An Example of Uncertainty in Sea Level Pressure Reduction

Abstract

Difficulty analyzing mesoscale features in California and Nevada for a 1991 case study prompted a review of techniques for sea level pressure (SLP) reduction and an evaluation of the performance of the various techniques for the U.S. west coast states at 0000 UTC 30 November 1991. The objective of any SLP reduction procedure is to provide a pressure field that portrays meteorological features rather than terrain features, a difficult goal to meet in this region given the steep terrain gradients on the western slopes of the Sierra Nevada range. The review and evaluation are performed both for techniques applicable at individual stations and for techniques applicable at grid points in a model analysis or forecast. When using station data, one would like to perform a manual or objective analysis of SLP with the greatest number of stations possible by adding stations that report only altimeter setting to the stations that report both SLP and altimeter setting. The results of the comparison show that the incorporation of altimeter-setting stations into an analysis of SLP was found to be practical only at elevations less than 300 m. Above this, the standard reduction includes empirical corrections that cannot be easily duplicated, and the other reduction techniques yielded values that varied over a large enough range that the uncertainty associated with the choice of technique becomes too great to permit the analysis of weak mesoscale features. At such low elevations, the various techniques examined gave similar results; therefore, the simple reduction is recommended. In elevated plateau regions, a pressure analysis on a geopotential surface at approximately the mean terrain height is recommended to minimize reduction errors. No satisfactory solution was found for regions with steep terrain gradients. Computing SLP from model objective analyses or forecasts that are in the model’s native vertical coordinate, typically the terrain-following sigma coordinate, poses a different set of problems. The model terrain field is usually smoothed and so contains regions where it differs significantly from the actual terrain. This is sufficient in itself to yield reduction errors that have a coherent mesoscale signature. In addition, SLP fields computed using available techniques vary widely in areas of higher terrain elevation, sometimes producing mesoscale features that suspiciously coincide with terrain features and so suggest reduction error. These mesoscale pressure artifacts are also often associated with unrealistic geostrophic wind speed maxima. The Mesinger method of defining the below-ground temperature field by horizontal interpolation across terrain features after interpolating the model sigma-level objective analyses to pressure surfaces worked best for this case. It produced values that agreed reasonably well with the manual SLP analysis and with the 1300-m pressure analysis over Nevada, without generating an artificial geostrophic wind speed maximum.