Using Facsimile and NODDS Products

Using the Current 500-hPa Chart In the deepening of lows there must be removal of air at high levels due to divergence in the 400- to 200-hPa stratum, resulting in stratospheric warming. Insufficient inflow at very high levels to compensate the subsidence results in 500-hPa contour falls. This is roughly the mechanism thought to be responsible for the development of low-pressure systems. The high-level decrease in mass overcompensates the low tropospheric increase in density; the high-level effect thus determines the reduction of pressure at the surface when lows are intensifying. 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. Using Facsimile and NODDS Products 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. Anticyclogenesis Indicators 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 3-12