The theory and use of upper air thickness patterns in forecasting

Abstract

AbstractThe combination of the ordinary synoptic chart with the 1,000—500 mb thickness pattern, and the implied 500 mb contour chart, provides a first approximation to a three‐dimensional representation of the troposphere. Although incomplete, the representation permits the introduction of dynamical and thermodynamical theory and provides a basis for scientific forecasting. The forecasting of thickness patterns, in effect of the temperature field, is a thermodynamical problem. The theory of advective, adiabatic and non‐adiabatic temperature change is discussed. A useful approximation to advection is obtained by regarding the thickness lines as material lines embedded in the surface geostrophic wind field. Climatological monthly means and extremes of thickness are given; these provide a practical framework to the forecast. The dynamical development is related with the geometry of the thickness patterns as indicating the vertical variation of vorticity transport. Attention is directed to the development field associated with thermal troughs and ridges, confluence and difluence, and certain combinations of these. When the dynamical development so produced sets up a circulation which by advection maintains or increases the thermal distortion in the same sense, the situation is self‐maintaining or self‐developing. The warm sector depression, the baroclinic anticyclone and the jet stream complex are examples of such systems. A step‐by‐step procedure of forecasting prebaratic and pre‐thickness charts, which has been used in experimental four‐day forecasts, is outlined.