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may become visible (e.g., the lenticular clouds in Fig. 4). Viewed from space (Fig. 6),
the wavelike nature of the gravity waves is readily apparent. The waves embedded
within these flows result from the response to the forced ascent: After the air is
carried aloft as high as possible, the lifted air is cooler (and heavier) than the
surrounding air and displaced back toward its original level by gravity.
Figure 5 (see color insert) Fog channelled through a gap in the coastal mountains of
northern California (photo by J. Horel). See ftp site for color image.
Figure 6 Mountain waves generated by southwesterly flow traversing the mountain ranges
of the southwestern United States. This visible satellite image was taken at 2200 Universal
Time Coordinated (UTC), February 9, 1999.
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TERRAIN-FORCED MESOSCALE CIRCULATIONS
When the cross-barrier flow of stable air is sufficiently strong, damaging down-
slope wind storms in the lee of major mountain barriers are co mmon during the
winter season at many locales around the globe (Whiteman, 2000). Local residents
refer to these wind storms by many different names (e.g., foehn in the European
Alps, bora near the Adriatic Sea, chinook to the east of the Rockies, Santa Ana in
southern California, canyon winds in Utah, zonda in Argentina, and oroshi in Japan).
While local topography modulates the intensity of these storms, damaging
downslope wind storms share one or more of the following common characteristics:
pronounced mountain waves, precipitation on the upwind side of the mountain range
coupled with adiabatic warming on the downwind side, strong cross-barrier pressure
gradient (high pressure up stream, low pressure downstream), inversion (temperature
increasing with height) in a layer above ridge crest, or wind reversal above the crest.
The wind and temperature structure across the Rocky Mountains during a particu-
larly damaging event is shown in Figures 5 and 6 in Chapter 27. Durran (1990)
summarizes how the change in flow characteristics across the barrier (Froude
number less than 1 upstream and greater than 1 downstream) may contribute to
further accelerati on of the wind in the lee of the barrier in a manner analogous to
a hydraulic jump.
Barrier jets form when stable, low-level flow impinges upon a mount ain range.
As the flow piles up in front of the barrier, it is deflected to the left in the Northern
Hemisphere as a result of a leftward-directed component of the pressure gradient
force. Northward-directed barrier jets have been observed along the Pacific coast
when the prevailing wind was from the west (Overland and Bond, 1995). Southward-
deflected barrier jets have been observed on the east slopes of the Rockies
and Appalachians during cold-air damming episodes (Dunn, 1987; Bell and
Bosart, 1988).
3 INFLUENCE OF TERRAIN UPON PRECIPITATION
Orographic Precipitation
Local variations in precipitation as a result of the height, relief, and aspect (i.e., slope
direction) of local terrain features can be striking: For example, annual precipitation
increases over a distance of 35 km from 16.2 in. (41 cm) at Salt Lake City, Utah, to
58.5 in. (149 cm) at Alta in the nearby Wasatch Mountains. One of the most dramatic
mesoscale variations in precipitation in the United States is evident in northwestern
Washington where precipitation on the windward side of the Olympic Mount ains is
over 120 in. (305 cm) yet drops to less than 30 in. (76 cm) in the lee of those
mountains (see Fig. 1).
Hills or mountains deflect air near the surface upward or downward, depending
on the direction of the air flow relative to the slope of the topography. Banta (1990)
notes that mountains have two major roles in forming clouds and precipitation: first,
as obstacles to the flow and, second, as high-level heat sources during the day that
cause the winds to converge toward the mountain. Clouds are likely to be generated
3 INFLUENCE OF TERRAIN UPON PRECIPITATION 567
if the ascent caused by either of these mechanisms is strong and there is sufficient
moisture present in the air stream. If the ascent carries the cloud water and ice high
enough such that the air becomes supersaturated, then the excess water and ice in
the cloud may begin to fall and eventually may be deposited at the surface as
precipitation.
Figure 7 summarizes the physical processes that control the development of
orographic precipitation (Houze, 1993):
Upslope Condensation Stable ascent of saturated air is forced by flow over
mountains.
Orographic Convection Lifting induced by ter rain leads to convective release
of instabilities present in the flow (e.g., Fig. 8). Orographic convection can be
further subdivided into the following:
Upslope and Upstream Triggering Topographically induced motions and
upstream blocking trigger convection leading to precipitation on the wind-
ward slope of the barrier.
Thermal Triggering Daytime heating produce s an elevated heat source
with local convergence near the top of the hill or mountain.
Lee-Side Triggering Low-Froude-number flow around a hill or isolated
mountain leads to convergence in the lee of the obstacle.
Lee-Side Enhancement of Deep Convection Flow across a mountain range
converges with low-level thermally induced upslope flow.
Figure 7 Mechanisms of orographic precipitation (adapted from Houze, 1993); (a) seeder-
feeder; (b) upslope condensation; (c) upslope triggering; (d) upstream triggering; (e) thermal
triggering; ( f ) lee-side triggering; (g) lee-side enhancement.
568
TERRAIN-FORCED MESOSCALE CIRCULATIONS
Seeder-feeder Convective cells aloft produce cloud water or ice that fall into
lower cloud decks; the falling cloud particles grow at the expense of the water
content of the lower clouds.
Lake Effect Snow
During the cool season, enhanced precipitation is often observed in the lee of major
water bodies such as the Great Lakes (Niziol et al., 1995). As shown in Figure 11 of
Chapter 27, the greatest snowfall near the Great Lakes occurs where the prevailing
winds blow across the longest fetch of a lake and is enhanced by local orography
such as the Tug Hill plateau downwind of Lake Ontario (Niziol et al., 1995). A
crippling snowstorm for the Buffalo, New York, area and many other locales down-
wind of Lakes Erie and Ontario occurred during November 20–23, 2000. Loc al total
snowfall amounts were as large as 31 in. (79 cm) downwind of Lake Erie and 28 in.
(71 cm) downwind of Lake Ontrario. Chap ter 27 reviews the synoptic-scale and
mesoscale weather associated with lake effect snowstorms in the Great Lakes area.
Residents far from the shores of the Great Lakes contend occasionally with
narrow lake effect snow bands that extend a considerable distance inland (Nizio l
et al., 1995). However, the greatest impact of lake effect snowstorms tends to be
closer to the shores of the Great Lakes. Ing redients that determine the intensity and
structure of lake effect snowstorms include the synoptic setting, the upstream fetch
(distance the air travels over water) and degree to which the water body is covered by
snow or ice, instability in the boundary layer, wind shear, moisture availability,
differences in surface roughness between the water and land surfaces,
offshore-directed land breezes, and presence of upstream lakes or downstream
orography.
Figure 8 Convection developing over the Wasatch Mountains in the aftern oon (photo by
J. Horel). See ftp site for color image.
3 INFLUENCE OF TERRAIN UPON PRECIPITATION 569
Enhanced snowfall occurs as well downwind of smaller water bodies, such as the
Great Salt Lake, that can have serious societal impacts (Steenburgh et al., 2000). For
example, a wind-parallel band developed on November 27, 1995, over the Great Salt
Lake and produced localized snow accumulations of 10 in. (25 cm) downstream
(Fig. 9).
4 OTHER IMPACTS OF SURFACE INHOMOGENEITIES
Anthes (1984) and Zeng and Pielke (1995) demonstrate that variations in landscape
characteristics can strongly affect the atmospheric boundary layer structure, forma-
tion of convective clouds, and fluxes of heat and moisture on the mesoscale. Meso-
scale flows can be generated directly by differences in surface temperature and heat
and moistur e fluxes. In addition, biophysical processes that control moisture avail-
ability within the vegetative canopy can influence mesoscale circulations.
Pielke and Segal (1986) summarize the impact of horizontal contrasts in snow
cover upon mesoscale circulations. Snow cover increases the albedo, reduces
roughness, and alters surface fluxes of heat and moisture relative to nearby
snow-free regions.
Urban areas affect the structure of the atmosphere and weather in a variety of
ways (Dabberdt et al., 2000). Urban heat islands result from the combined effects of
modified thermal and radiative properties of the surface and anthropogenic sources
of sensible heat and moisture. The variability of surface roughness in urban areas
affects the exchange of heat, mass, and momentum between the surface and the
Figure 9 Example of lake effect snow band downwind of the Great Salt Lake, Utah. Lowest-
elevation (0.5
) base-reflectivity analysis at 1300 UTC, November 27, 1995. Reflectivity
shading based on scale at left. (From Steenburgh et al., 2000.)
570
TERRAIN-FORCED MESOSCALE CIRCULATIONS
atmosphere. As a result of structures and pavement, hydrological proces ses are
altered substantially.
Residents of urban areas are susceptible to the interactions of mesoscale and
convective-scale weather phenomena, as illustrated in Figure 10, the F2 tornado
that traversed downtown Salt Lake City on August 11, 1999 (Dunn and Vasiloff,
2001). It has been speculated (e.g., Landsberg, 1981; Bornstein and Lin, 1999) that
large urban areas may affect the origin, strength, and movement of convective storms
and other weather systems.
5 PREDICTABILITY OF TERRAIN-FORCED MESOSCALE
CIRCULATIONS
As mentioned earlier, Pielke (1984) divided mesoscale atmospheric systems into two
groups: terrain-induced mesoscale systems and synoptically induced mesoscale
systems. He stated that the for mer are easier to simulate because they are forced
by geographically fixed features in the underlying terrain. Paegle et al. (1990) also
suggest that terrain-forced circulations may be inherently more predictable than
synoptically induced flows, which are more sensitive to errors in the data used to
specify the initial conditions of a numerical simulation. However, Paegle et al.
(1990) note that the modeling of terrain-induced flows is difficult and susceptible
to inaccurate numerical treatment of physical processes such as turbulent mixing,
Figure 10 Category F2 tornado of August 11, 1999, in downtown Salt Lake City, UT (photo
courtesy of Department of Meteorology, University of Utah, Salt Lake City, UT).
5 PREDICTABILITY OF TERRAIN-FORCED MESOSCALE CIRCULATIONS 571
radiative heating, cloud processes, and soil transfer and the numerical errors arising
from steep terrain slopes.
From ALPEX (Kuettner, 1981) to CASES (LeMone et al., 2000) and MAP
(Bougeault et al., 2001), field programs have been critical to improve the numerical
simulation and prediction of mesoscale circulations forced by variations in the
underlying surface. Comprehensive datasets are required to provide validation of
the model simulations and lead to improved treatment of relevant physical processes
in high-resolution numerical weather prediction models.
Considerable debate remains in the mesoscale modeling community regarding the
inherent predictability of terrain-induced circulations. As noted by Mass and Kuo
(1998), mesoscale predictability in regions of complex terrain may be enhanced due
to the relatively deterministic interactions between the synoptic-scale flow and the
underlying terrain. Nonetheless, while models are increasingly capable of simulating
physically realistic responses to flow over terrain, a corresponding increase in
forecast skill has not always been evident (e.g., Colle et al., 2000).
REFERENCES
Anthes, R. A. (1984). Enhancement of convective precipitation by mesoscale variations in
vegetative covering in semi-arid regions, J. Appl. Meteor. 23, 541–554.
Banta, R. M. (1990). The role of mountain flows in making clouds, in W. Blumen (Ed.),
Atmospheric Processes over Complex Terrain, Meteor. Monogr. 23(45), 229–282.
Barry, R. G. (1992). Mountain Weather and Climate, 2nd ed., London, Routledge.
Bell, G. D., and L. F. Bosart (1988). Appalachian cold-air damming, Mon. Wea. Rev. 116,
137–162.
Bornstein, R., and Q. Lin (1999). Urban heat islands and summertime convective
thunderstorms in Atlanta, Atmos. Environ. 34, 507–516.
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and H. Volker (2001). The MAP special observing period, Bull. Am. Meteor. Soc. 82,
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Chen, W. Y. (1977). Analysis of vorticity and divergence fields and other meteorological
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Dabberdt, W. F., J. Hales, S. Zubrick, A. Crook, W. Krajewski, J. C. Doran, C. Mueller, C. King,
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Dunn, L. (1987). Cold air damming by the Front Range of the Colorado Rockies and its
relationship to locally heavy snows, Wea. Forecasting 2, 177–189.
Dunn, L., and S. Vasiloff (2001). Tornadogenesis and operational considerations of the 11
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Durran, D. R. (1990). Mountain waves and downslope winds, in W. Blumen (Ed.), Atmospheric
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Houze, R. A., Jr. (1993). Cloud Dynamics, San Diego, CA, Academic.
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793–805.
Landsberg, H. E. (1981). Urban Climate, New York, Academic.
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Mass, C. F., and Y.-H. Kuo (1998). Regional real-time numerical weather prediction: Current
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Niziol, T. A., W. R. Snyder, and J. S. Waldstreicher (1995). Winter weather forecasting
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REFERENCES 573
CHAPTER 29
SEVERE THUNDERSTORMS AND
TORNADOES
H. BROOKS, C. DOSWELL III, D. DOWELL, R. HOLLE, B. JOHNS,
D. JORGENSON, D. SCH ULTZ, D. STENSRUD, S. WEISS, L. WICKER,
AND D. ZARAS
1 INTRODUCTION
Severe thunderstorms and tornadoes are phenomena that can occur at almost any
place on the planet. Unlike hurricanes and synoptic-scale cyclones, these local
storms affect areas of 10 to 100 km
2
(e.g., the size of a typical U.S. city ) and last
a few minutes to several hours. Nevertheless, these storms can produce devastating
damage that rivals any other atmospheric storm on ear th. Tornadoes kill nearly three-
dozen people in the United States per year, and recent tornadoes such as the May 3,
1999, Oklahoma City tornado can cause more than $1 billion in damage. However,
the real killers from severe storms are actually flash floods and lightning, as the
fatality rate from these events is more than 200 people every year. Therefore, timely
forecasts and warnings of severe weather are crucial for mitigating damage and
protecting the public.
2 CLIMATOLOGY OF SEVERE THUNDERSTORMS
Severe weather associated with thunderstorms affects almost all of the planet
and represents a significant threat to life and property in many locations. The
definition of what is considered ‘‘severe’’ depends on operational forecasting
considerations that vary from country to country but typically incl udes phenomena
Handbook of Weather, Climate, and Water: Dynamics, Climate, Physical Meteorology, Weather Systems,
and Measurements, Edited by Thomas D. Potter and Bradley R. Colman.
ISBN 0-471-21490-6 # 2003 John Wiley & Sons, Inc.
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