ADVECTION OF AIR AND THE FORECASTING OF PRESSURE CHANGES

Abstract

The advection method of preparing prognostic pressure charts was tested using a number of different procedures. First, it was assumed that the density change at a given level can be regarded as representative of the density change throughout a layer which includes this level. Accordingly, isopycnic lines were drawn for five levels between sea level and 14,000 meters, and forecasts of the density changes at these levels were prepared on the hypothesis that the air moves with the geostrophic wind velocity, preserving its density. The sea level pressure change is obtained by adding the density changes for all levels after making allowance for the thickness of each layer. The effects of advection above 14,000 meters were neglected. The examples selected for a test gave poor results, and a statistical check showed that, even with perfect forecasts of the density, satisfactory results cannot be expected because the density changes at a given level are not sufficiently representative of the density changes in the whole layer. In the major part of the study the pressure forecasts were based on weight charts for four layers, from sea level to 13,000 meters. Since the weight, represented by the pressure difference between the bottom and the top of the layer, is proportional to the mean density of the layer, this method appears more satisfactory than the one studied first. To obtain prognostic weight changes, the motion of the weight lines has to be known. Three different methods for the determination of the motion of the weight lines were tested: The motion was assumed to be geostrophic. The trajectory of the air was found by successive approximations taking into account the variation of the pressure distribution during the forecast interval. The trajectory of the air was found under the assumption that the absolute vorticity is conserved (Rossby method). In all three cases the weight was considered as a conservative property of the air. The sea-level pressure changes were obtained by adding the prognosticated weight changes in all layers and the pressure change at the top level. The prognostic pressure changes thus prepared were quite unsatisfactory, and statistical checks gave correlation coefficients between the observed and prognostic pressure changes which were too small to be of forecasting significance. It is concluded that the advection method used by itself cannot be recommended as a forecasting tool. In addition to advection, other factors such as divergence and vertical motions make themselves felt too. Their effects have also to be taken into account if satisfactory prognostic charts are to be constructed in a purely objective fashion.