Stratospheric and Upper Tropospheric Decrease in Mass

The chief cause of deepening lows is the decrease in mass in the upper troposphere and the lower stratosphere. With rapidly deepening lows, it is known that the change in mass in the stratosphere contributes as much to the local surface pressure change as do the tropospheric changes in density, if not more. Warming is frequently observed in the stratosphere over deepening surface lows, pointing to subsidence in the lower stratosphere. This warming is accompanied by lowering heights of constant pressure surfaces in the lower stratosphere, indicating a decrease in mass at high levels. See figure 3-8.

Deepening, to a large extent is controlled by mass changes in the upper atmosphere. For example, it has been shown that the lower two-thirds (below about 300-hPa level) of the central column become colder and denser as the areas of low pressure deepen, while the upper one-third of the column becomes warmer. The upper mass decreases by an amount sufficient to counteract the cooling in the lower layers, plus an additional amount to deepen the low. The preferred region for deepening of lows is in the top third of the atmospheric column or, roughly, the stratosphere. See figure 3-8.

Facsimile and NODDS products currently contain prognostic 500 mb, 1000- to 500-mb thickness, and 500-mb vorticity charts. These charts can be used in making predictions of advective changes, thickness patterns, and subsequent changes to the surface pattern.

DEEPENING OF LOWS RELATIVE TO WEATHER TYPES

Weather types were discussed previously under the section Movement of Low-Pressure Systems. This method can also be used to forecast changes in intensity of pressure systems, as each system or type has its own average movement plus average deepening or filling.

DEEPENING OF LOWS IN RELATION TO NORMAL STORM TRACK

Lows whose tracks deviate to the left of the normal track frequently deepen. In general, the normal track of a low is parallel to the upper flow. If a low deviates to the left of normal, it crosses upper contours (assuming an undisturbed upper current) and becomes superimposed by less mass aloft, resulting in deepening of the low. As long as this crossing of upper contours is unaccompanied by sufficient compensatory cooling at the surface low center, the system will deepen.

RELATION BETWEEN DEEPENING LOWS AND MOVEMENT

There is little basis for the rule that deepening storms move slowly and tilling storms move rapidly. The speed of movement of a low, whatever its intensity, is dependent upon the isallobaric gradient and other factors. The magnitude of the surface isallobaric gradients depends upon the low-level advection, the magnitude of the upper-level height changes, and the phase relation between the two levels.

FORECASTING THE INTENSITY OF SURFACE HIGHS

The following section will deal with atmospheric factors aloft and how they affect surface anticyclogenesis. This section will also discuss rules for forecasting the intensity of surface highs.

In the case of developing dynamic anticyclones, cooling takes place at about 200 hPa and above. This cooling is due to the ascent of air, resulting from convergence in the 400- to 200-hPa stratum. Incomplete outflow at very high levels causes piling up of air above fixed upper levels, resulting in high-level pressure rises. At the same time, warming occurs in the lower troposphere. This warming sometimes occurs very rapidly in the lower troposphere above the surface levels, which may remain quite cold. A warming of 10C per day at the 500-hPa level is not unusual. Such a rate of warming is not entirely due to subsidence but probably has a considerable contribution from warm advection. However, continuity considerations suggest that the convergence in the 400- to 200-hPa stratum produces some sinking and adiabatic warming in the lower troposphere. See figure 3-9.

Thus, in the building of anticyclones, there must be a piling up of air at high levels due to horizontal velocity convergence in the 400- to 200-hPa stratum, which results in the stratospheric cooling observed with developing anticyclones. Insufficient outflow at very high levels results in an accumulation of mass. This is roughly the mechanism thought to be responsible for the development of high-pressure systems. The high-level increase of mass overcompensates the low tropospheric decrease of density, and the high-level effect thus determines the sign of increase of pressure at the surface when highs are intensifying. See figure 3-9. 

The development of anticyclones appears to be just the reverse of the deepening of cyclones. Outside of cold source regions, and frequently in cold source regions, high-level anticyclogenesis appears to be associated with an accumulation of mass in the lower stratosphere accompanied by ceding. In many cases this stratospheric cooling maybe advective, but more frequently the cooling appears to be clearly dynamic; that is, due to the ascent of air resulting from horizontal convergence in the upper troposphere.

Studies of successive soundings accompanying anticyclogenesis outside cold source regions show progressive warming throughout the troposphere. This constitutes a negative contribution to anticyclogenesis. In other words, outside of cold source regions, during anticyclone development, the decrease in DENSITY in the troposphere is overcompensated by an increase in

mass, and generally accompanied by cooling in the stratosphere. This is analogous to the deepening of lows where the decrease in mass, generally accompanied by warming at high levels, overcompensates the cooling in the troposphere. The evidence, therefore, indicates that high-level changes, undoubtedly due to dynamic mechanisms in the upper troposphere, are largely responsible for deepening and filling of surface pressure systems. This fact is of considerable prognostic value if the dynamic processes that induce these mass and density changes can be detected on the working charts.

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

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