An Improved Technique for Computing the Horizontal Pressure-Gradient Force at the Earth's Surface

Abstract

On conventional surface analyses, sea level isobars allow a forecaster to compute the horizontal pressure-gradient force at sea level, which for much of the world is fairly close to the earth's surface. However, over elevated terrain the procedures invoked in reducing the observed station pressures to sea level can give results that are markedly different from those obtained directly from surface data, as shown in an earlier paper by the author. This paper represents an extension of that work. A revised variable reference atmosphere is developed and used as an aid in computing the horizontal pressure-gradient force on a smoothed terrain approximating the earth's surface. Surface observations are input to the procedure, and surface geostrophic winds, or alternatively, stream and potential functions of the surface geostrophic wind, are output. The methods described herein are currently in operational use at the National Meteorological Center. Charts are shown for summer situations over the Great Plains when and where they have especial utility, as well as for a winter case where the reduction-to-sea level process led to very strong gradients over elevated terrain on sea level pressure charts.