Geoscience Reference
In-Depth Information
The relative importance of these processes can influence the overall structure and
shape of the cloud and the extent and severity of any precipitation that may be
generated by them.
Cloud formation processes
Thermal convection
Whether cloud develops, and the extent and form in which it develops, depends
on the lapse rate of the advected atmosphere, i.e., on the Environmental Lapse
Rate  (
G e ). The Environmental Lapse Rate is the lapse rate of the incoming air mass
and so is location-specific and changes with time because it is determined by the
history of inputs and outputs to the atmosphere upstream. It might be measured
using a radiosonde attached to a balloon released into the atmosphere to provide
data for weather forecasts, including forecasts of cloud cover. The Environmental
Lapse Rate will usually include an inversion in potential temperature at the
tropopause which can stop the ascent of moist air and thus limits the vertical
extent of cloud. Near the ground the Environmental Lapse Rate is rarely equal to
(and is usually less than) the dry adiabatic lapse rate,
, this being the approximate
rate at which buoyant air cools when it ascends quickly, see Chapter 3.
The potential for cloud formation depends on the height dependent interrela-
tionship between three lapse rates, specifically:
G
The environmental lapse rate ,
G e - the (measured or model-calculated)
lapse rate of the air overlying the location of interest into which cloud might
be seeded.
The dry adiabatic lapse rate ,
- the rate at which air cools if is moved
upward in the atmosphere adiabatically (see Chapter 3).
G
The moist adiabatic lapse rate ,
G m - the rate at which saturated air cools if it
is moved upward in the atmosphere adiabatically.
Recall that the dry adiabatic lapse rate (
G
= g / c p ) is constant with height but that
the moist adiabatic lapse rate,
, increases
with temperature (see Equation 3.10). In practice, temperature decreases with
height inside a cloud, consequently the moist adiabatic lapse rate and therefore the
rate of cooling of the atmosphere increases with height.
Figure 10.4 illustrates the rate of cooling of a buoyant parcel of air (created by
surface warming) that is seeded into the atmosphere with three different
Environmental Lapse Rates which we refer to as Case 1, 2, and 3. In all three cases
the air parcel initially rises at the dry adiabatic lapse rate. In Case 1, the air is so dry
that the dry adiabatic rate intersects the environmental lapse rate before the air has
cooled enough to saturate, consequently no cloud develops. In Cases 2 and 3 the
air is initially moister. Consequently, the parcel rises and cools at the dry adiabatic
G m , in addition to being smaller than
G
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