Transcript Mountain_Met_280_Lecture_3

Geographical Controls of
Mountain Meteorological Elements
Latitude
Continentality
Altitude
Topography
Role of Topography
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
Topography- Temperature on Mountain Summits
From several studies, the primary control of free airsummit temperature difference seems to be the
atmospheric temperature structure.
Peppler (1931) found that, in the Alps, mountain
temperatures are closest to those in the free air when
the lapse rate is nearly adiabatic between 1-3 km.
With isothermal or inversion conditions, temperatures
in both summer and winter are considerably loweer
on mountain summits.
Topography- Temperature on Mountain Summits
Pepin et al. (1999) showed for stations in the
Pennines, lapse rates are determined by
atmospheric temperature and moisture level,
cloudiness/solar radiation, and wind speed.
Changes in lapse rate can result from changes in the
frequency of cyclonic/anticyclonic circulation
regimes.
Rolland (2003) analyzed 640 stations in the AustrianItalian Alps and showed that many earlier studies are
biased by the use short records and limited number
of stations.
Rolland, C. 2003. Spatial and seasonal variations of air temperature lapse rates
in alpine regions. Journal of Climate 16:1032–1046.
Topography- Temperature on Mountain Summits
Rolland (2003) used stations that ranged in altitude
from below 100 m to over 2000 m, and identified
valley and slope sites.
Lapse rates are calculated using simple linear
regression of temperature with altitude in four station
groups.
Fig. 2.14
Topography- Temperature on Mountain Summits
The influence of nocturnal inversions at night and of nearadiabatic conditions by day, in addition to effects of foehn
winds and katabatic drainage winds, led von Ficker (1926)
to declare that “true” lapse rates cannot be determined in
mountain regions.
A unique approach of measurement was performed by
Brocks (1940) in which he determined density differences
in layers over the Austrian Alps.
Brocks found that the diurnal amplitude of lapse rate
decreases with altitude more rapidly in the free air over
the plains than over the mountains.
Brock also found that the mountain atmosphere extends
above the mean ridge height. Similar to the model by
Ekhart (1948).
Topography- Temperature on Mountain Summits
Important to distinguish between the effects of local
topography which cause diurnal changes in lapse
rate and large-scale topographic effects that modify
the atmospheric structure.
Tabony (1985) outlines three idealized topographic
situations:
Fig 2.15
1. Isolated mountain
2. Plateau of limited extent
3. Extensive plateau
Topography- Temperature on Mountain Summits
Topography- Temperature on Mountain Summits
1913 von Hahn noted that temperatures observed at
summit stations, on average, are lower than those in
the free air at the same level.
Later studies (McCutchan 1983; Richner an Phillips 1984)
show that mountain peaks are warmer in the afternoon
and colder in the early morning.
Topography- Temperature in Mountain Environments
Dobrowski, Abatzoglou, Greenberg, and Schladow (2009)
suggest that air temperature in mountain environments (in
their study area—Lake Tahoe basin) is driven primarily by
regional temperature patterns.
They mention, but neglect the role of stability and
decoupling due to cold air pooling in enclosed terrain.
They use “Free Air” estimates from NARR which provides
assimilated temperature data at 3 hr and 32 km
resolution.
Please read and write a discussion on this paper for
Monday. Think about techniques used and are these valid
when comparing the ideas of Ekhart (1938).
Topography- Wind Speeds on Mountain Summits
The effect of mountains on wind flow over them has
aroused early interest in the topic since peoples
have occupied mountainous areas.
Georgii (1922) argued that wind speeds on summits
generally increase above mountain summits up to a
level of 30% of the mountain altitude. He termed
this effect as the ‘influence height’.
This was argued by A. Wagner that it is not a good
generalization.
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- Wind Speeds on Mountain Summits
Topography- Wind Speeds on Mountain Summits
There are two basic factors which affect wind
speeds on mountain summits (Barry 1992).
These operate in opposition to each other.
1. The vertical compression of airflow over a
mountain causes acceleration of the air.
2. Frictional effects cause retardation.
Frictional drag in the loer layers of the atmosphere is
caused partly by ‘skin friction’ (shear stress), due
to small-scale roughness elements (< 10 m).
Topography- Wind Speeds on Mountain Summits
Additional frictional effects are caused by ‘form drag’
which is due to topographic features 0.1-1 km in
size that set up dynamic pressure perturbations.
In mountain areas, form drag contributes the largest
proportion to the total friction.
Over simple 2-D terrain, drag increases in proportion
to the slope2, up to the point where flow separation
takes place (Taylor et al. 1989).
Topography- Wind Speeds on Mountain Summits
Balloon measurements in the central Alps indicate
that drag influences extend up to about 1 km above
the local mean ridge altitude of 3 km.
Special soundings made during ALPEX (Alpine
Experiment) show that the airflow over the central
Swiss Alps is decelerated up to about 4 km (600 mb).
Ohmura (1990) suggests that momentum transfer
between the atmosphere and the mountains takes
place from about 4 km down to 500 m below ridge
tops.
Topography- Wind Speeds on Mountain Summits
Many of the wind profiles indicated a wind maximum
at 1.5 km above the ridges with speeds greater than
over the adjacent lowlands.
Schell (1936) attempted to explain contrasting
observations with tethered balloons on three
summits in the Caucasus at 1300 m. He concluded
that in the case of an isolated peak, or and exposed
ridge, the compressional effect outweighs frictional
retardation.
Resulting in stronger winds up to about 50-100 m
over the summit than overlying free air.