Geography Reference
In-Depth Information
Fig. 3.10
Grid for estimation of the horizontal
divergence.
3.5.2
The Adiabatic Method
A second method for inferring vertical velocities, which is not so sensitive to
errors in the measured horizontal velocities, is based on the thermodynamic energy
equation. If the diabatic heating J is small compared to the other terms in the heat
balance, (3.6) yields
∂T
∂t
+
u
∂T
v
∂T
∂y
S
−
1
p
ω
=
∂x
+
(3.42)
Because temperature advection can usually be estimated quite accurately in
midlatitudes by using geostrophic winds, the adiabatic method can be applied
when only geopotential and temperature data are available. A disadvantage of
the adiabatic method is that the local rate of change of temperature is required.
Unless observations are taken at close intervals in time, it may be difficult to
accurately estimate ∂T /∂t over a wide area. This method is also rather inaccurate
in situations where strong diabatic heating is present, such as storms in which
heavy rainfall occurs over a large area. Chapter 6 presents an alternative method
for estimating ω, based on the so-called
omega equation
, that does not suffer from
these difficulties.
3.6
SURFACE PRESSURE TENDENCY
The development of a negative surface
pressure tendency
is a classic warning of
an approaching cyclonic weather disturbance. A simple expression that relates the
surface pressure tendency to the wind field, and hence in theory might be used as
the basis for short-range forecasts, can be obtained by taking the limit p
→
0in
(3.39) to get
p
s
ω (p
s
)
=−
(
∇·
V
) dp
(3.43)
0
