Transcript Mountain_Met_280_Lecture_2

Geographical Controls of
Mountain Meteorological Elements
Latitude
Continentality
Altitude
Topography
Latitude
Determines length of day and angle of incoming sunlight and, thus,
amount of solar radiation received
• In equatorial regions, day length & solar angle change little with
season.
Little seasonal variability, mostly diurnal changes.
• In polar regions, the sun does not rise at all in winter. In the summer it
never sets, although remaining low in sky. Big seasonal changes, small
diurnal changes.
• In mid-latitudes, seasonal and diurnal changes.
Also determines site’s exposure to latitudinal belts of high and low
pressure
• High pressure - subsidence
• Low pressure - convection
Day length vs latitude
(Whiteman, 2000)
Impacts of Latitude
Net radiation (incoming – outgoing) and temperature
decrease as latitude increases
Elevation of treeline/snowline decreases
poleward
Belt of alpine vegetation and permanent snow and
ice are lower on mountains at high latitude versus
the tropics
Snow lines and timberlines
Continentality
• The degree to which a point on the earth’s surface is in all respects
subject to the influence of a landmass.
• Continental locations experience larger diurnal and seasonal
temperature changes than locations on or near large bodies of water
because land surfaces heat and cool more quickly than oceans.
• Interior locations experience more sunshine, less cloudiness, less
moisture and less precipitation than coastal areas.
• Precipitation is especially heavy on the windward side of coastal
mountain ranges oriented perpendicular to prevailing winds from
the ocean. Marine air lifted up a mountain range releases much of
its moisture as precipitation. As a result, far less precipitation is
received on the leeward side.
Continentality
Arises from differences in heat capacity and heat
conduction of soils vs. water
• Water able to store more heat
• Soils less
Degree of continentality expressed by annual range of
mean monthly temperature
Continentality
High mountains also protrude into the middle of the
troposphere where the atmospheric circulations may
differ considerably from that at sea level.
Mountain ranges located in semi-arid macroclimatic
zones, may have distinctly different climatic and
vegetation characteristics from the adjacent lowlands.
Extensive mountain massifs and high plateaus set up
their own large-scale and local-scale circulations. Such
large-scale effects on diurnal and seasonal circulations
(‘plateau monsoons’) have been demonstrated for the
Tibetan Plateau and the mountain ranges of the
south-western US (Tang and Reiter 1984).
Continentality
The summer-time plateau wind circulation in the
Great Basin area of the US, reverses diurnally over a
depth of 2 km.
Gao and Li (1981) show that the Tibetan Plateau
creates a lateral boundary layer that enlarges its
effective dimensions.
Altitude
Incoming solar radiation increases with altitude
• Changes in air temperature at high altitudes are small,
however, because of smaller amount of land area at
higher altitudes
• Air temperature usually decreases with altitude (-6.5°C/km)
• Moisture in air usually decreases with altitude
• Wind speed usually increases with altitude
• Air density and atmospheric pressure decrease
exponentially with altitude
Altitude
Distribution of state variables (p,ρ,T,u) depends strongly
on height in free atmosphere and as function of terrain
height
• Vapor pressure of water and radiation also vary
strongly with height
(Whiteman, 2000)
Solar Radiation at Altitude
Mountain observatories were of special importance
in early studies of solar radiation and the solar
constant.
Sonnblick Observatory, Austria
Solar Radiation at Altitude
In 1875, J. Violle made the earliest mountain-top measurements on
Mont Blanc.
Langley (1884) made actinometer observations on a special
expedition to Mt. Whitney, California in 1881, but their estimates of
the solar constant were higher than the currently assumed value of
1368 W m-2.
Early estimates of the solar constant were obtained by extrapolating
actinometer (later pyrheliometer) measurements of integrated solar
radiation, made at different path lengths.
The alternative method of spectrobolometry, pioneered by S.
P.Langley on Whitney (4420 m) was gradually perfected by C.G.
Abbott.
This involved observations of relative intensity in narrow spectral
bands at different solar angles. Transmission coefficients are thereby
determined for each ray.
Optical air mass
The path length of the solar beam through the atmosphere expressed in
terms of optical air mass, m
m = 1/sin θ
where θ is the solar altitude.
At sea level the relationship between optical mass and solar altitude is:
for m = 1, θ = 90°, m = 2, θ = 30°, m = 4, θ = 14°.
For comparative radiation calculations at different altitudes, the absolute
optical air mass M= m (p/po) where p is the station pressure and po =
1000 mb, is used for the effects of air density on transmission.
Thus at 500 mb a value of 2 for M corresponds to m = 4 and θ = 14°.
For an ideal (pure, dry) atmosphere, the direct solar radiation received
at the 500 mb level (5.5 km) is 5-12% greater than at sea level. This
corresponds to an average increase of 1-2 % km-1.
Optical air mass
(Barry 1992)
Solar radiation vs. Altitude
(Barry 1992)
Seasonal variation of short and longwave radiation
(Barry 1992)
(Barry 1992)
Solar Radiation at Altitude
Following new spectrobolometer measurements with UV
filters on Mt. Whitney in 1909-10, Abbott and Fowle (1911)
estimated a mean solar constant of 1343 W m-2 .
Pyrheliometer data obtained 30 years later by US Weather
Bureau on Mt. Evans, Colorado gave 1349 W m-2.
These estimates are within 2% of modern value derived
from satellite measurements.
Solar Radiation at Altitude
The depletion of the direct solar beam irradiance by
atmospheric absorption and backscatter is referred to as
the relative opacity of the atmosphere or its turbidity.
Valko(1980) shows that the turbidity at Swiss mountain
stations is typically four to five times less than that in the
lowlands.
Dirmhirn (1951) made in-depth studies of the effect of
altitude on diffuse (sky) radiation. Under cloudless skies,
the sky radiation decreases with altitude owing to the
reduction in air density and therefore scattering, but
multiple reflections from adjacent peaks may obscure this
to some extent, especially when there is snow cover.
(Barry 1992)
Role of Topography
Topography- Wind Speeds on Mountain Summits
Wind observations on mountain summits and in the
free air was carried out by Wahl (1966). From data
for European stations, in general, speeds on
summits average approximately half of the
corresponding free-air values.
Topography- Temperature on Mountain Summits
Topography- Temperature on Mountain Summits
During the eighteenth century there was still
considerable controversy as to the cause of the
general temperature decrease with height.
De Sussure, was the first physical scientist to
approach a realistic explanation of the cause of cold
in mountains. (Barry, 1978).
Read: Barry 1978