Geoscience Reference
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rainfall amounts. A useful measure of instability
in mesoscale storms is the bulk Richardson
Number (Ri) which is the (dimensionless) ratio of
the suppression of turbulence by buoyancy to the
generation of turbulence by vertical wind shear in
the lower troposphere. A high value of Ri means
weak shear compared to buoyancy; Ri > 45 favors
independent cell formation away from the parent
updraft. For Ri < 30, strong shear supports a
supercell by keeping the updraft close to its
downdraft. Ri < 10 indicates weak instability and
for strong vertical shear.
Tornadoes, which often develop within MCSs,
are common over the Great Plains of the United
States, especially in spring and early summer (see
Figure 9.32
and
Plate 9.3
). During this period,
cold, dry air from the high plateaux may over-
ride maritime tropical air (see Note 1). Subsidence
beneath the upper tropospheric westerly jet
(
Figure 9.33
) forms an inversion at about
1500-2000m, capping the low-level moist air. The
moist air is extended northward by a low-level
southerly jet (cf. p. 261) and, through continuing
advection the air beneath the inversion becomes
progressively more warm and moist. Eventually,
the general convergence and ascent in the
depression trigger the potential instability of
the air, generating large cumulus clouds, which
Newcastle
HOT
DRY
1500 -
6000m
Sydney
5
3
1
Tasman Sea
Canberra
0
100
km
Figure 9.30
Conditions associated with the severe
thunderstorm near Sydney, Australia, on 21 January 1991.
The contours indicate the mean annual number of severe
thunderstorms (per 25,000km
2
) over eastern New South
Wales for the period 1950-1989.
Source:Based on Griffiths et al. (1993). After Eyre (1992). Reproduced by
kind permission of the NSW Bureau of Meteorology, from Weather by
permission of the Royal Meteorological Society. Copyright ©.
Overshooting top
15
Anvil
Back-sheared
anvil
10
Mammatus
Mammatus
Motion
Cold air
Flaking line
towers
5
Wall cloud
Tail cl
o
ud
Horizon
0
SW
NE
Forward-flank gust front
Debris
Light rain
Precipitation-free
cloud base
Heavy rain
Warm
air
Small hail
Rear-flank
gust front
0
50
Large hail
Precipitation
curtain
Tornado
km
Figure 9.31
A supercell thunderstorm.
Source: After the National Severe Storms Laboratory, USA and H. Bluestein; from Houze and Hobbs (1982). Copyright © Academic
Press, reproduced by permission.
