to the west or northwest of the precipitation regions, at the 'local' grid point. These small-scale height anomalies were found to rep- resent traveling short waves that were missed by the circulation typing scheme. The correlation fields between monthly precipitation and concurrent 700 hPa heights revealed a westward shift of the height minima to a location west of 100°W, to a 'remote' grid point. The nature of these remote min- ima was examined using the 10 wettest and 10 driest months. Decomposition of the height anomalies at the remote grid points into between-type and within-type contributions revealed that while some of the negative heights during wet months are due to an increase in the frequency of circulation pat- terns with below normal heights (such as troughs or weak ridges), more than two-thirds of the con- tribution comes from height depressions superimposed upon the larger-scale patterns and most of these are associated with positive vorticity. This leads to the conclusion that the correlation minima west of 100° W at the monthly time scale reflect the frequent generation of short waves during wet months over the well-known North American cyclogenetic region east of the Rocky Mountains. These results have 2 implications with respect to statistical downscaling. First, regarding techniques designed to extract information from a single grid point: the optimum grid point location may be a function of the time scale under consideration. Second, regarding techniques designed to extract information from large-scale circulation patterns: the difficulties in capturing the very important but small short waves limit their usefulness.