Cloudiness and precipitation in relation to frontal lifting and horizontal convergence
Austin, James Murdoch
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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 can be determined from the velocity of the front and the horizontal wind velocity. The wind velocities in the free atmosphere can be obtained with a reasonable degree of accuracy, but the front velocity cannot always be evaluated with the same accuracy. Because it is important to determine the magnitude or at least the sign of the vertical velocities, a kinematical study will be made of the displacement of frontal surfaces. (2) Horizontal Convergence. In 1931 Giao published an explanation of the origin of clouds and precipitation in the vicinity of moving cyclones, without introducing the concept of frontal surfaces. Giao claimed that most condensation phenomena could be attributed to the cooling produced by local pressure changes and the horizontal convergence which arises from the meridional component of the wind velocity. The conclusions are conveniently summarized by Haurwitz, together with a short discussion of Giao's theory. Since some systems of clouds and precipitation types may result solely from convergence in the horizontal flow, an attempt wil be made to evaluate this effect quantitatively.
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