Cloudiness and Precipitation in Relation to Frontal Lifting and Horizontal Convergence PDF Download

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Book Description
The physical processes which result in the formation of clouds and the production of precipitation have been described by numerous meteorologists. The genetical classification has been summarized by Petterssen as follows: (1) Clouds and precipitation types which form in unstable air masses, (2) Clouds and precipitation types which form in stable air masses, (3) Clouds and precipitation types which form in connection with quasi-horizontal inversions in the free atmosphere, and (4) Frontal clouds and precipitation forms. In general, the procedure adopted for the forecasting of these meteorological elements can also be subdivided into the same four categories. Since the distribution of the elements in the atmosphere is never constant with time, the problem of forecasting cloudiness and precipitation is sometimes diffcult. A detailed discussion of qualitative and quantitative prognostic methods can be found in various standard meteorological texts. However, present methods are not entirely adequate and it is the aim of this investigation to contribute to the solution of the forecast problem. This study has been divided into two main sections, viz., (1) Clouds and precipitation types which occur at frontal surfaces as the result of frontal lifting, and (2) Clouds and precipitation types which occur within air masses as the result of horizontal convergence within the wind field. (1) Frontal Surfaces. In a recent study of fronts and frontogenesis, Petterssen and Austin have investigated the processes that tend to create wind shear, or vorticity, along fronts. It was found that, in general, an increase or decrease in shear is accompanied by a change in the vertical velocity field at a front. Since cloudiness and precipitation arise from the ascending velocities at frontal surfaces, the possibility of forecasting the change in horizontal shear at a front will be investigated. Furthermore, it has been shown that the vertical velocity of either air mass at a frontal surface ca.