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
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
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
. 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.