In general, an orographic barrier increases the extent and duration of cloudiness and precipitation on the windward side, and decreases it on the leeward side.

AIR MASS CLOUDINESS AND PRECIPITATION

If an air mass is lifted over an orographic barrier, and the lifting is sufficient for the air to reach its lifting condensation level, cloudiness of the convective type occurs. If the air is convectively unstable and has sufficient moisture, showers or thunderstorms occur. The preceding situations occur on the windward side of the barrier.

Curvature (path of movement) of the flow aloft also affects the occurrence of cloudiness and precipitation. In a cool air mass, showers and cumulus and stratocumulus clouds are found in those portions of the air mass that are moving in a cyclonically curved path. In a warm air mass, cloudiness and precipitation will be abundant under a current turning cyclonically or moving in a straight line. Clear skies occur where a current of air is moving from the north in a straight line or in an anticyclonically curved path. Also, clear skies are observed in a current of air moving from the south if it is turning sharply anticyclonically. Elongated V-shaped troughs aloft have cloudiness and precipitation in the southerly current in advance of the troughs, with clearing at and behind the trough. These rules also apply in situations where this type of low is associated with frontal situations.

Cellular cloud patterns (open or closed), as shown by satellite imagery, will aid the forecaster in identifying regions of cold air advection, areas of cyclonic, anticyclonic, and divergent wind flow.

Open cellular cloud patterns are most commonly found to the rear of cold fronts in cold, unstable air. These patterns are made up of many individual cumuliform cells. The cells are composed of cloudless, or less cloudy, centers surrounded by cloud walls with a predominant ring or U-shape. In the polar air mass, the open cellular patterns that form in the deep, cold air are predominately cumulus congestus and cumulonimbus. The open cells that form in the subtropical high are mainly stratocumulus, cumulus, or cumulus congestus clusters. For open cells to form in a polar high, there must be moderate to intense heating of the air mass from below.

Figure 4-8.-Open cells on satellite Imagery.

When this polar air mass moves out over the water, the moist layer is shallow and capped by a subsidence inversion near the coast. Further downstream the vertical extent of the moist layer and the height of the clouds increases due to air mass modifications by the underlying surface. In figure 4-8, the open cells behind a polar front over the North Atlantic indicate cold air advection and cyclonic curvature of the low-level wind flow. Vertical thickness of the cumulus at A is small, but increases eastward toward B.

Figure 4-9 shows a large area of the subtropical high west of Peru covered with open cells. These are not associated with low-level cyclonic flow or steady cold air advection.

This information is now available on CD in Adobe PDF Printable Format

Privacy Statement - Press Release - Copyright Information. - Contact Us - Support Integrated Publishing