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.
Rights
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