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Up to 15 km,
or m
o
re in tropical cyclones
Neutrally buoyant cooling air
driven poleward
E
S
gives long wave
reradiation
Subtropical
jet stream
STJ
Radiative
sinking
-
E
L
gives +
E
S
+
E
P
Warm, saturated air rises and cools
along saturated adiabatic lapse rate,
water condenses
Descending cool dry air
warms up along dry adiabatic
lapse rate
IDEALIZED HADLEY CELL
-
E
P
gives +
E
S
Convection in tropical
thunderstorms
Plus tropical
cyclones
Clear subtropical skies enables
short wave heating and evaporation
of ocean water
+
E
L
+
E
S
Dry
Wet
High
p
Trade winds
Low
p
30
°
0
°
Fig. 6.9
Energy transfers in an idealized Hadley cell.
in all latitudes as air masses move over elevated topogra-
phy. Consider a dry, slowly ascending mass in which we
have no
E
L
term. The increase in
E
P
due to ascent must be
accompanied by a fall in temperature so that
E
S
can
decrease. We can think of this easily in physical terms with
a little help from kinetic theory and the gas laws
(Sections 3.4 and 4.18), since as the dry air slowly rises it
moves into regimes of lower pressure where work must be
done in expansion and temperature falls; vice versa for
falling air. This change of
T
with height is known as the
dry adiabatic lapse rate,
about 1 K per 100 m. When water
vapor is present in ascending air, the lapse rate is reduced
because as cooling takes place the air becomes saturated,
condensation occurs, and latent heat is released, becoming
sensible heat. This
saturated adiabatic lapse rate
varies
with
T
, being approximately equal to the dry rate for cold
air, but much less for warm air. A typical average value
would be around 0.6 K per 100 m.
monthly timescale in the ITCZ in the Indian Ocean,
southeast Asia, and southwest Pacific is due to a curious
linked atmosphere-ocean phenomenon known as the
Madden-Julian Oscillation
. Recent decadal warming of
the tropical oceans, particularly the Indo-Pacific, is
thought to be the teleclimatic link that has forced changes
in higher-latitude climate systems (discussed later for
North Atlantic).
As it is cooled by radiative heat loss due to emission of
longwave radiation gained by its high sensible heat content
the equatorial air sinks, with a transfer of potential energy
to sensible heat according to the dry adiabatic lapse rate.
Thus the dry cloudless subtropical deserts have their radi-
ation deficits, due to high infrared radiative heat loss from
the high-albedo/low-cloud-cover environment, compen-
sated by the lateral transfer of heat from the equator. Over
the subtropical oceans the low-humidity air and cloudless
conditions enable shortwave solar radiation to warm and
evaporate seawater. The trade winds then complete the
Hadley circuit, transporting latent and sensible heat in the
form of moist, near-saturated winds returning once more
to the ITCZ at low levels as the generally steady and
dependable easterly trade winds. The convergence or con-
fluence of these then causes general upwelling in the equa-
torial troposphere. Winds in the ITCZ itself are usually
light: the area of the doldrums.
The general Hadley cell circulation is thus clockwise in
the northern hemisphere and anticlockwise in the south-
ern hemisphere. The warm low-density equatorial air cre-
ates a low-pressure zone at the tropics, which shifts north
and south of the equator with the ITCZ during the course
of the year. The descending and cooling high-density sub-
tropical air creates the zonal high-pressure belts marked on
the continents by the great trade wind deserts.
6.1.5
Low-latitude circulation and climate
In the simplest representation of low-latitude circulation,
termed a
Hadley cell
, between about 30
latitude north
and south of the equator (Fig. 6.10), warm moist rising air
originates at the equatorial
intertropical convergence zone
(ITCZ), where incoming radiative heating is at its most
effective and the Coriolis force is minimal. This warm air
uplifts the higher atmosphere air column, increasing the
pressure at any height and causing an outflow into higher
latitude areas. The warm saturated equatorial air ascends
mostly in convective thunderstorms. As upflowing air is
cooled, precipitation releases latent heat energy.
Periodicity of deep convective rainfall on a roughly
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