FRONTAL AND OROGRAPHIC CLOUDINESS AND PRECIPITATION

Cyclones are important generators of precipitation in the Tropics as well as in midlatitudes. Factors to be considered in arriving at an accurate forecast are listed below; these factors are not listed in any order of importance: The source region of the parent air mass. Nature of the underlying surface. The type and slope of the front(s). Wind and contour patterns aloft. Past speed and direction of movement of the low or front(s). Familiarization with the normal weather patterns. As pointed out earlier, a thorough understanding of the physical processes by which precipitation develops and spreads is essential to an accurate forecast. FRONTAL AND OROGRAPHIC CLOUDINESS AND PRECIPITATION There are unique cloud and precipitation features and characteristics associated with the cold and warm fronts, as well as orographic barriers. The following text discusses these features and characteristics. Cold Front You will find it helpful to use constant pressure charts in conjunction with the surface synoptic situation in forecasting cold frontal cloudiness and precipitation. When the contours at the 700-hPa level are perpendicular to the surface cold front, the band of weather associated with the front is narrow. This situation occurs with a fast-moving front. If the front is slow moving, the weather and precipitation will extend as far to the rear of the front as the winds at the 700- hPa level are parallel to the front. In both of the above cases, the flow at 700 hPa also indicates the slope of the front. Since the front at the 700-hPa level lies near the trough line, it is apparent that when the flow at 700 hPa is perpendicular to the surface front, the 700-hPa trough is very nearly above the surface trough; hence, the slope of the front is very steep. When the 700-hPa flow is parallel to the surface front, the 700-hPa trough lies to the rear of the surface front and beyond the region in which the flow continues parallel to the front. Consequently, the frontal slope is more gradual, and lifting is continuing between the surface and the 700- hPa level at some distance to the rear of the surface front. Another factor that contributes to the distribution of cloudiness and precipitation is the curvature of the flow aloft above the front. Cyclonic flow is associated with horizontal convergence, and anticyclonic flow is associated with horizontal divergence. Very little weather is associated with a cold front if the mean isotherms are perpendicular to the front. When the mean isotherms are parallel to the front, weather will occur with the front. This principle is associated with the contrast of the two air masses; hence, with the effectiveness of lifting. Satellite imagery provides a representative picture of the cloud structure of frontal systems. Active cold fronts appear as continuous, well-developed cloud bands composed of low, middle, and high clouds. This is caused by the upper wind flow, which is parallel, or nearly parallel, to the frontal zone (fig. 4-5). Figure 4-5.—An active cold front. 4-4