Download HIGHLIGHT 3: MESOSCALE TEMPERATURE PATTERNS IN THE

HIGHLIGHT 3: MESOSCALE TEMPERATURE PATTERNS IN THE ROCKY MOUNTAINS
AND FOOTHILLS REGION OF SOUTHERN ALBERTA
Research Cluster: Ecohydrology, Meteorology and Watersheds
Project Coordinator: Shawn Marshall, Rachel Cullen, Shannon Fargey, Kara Paraczek, Brian
Horton
Introduction
Surface temperature is relevant to a broad range
of Earth surface processes, but estimating localand regional-scale temperature variability proves
to be a difficult task in regions of complex
terrain. Surface environment, terrain features,
and atmospheric conditions play a complicated
role in dictating spatial and temporal patterns of
surface temperature. Topographic effects can
lead to processes that influence heat transfer
between the surface and the atmosphere, such
as valley winds and cold air drainage. Variations
in surface conditions (e.g. snow, rock, forest
environments) also affect the surface energy
balance, impacting air temperature and other
meteorological processes. In addition, synoptic
weather systems migrating into a region can
greatly alter temperatures. Local climate drives
many ecological and hydrological processes,
dictates much in our daily lives, and impacts the
livelihood of our society; therefore, examining
the patterns of temperature variability on a
‘landscape’ scale is important on many levels.
The research group at the University of Calgary
has been collecting data through the Foothills
Climate Array (FCA) since 2004 in order to
examine temperature patterns in the Rocky
Mountains and Foothills region of Southern
Alberta. Of particular focus are: i) seasonal
temperature patterns, ii) monthly lapse rates
(variation of temperature with altitude), and iii)
daily temperature patterns, and how these are
affected by cold, continental air masses that
descend over the region and westerly, lee-slope
(chinook) winds that are associated with
anomalously warm temperatures. This study
examines the results from a network of 280
temperature loggers installed over an area of
24,000 km², covering a broad range of altitudes
and surface environments over an elevation
range from 820 to 2950 m above sea level
(Figure 1). The network of stations begin at the
continental divide, the provincial boarder
between Alberta and British Columbia, traverses
the eastern slopes of the Canadian Rocky
Mountains and ends in the agricultural prairie
lands approximately 50 km east of Calgary,
Alberta. The network samples alpine terrain,
forested slopes, prairie farmland, and urban
sites (Figure 2).
The aim of the study is to better understand
mesoscale temperature patterns in this complex
region, in order to improve weather forecasts,
ecological and hydrological models, and ‘climate
downscaling’ scenarios from global climate
models and reanalyses. The latter involves
estimation of detailed temperature and
precipitation patterns over the landscape based
on larger-scale meteorological or climate change
scenarios, which cannot explicitly resolve the
terrain.
.
Figure 1: Map of the Foothills Climate Array (FCA) study area
Due to its position in the eastern (lee) slopes of
the Rocky Mountains; the study region is
temperate, continental, and arid, with
precipitation rates rapidly decreasing with
distance from the continental divide. The
geographic position allows for strong sensitivity
of regional weather systems to the position and
strength of the polar jet stream. The Rocky
Mountains also influence the characteristics of
the polar jet stream and westerly flow across the
region. Seasonal variability is common across all
seasons, with winter temperature fluctuations
most pronounced during alterations of northerly
(Arctic) and southwesterly (Pacific) advection
producing temperature range from about 35°C
to +15°C from December to February on a time
scale of days to weeks. Cullen and Marshall
(2011) set out to understand the regional
patterns of temperature as a function of different
prevailing weather systems, in particular chinook
systems and polar air masses. These have
contrasting east-west and altitudinal structure in
the study region, leading to relatively predictable
but unconventional regional temperature
patterns.
Continental polar air masses are cold, dry, highpressure systems that sweep down into
southern Canada several times each winter. In
our study region they move in from the north and
east and are characterized by temperatures
below 15C in the winter months. Chinooks are
regional-scale westerly winds that adiabatically
warm as they descend from high elevations
along the continental divide. These frequent the
lee slopes of the Rockies and are associated
with strong winds, low humidity, and unseasonal
warmth. They are generated by strong westerly
flow of moist Pacific air that has been warmed
via condensation and rainout on the windward
slopes. Low to moderate pressures on the
eastern slopes permit this westerly airflow to
descend to ground level and displace cooler,
continental air masses. Cullen and Marshall
(2011) analyze a set of 100 polar air mass days
and 102 chinook days over the period 20052009.
Figure 2. Sample FCA weather stations, showing typical agricultural regions west of a) Airdrie, b)
Longview, and c) Madden, and two mountain sites in Kananaskis Country, d) near Moose Mountain and e) Mt. Evan
Thomas. From Fargey (2007).
Mean annual temperatures in the region show
an east-west gradient in all seasons, much as
one would expect from the higher elevations to
the west. Other, more subtle, spatial patterns
emerge, such as an urban heat island effect
associated with the city of Calgary. Although this
is present year-round, it is strongest in the
summer, due to retention of daytime heating.
Mean winter temperature patterns show a
departure from the basic elevation control on
temperatures. Cold temperatures prevail on the
eastern side of the study region and the warmest
temperatures occur along a north-south band
through the middle of the array. Cooler
temperatures prevail again to the west. Daily
temperature variability is lowest in the summer
and highest in the winter, and is less
pronounced in the foothills and mountain section
of the study region. This is consistent with a less
continental regime and stronger westerly
(Pacific) influences in the mountains. There is
reduced influence from continental polar air
masses in the western portion of the study area,
particularly in the winter.
Different portions of the study regions are
affected by different air masses and weather
systems in a non-uniform way. The northeast
section of the study area is more subject to cold
air masses that descend from northern Canada,
giving anomalously cold temperatures (Figure
3). Cold anomalies exceed 10C on the eastern
(prairie) part of our study region at these times,
and are closer to 6C in the mountains.
Inversions (warming with altitude) are common
during these events, as the cold air masses are
shallow and don’t always penetrate deeply into
the foothills. Chinook conditions exhibit complex
and non-uniform spatial patterns, and are
strongest in southern and central portions of the
study region (Figure 3). These are lower
elevation foothills sites, which permit a greater
degree of adiabatic warming. Mean chinook
temperature anomalies reach more than 8C
above mean winter temperatures.
Lapse rates – cooling with elevation – are
exceptionally divergent for the two weather
systems. Chinooks are associated with a steep
linear lapse rate, 6° to 8°C km1, consistent
with larger warm anomalies at the low-elevation
sites on the eastern portion of the grid. This is
steeper than the mean winter lapse rate,
indicating that chinook events drive values that
more closely resemble free-air lapse rates. In
contrast, lapse rates under the influence of cold
air masses are marked with deep inversion
structure and a positive temperature gradient
extending to elevations of 2000 m or more, on
average.
Figure 3. Chinook (left hand side) and continental polar (right hand side) temperature anomalies in the region. (a)
Mean temperature from 102 winter chinook events, 2005-2009. (b) Mean temperature from 100 cold air mass
incursions, 2005-2009. (c) and (d) indicate temperature anomalies relative to the mean winter temperatures .
Conclusions
Anomalous seasonal warmth associated with
chinooks is strongest at intermediate altitudes in
the foothills region, with less warming along the
western and eastern edges of the study area.
This pattern explains some of the spatial and
altitudinal temperature structure observed in the
region in the winter, but it largely opposes the
spatial structure induced by continental polar air
masses. Average winter temperature patterns
are a complex composite that reflects the
relative frequency of these two influences,
amongst other weather systems.
Rates of cooling with altitude are highly variable
and are influenced by the prevailing weather
system. Mean seasonal and annual lapse rates
are less steep than typical free-air
(environmental) values of 6.5°C km1,
particularly in winter months. Winter temperature
inversions contribute to weaker lapse rates.
Typical cooling rates are closer to 4.5°C km1,
and are considerably less in the winter (ca.
2.5°C km1). This is important for extrapolation
of observed temperatures from valley-bottom
weather stations (e.g., in Banff or at the field
station) to higher elevations for ecological,
hydrological, or glaciological models.
References
Cullen, R.M. and S.J. Marshall. 2011. Mesoscale temperature patterns in the Rocky Mountains and
foothills region of southern Alberta. Atmosphere-Ocean 49(3): 189 to 205
Fargey, S. 2007. Spatial evaluation of rain events and growing season rainfall patterns in southwestern
Alberta, 2006 – 2006. M. Sc. Thesis, University of Calgary, 225 pp.