INDICATIONS ALOFT FOR DEEPENING AND FILLING OF SURFACE LOWS

surrounding it will tighten and tend to intensify the old masked warm front (fig. 3-7, step 3). Later, as the wave moves rapidly eastward, it will pickup this intensified warm front and begin to occlude (fig. 3-7, step 4). This occlusion deepens as much as 10 to 15 hPa in 12 hours. The resultant rapid deepening and increase in cyclonic circulation results in a portion of the original polar front discontinuity between the new and the old cyclone being washed out (fig. 3-7, step 5). INDICATIONS ALOFT FOR DEEPENING AND FILLING OF SURFACE LOWS There are numerous atmospheric factors aloft that affect the central pressure of surface lows. The following text discusses a few of these factors. Temperature Advection Changes The role of temperature advection in contributing to the pressure or height changes can be misleading. On the one hand, low-level (usually the 1,000- to 500-hPa stratum) warm air advection is frequently cited as responsible for the surface pressure falls ahead of moving surface lows (the converse for cold air advection); on the other hand, warm air advection is frequently associated with rising heights in the upper levels. The pressure change at the SURFACE is equal to the pressure change at some UPPER LEVEL, plus the change in mass of the column of air between the two. That is, if the pressure at some upper level remains UNCHANGED and the intervening column is replaced with warmer air, the mass of the whole atmospheric column (and consequently the surface pressure) decreases, and so does the height of the 1,000-hPa surface. As an example, assume that warm air advection is indicated below the 500-hPa surface (5,460 meters) above a certain station. If no change in mass is expected above this level, the height of the 500-hPa level (5,460 meters) will remain unchanged. Suppose the 1,000- to 500-hPa advection chart indicates that the 5,400-meter thickness line is now over the station in question and will be replaced by the 5,490-meter thickness line in a given time interval; that is, warm air advection of 90 meters. The consequence is that the 1,000-hPa surface, which is now 60 meters above sea level, will lower 90 meters to 30 meters below sea level, and the surface pressure will decrease a corresponding amount, about 11 hPa (7.5 hPa approximately equals 60 meters). Whenever the surface pressure is less than 1,000 hPa, the 1,000-hPa surface is below the ground and is entirely fictitious. In view of the above description of advective temperature changes, the following rules may apply: . Warm air advection between 1,000 and 500 hPa induces falling surface pressures. . Cold air advection between 1,000 and 500 hPa induces rising surface pressures. Indications of Deepening From Vorticity Cyclogenesis and deepening are closely related to cyclonic flow or cyclonic vorticity aloft If you recall from the discussion of vorticity in chapter 1, vorticity is the measure of the path of motion of a parcel plus the wind shear along the path of motion. Thus, we have the following rules for the relationship of vorticity aloft to the deepening or falling of surface lows: @ Increasing cyclonic (positive) relative vorticity induces downstream surface pressure falls. Q Increasing antic cyclonic (negative) relative vorticity induces downstream surface pressure rises. l A wave will be unstable and deepen if the 700-hPa wind field over it possesses cyclonic relative vorticity. . A wave will be stable if the 700-hPa wind over it possesses anticyclonic vorticity. . If there are several waves along a front, the one with the most intense cyclonic vorticity aloft will develop at the expense of the others. This is usually the one nearest the axis of the trough. Deepening of Lows Relative to Upper Contours The amount of deepening of eastern United States lows moving northeastward into the Maritime Provinces of Canada frequently can be predicted by estimating the number of contours at the 200- or 300-hPa level that would be traversed by the surface low during the forecast period. For a close approximation, multiply the 200-hPa current height difference in tens of meters by 3/4 to obtain the surface pressure change in hectopascals. For example: a 240-meter height difference at 200 hPa results in a pressure change of 18 hPa at the surface. 24 x 3/4 = 18 hPa pressure change If FALLING heights are indicated aloft, the AMOUNT of fall need not be estimated. The deepening of the surface low and greater advective cooling, associated with the occlusion process, appear to compensate for the upper height falls. 3-10