The Use of Isentropic Analysis in Short Term Forecasting

Abstract

S the U.S. Weather Bureau, Navy, and Army expanded their programs of obtaining records of temperature, pressure, humidity, and wind in the free atmosphere, meteorologists have attempted to find methods of using these data to best advantage. While maps of atmospheric pressure at various levels have been found especially helpful in weather forecasting, the use of temperatures and humidities on such constant level charts has hardly done more than make atmospheric disturbances seem more complicated in vertical structure than surface weather analyses would indicate. Considering that air moves in the vertical as well as horizontal plane and that vertical displacements lead to adiabatic temperature changes, it is not surprising to find that any chart on which temperatures and humidities are compared level for level is unsatisfactory for tracing the movements of large-scale air currents from day to day. In order to eliminate the effects of these adiabatic changes Rossby suggested the use of two elements which are invariant with adiabatic processes—specific humidity and potential temperature. Since, in a stable atmosphere air particles tend to flow along surfaces of potential temperature and at the same time retain their specific humidity, Rossby suggested that charts be constructed along surfaces of constant potential temperature, which are identical with isentropic surfaces. Further support for the use of such isentropic charts is found in the logical assumption that horizontal wind shearing stresses should be chiefly operative along surfaces of potential temperature. While these lateral shearing stresses until recently have been almost completely neglected in the study of atmospheric flow, much evidence is accumulating in support of the hypothesis that these stresses are frequently of an order of magnitude large eough to produce important hydro dynamical effects. The function of the isentropic chart is thus to serve the two-fold purpose of (1), facilitating the study of the identification and thermodynamic modification of large-scale air currents, and (2), presenting a clearcut picture of the major patterns of flow of atmospheric currents through the configurations of lines of constant moisture.