Geoscience Reference
In-Depth Information
ATMOSPHERIC CIRCULATIONS
There are fundamental differences between atmospheric circulations in middle
and high latitudes and in the tropics because of the latitude dependence of the
Coriolis force (Exercise 6.4). Poleward of about 20°latitude, depending on the
season, the large-scale flow is approximately in geostrophic balance, as seen in
the figures of chapter 2 . At low latitudes, thermally driven circulation systems
dominate in the presence of weaker Coriolis forces and strong condensational
heating.
7.1 THERMALLY DIRECT CIRCULATIONS
Thermally direct circulations transfer heat from warmer regions to cooler re-
gions. They arise in association with differences in surface temperature and,
therefore, low-level geopotential height and pressure gradients.
Figure 7.1 illustrates the basic features of a thermally direct circulation. The
dashed lines indicate geopotential height contours and the arrows show the
direction of the flow. Four regions are numbered to facilitate discussion.
The atmospheric temperature over the warmer surface (Region 1) is in-
creased by enhanced heat fluxes from the surface ( chapter 5 ) and, as a re-
sult, the thickness (distance between geopotential height lines; see Eq. 6.44)
is greater than the thickness over the cooler surface (Region 3). The resulting
horizontal geopotential height (and pressure) gradients drive flow down the
gradients in Regions 2 and 4. The circuit is closed as air subsides over the sur-
face high and rises over the surface low.
Part of the heat transport by a thermally direct circulation takes place in the
form of sensible heat fluxes. Horizontal sensible heat fluxes occur when warm
air is transported to a cooler region, and when cooler air is transported to a
warmer region. As illustrated in Figure 7.1, the horizontal branches (Regions
2 and 4) of the circulation are responsible for these sensible heat transports.
Latent heat fluxes also play an important role in thermally direct circulation
systems. Strong evaporation occurs in Region 4. The air is dry, which enhances
evaporation (Eq. 5.21), and the release of latent heat as a result of convection
is suppressed by subsidence (sinking air) in the down-branch of the circulation.
Thus, the specific humidity of the air increases as it flows into the thermal low,
where it rises to form the up-branch of the thermally direct circulation. As the
warmed parcels of moist air rise they adjust to the exponentially decreasing en-
vironmental pressure (Eq. 6.39) and expand and cool adiabatically (Eq. 6.37).
When the temperature decrease is sufficiently large, the parcels reach saturation
specific humidity ( Fig. 2.32) and the water vapor in the parcels condenses. The
 
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