Abstract
A method for station or grid point reduction of surface pressure to sea level or some other level is presented that shows improvement over the standard reduction method in the western United States. This method (MAPS SLP-Mesoscale Analysis and Prediction System sea level pressure) uses the 700 hPa temperature to estimate an “effective” surface temperature from which the temperature of the hypothetical layer beneath the ground is estimated. The use of this “effective” temperature instead of the observed surface temperature is responsible for the improved reduction since it varies more smoothly over space and time and is more representative of the temperature variation found above the boundary layer.
The MAPS SLP reduction was compared with the standard reduction and altimeter setting reduction in statistical comparisons of geostrophic wind estimates with observed winds and in a case study. A 21-month comparison between geostrophic and observed winds was made over different geographical regions, times of day, rotation angles and seasons. The results showed that the MAPS SLP reduction performed better than the standard reduction in the western United States, but not in other regions with generally low elevation. In general, the correlation between sea level geostrophic winds and observed winds was found to be dependent on the Froude number. A statistical comparison using a smaller sample between MAPS SLP and the Sangster geostrophic wind, which is not a station reduction, showed similar skill over the western United States. The case study also showed that the pattern over the western United States was more coherent and less anomalous with MAPS SLP that with the other reductions.
