Geoscience Reference
In-Depth Information
Radiation and heat balance
It is possible to chart general trends in principal meteo-
rological parameters with altitude ( Table 24.1 ), provided
that we appreciate considerable variations with latitude,
seasonality, continentality, aspect, topography and surface
conditions. Cooling with altitude at an average environ-
mental lapse rate of 6·5 C km -1
Moisture balance and precipitation
The principal mountain impact on moisture balance is
orographic enhancement of precipitation by triggering
conditional instability during forced ascent. This was
formerly thought to be exaggerated, since precipitation
from onshore winds occurs anyway at the coast, but it is
now appreciated that mountains also retard mid-latitude
cyclones and thereby accentuate convergence and uplift.
Windward coastal mountains undoubtedly experience
some of Earth's highest precipitation levels and intensi-
ties. Precipitation may increase up to 3-4 km in middle
latitudes at frontal disturbances but only up to 2-3 km
in the tropics, marking the more limited vertical extent
of warm tropical clouds (see Figure 5.16 ). Mean precipi-
tation maxima are found between 0·5 km and 1·0 km,
0·7 km and 1·5 km, and at 3 km in equatorial, tropical
maritime and high-latitude mountains respectively. Above
this the atmosphere becomes drier, as available water
vapour is consumed in cloud and precipitation forma-
tion, and may be arid. Mountains may also enhance
convection, especially inland in summer, and provide
high-level moisture sources. Effective wind speed increases
over exposed surfaces and either has a desiccating effect
or provides a source of moisture by advection and sublim-
ation ( Plate 24.1 ).
Orographic effects may be greater in winter, and
individual cells within frontal systems are capable of
'dumping' rain at rates of, or even above, 50-200 mm hr -1 .
would reduce sea-level
temperatures by 30-35
C at 5 km aloft. However, surface
temperatures in the mountain atmosphere are compli-
cated by other changes in radiation, heat and moisture
balances compared with the adjacent free atmosphere.
Latent heat exchange triggered by orographic effects
provides an additional heat source which may exceed
advective (sensible) heat flux, except in windward coastal
ranges. Latent heat release on condensation reduces
adiabatic cooling by 6-8
C at 5 km in mid- and high-
latitude mountains and by 3-5
C in low-latitude moun-
tains, where clouds are less extensive vertically.
Atmospheric density and thickness decrease with altitude,
whilst transparency increases as aerosol density and
absolute humidity fall as they become involved in cloud
formation. Both short- and long-wave radiation fluxes
thus increase with altitude, the former by 7-10 per cent
km -1 in the Alps. Gains exceed losses, since the long-wave
flux remains more constant with lower ambient temper-
ature. This may be countered above snow and ice surfaces.
With albedos between 0·4 and 0·9, they reflect more short-
wave radiation and consume sensible heat during summer
melt. On balance, mountains may form high-altitude heat
sources. The Tibetan plateau, for example, experiences
temperatures 4-6 C above the zonal average free atmos-
phere at 5 km. Increasing latitude sees a general decline
in radiation balance and temperature. Stronger seasonal
contrasts towards the poles compare with stronger diurnal
contrasts at the equator.
Table 24.1 Variation of some standard properties of the
atmosphere with altitude
Altitude
P
r
T
SWR
AH
vr
(kg m -3 ) (°C)
(W m -2 ) (g m -3 )
(km)
(mb)
(mb)
0
1013
1·23
15·0
970
16·0
15·0
1
899
1·11
8·5
1,050
10·9
11·0
2
795
1·06
2·0
1,120
7·4
7·4
3
701
0·91
-4·5
1,175
4·5
4·0
4
616
0·82
-11·0
1,200
3·0
3·1
5
541
0·79
-17·5
1,220
1·9
2·0
Plate 24.1 Rime ice coating the windward side of a meteo-
rological screen and buildings, deposited by sublimation in a
cold moist air stream. See also Plate 5.3.
Photo: Ken Addison
Notes: P, pressure; r, density; T, temperature; SWR, short-wave
radiation (calculated assuming an overhead sun); AH, absolute humidity;
vr, vapour pressure.
 
 
Search WWH ::




Custom Search