Mesoscale Convective Systems

Mesoscale convective systems (MCSs) are intermediate in size and life span between synoptic disturbances and individual cumulonimbus cells (see Figure 9.26). Figure 9.27 shows the movement of clusters of convective cells, each cell about 1 km in diameter, as they crossed southern Britain with a cold front. Each individual cell may be short-lived, but cell clusters may persist for hours, strengthening or weakening due to orographic and other factors.

MCSs occur seasonally in middle latitudes (particularly the central United States, eastern China and South Africa) and the tropics (India, West and Central Africa and northern Australia) as either nearly circular clusters of convective cells or linear squall lines. The squall line consists of a narrow line of thunderstorm cells, which may extend for hundreds of kilometres. It is marked by a sharp veer of wind direction and very gusty conditions. The squall line often occurs ahead of a cold front, maintained either as a self-propagating disturbance or by thunderstorm downdrafts. It may form a pseudo-cold front between rain-cooled air and a rainless zone within the same airmass. Mid-latitude squall lines appear to

Weather System Spatial Scales

Figure 9.26 The spatial scale and life spans of mesoscale and other meteorological systems.

103 104 Distance scale (m)

Figure 9.26 The spatial scale and life spans of mesoscale and other meteorological systems.

Figure 9.27 Successive positions of individual clusters of middle-tropospheric convective cells moving across southern Britain at about 50 km hr-l with a cold front. Cell location and intensity were determined by radar.

Source: After Browning (1980).

Figure 9.27 Successive positions of individual clusters of middle-tropospheric convective cells moving across southern Britain at about 50 km hr-l with a cold front. Cell location and intensity were determined by radar.

Source: After Browning (1980).

form through one of two mechanisms: (1) a pressure jump that propagates as a bore; (2) the leading edge of a cold front aloft (CFA) acting on instability present to the east of an orographic lee trough. In frontal cyclones, cold air in the rear of the depression may overrun air in the warm sector. The intrusion of this nose of cold air sets up great instability, and the subsiding cold wedge tends to act as a scoop forcing up the slower-moving warm air (see Plate 11).

Figure 9.28 demonstrates that the relative motion of the warm air is towards the squall line. Such conditions generate severe frontal thunderstorms such as that which struck Wokingham, England, in September 1959. This moved from the southwest at about 20 m s-1, steered by strong southwesterly flow aloft. The cold air subsided from high levels as a violent squall, and the updraft ahead of this produced an intense hailstorm. Hailstones grow by accretion in the upper part of the updraft, where speeds in excess of 50 m s-1 are not uncommon, are blown ahead of the storm by strong upper winds, and begin to fall. This causes surface melting, but the stone is caught up again by the advancing squall line and re-ascends. The melted surface freezes, giving glazed ice as the stone is carried above the freezing level, and further growth occurs by the collection of supercooled droplets (see also Chapter 5, pp. 100 and 107).

Various types of MCS occur over the central United States in spring and summer (see Figure 9.29), bringing widespread severe weather. They may be small con-vective cells organized linearly, or as a large amorphous cell known as a mesoscale convective complex (MCC). This develops from initially isolated cumulonimbus cells. As rain falls from the thunderstorm clouds, evaporative cooling of the air beneath the cloud bases sets up cold downdrafts, and when these become sufficiently extensive they create a local high pressure of a few millibars' intensity. The downdrafts trigger the ascent of displaced warm air, and a general warming of the middle troposphere results from latent heat release. Inflow develops towards this warm region, above the cold outflow, causing additional convergence of moist, unstable air. In some cases a low-level jet provides this inflow. As individual cells become organized in a cluster along the leading edge of the surface high, new cells tend to form on the right flank (in the northern hemisphere) through interaction of cold downdrafts with the surrounding air. Through this process and the decay of older cells on the left flank, the storm system tends to move 10 to 20° to the right of the mid-tropospheric wind direction. As the thunderstorm high intensifies, a 'wake low', associated with clearing weather forms to the rear of it. The system is now producing violent winds, and intense downpours of rain and hail accompanied by thunder. During the triggering of new cells, tornadoes may form (discussed below). As the MCC reaches maturity, during the evening and night hours over the Great Plains, the mesoscale circulation is capped by an extensive (>100,000 km2) cold upper-cloud shield, readily identified on infra-red satellite images. Statistics for forty-three systems over the Great Plains in 1978 showed that the systems lasted on average twelve hours, with initial mesoscale organization occurring in the early evening (18:00 to 19:00 LST) and maximum extent seven hours later.

Linear Mesoscale Convective Complex

Figure 9.28 Thunder cell structure with hail and tornado formation.

Source: After Hindley (1977)

Figure 9.28 Thunder cell structure with hail and tornado formation.

Source: After Hindley (1977)

Figure 9.29 Schematic evolution of three convective modes on the US Great Plains showing several scales of cloud development (shading).

Source: Blanchard (1990, p. 996, fig. 2), by permission of the American Meteorological Society.

Figure 9.29 Schematic evolution of three convective modes on the US Great Plains showing several scales of cloud development (shading).

Source: Blanchard (1990, p. 996, fig. 2), by permission of the American Meteorological Society.

During their life cycle, systems may travel from the Colorado-Kansas border to the Mississippi River or the Great Lakes, or from the Missouri-Mississippi river valley to the east coast. A MCC usually decays when synoptic-scale features inhibit its self-propagation. The production of cold air is shut off when new convection ceases, weakening the meso-high and -low, and the rainfall becomes light and sporadic, eventually stopping altogether.

Particularly severe thunderstorms are associated with great potential vertical instability (e.g. hot, moist air underlying dryer air, with colder air aloft). This was the case with a severe storm in the vicinity of Sydney, Australia, on 21 January 1991 (Figure 9.30). The storm formed in a hot, moist, low-level airstream flowing northeast on the eastern side of the Blue Mountains escarpment. This flow was overlain by a hot, dry northerly airstream at an elevation of 1500 to 6000 metres, which, in turn, was capped by cold air associated with a nearby cold front. Five to seven such severe thunderstorms occurred annually in the vicinity of Sydney during 1950 to 1989.

On occasion, so-called super-cell thunderstorms may develop as new cells forming downstream are swept up by the movement of an older cell (Figure 9.31). These are about the same size as thunder cell clusters but are dominated by one giant updraft and localized strong downdrafts (Figure 9.32). They may give rise to large hailstones and tornadoes, although some give only moderate rainfall amounts. A useful measure of

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 25,000 km2) over eastern New South Wales for the period 1950 to 1989 based on Griffiths et al. (1993).

Source: After Eyre (1992). Reproduced by kind permission of the NSW Bureau of Meteorology, from Weather, by permission of the Royal Meteorological Society. Crown copyright ©.

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 favours independent cell formation away from the parent updraft. For Ri < 30, strong shear supports a super-cell by keeping the updraft close to its downdraft. Intermediate values favour multi-cell development.

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). During this period, cold, dry air from the high plateaux may override maritime tropical air (see Note 1). Subsidence beneath the upper tropospheric westerly jet (Figure 9.33) forms an inversion at about 1500 to 2000 m, capping the low-level moist air. The moist air is extended northward by a low-level southerly jet (cf. p. 208) 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 penetrate

Figure 9.31 A super-cell thunderstorm.

Source: After the National Severe Storms Laboratory, USA and H. Bluestein; from Houze and Hobbs (1982), copyright © Academic Press, reproduced by permission.

the inversion. The convective trigger is sometimes provided by the approach of a cold front towards the western edge of the moist tongue. Tornadoes may also occur in association with tropical cyclones (see p. 272) and in other synoptic situations if the necessary vertical contrast is present in the temperature, moisture and wind fields.

The exact tornado mechanism is still not fully understood because of the observational difficulties. Tornadoes tend to develop in the right-rear quadrant of a severe thunderstorm. Super-cell thunderstorms are often identifiable in plan view on a radar reflectivity display by a hook echo pattern on the right-rear flank. The echo represents a (cyclonic or anticyclonic) spiral cloud band about a small central eye and its appearance may signal tornado development. The origin of the hook echo appears to involve the horizontal advection of precipitation from the rear of the mesocyclone. Rotation develops where a thunderstorm updraft interacts with the horizontal airflow. Provided that the wind speed increases with height, the vertical wind shear generates vorticity (Chapter 6B.3) about an axis normal to the airflow, which is then tilted vertically by the updraft. Directional shear also generates vorticity that the updraft translates vertically. These two elements lead to rotation in the updraft in the lower-middle troposphere forming a meso-low, 10 to 20 km across. Pressure in the meso-low is 2 to 5 mb lower than in the surrounding environment. At low levels, horizontal convergence increases the vorticity and rising air is replenished by moist air from progressively lower levels as the vortex descends and intensifies. The meso-low shrinks in diameter and the conservation of momentum increases the wind speed. At some point, a tornado, sometimes with secondary vortices (Figure 9.34), forms within the meso-low. The tornado funnel has been observed to originate in the cloud base and extend towards the surface (Plate 20). One idea is that convergence beneath the base of cumulonimbus clouds, aided by the interaction between cold precipitation downdrafts and neighbouring updrafts, may initiate the funnel. Other observations suggest that the funnel forms simultaneously throughout a considerable depth of cloud, usually a towering cumulus. The upper portion of the tornado spire in this cloud may become linked to the main updraft of a neighbouring cumulonimbus, causing rapid removal of air from the spire and allowing a sharp pressure decrease at the surface. The pressure drop is estimated to exceed 200 to 250 mb in some cases, and it is this that makes the funnel visible by causing air entering the vortex to reach saturation. Over water, tornadoes are termed waterspouts; the majority rarely attain extreme intensities. The tornado vortex is usually only a few hundred metres in diameter and in an even more restricted band around the core the winds can attain speeds of 50 to 100 ms-1. Intense tornadoes may have multiple vortices rotating anticlockwise with respect to the main tornado axis, each following a

Figure 9.32 Tornado characteristics in the United States. (A) Frequency of tornadoes (per 26,000 km2) in the United States, 1953 to 1980. (B) Monthly average number of tornadoes (1990 to 1998). (C) Monthly averages of resulting deaths (1966 to 1995).

Sources: (A) From NOAA (1982). (B) and (C) After NOAA - Storm Prediction Center.

' MOIST TONGUE

' MOIST TONGUE

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  • mezan
    Is climate change causing an increase in mesoscale convective system thunderstorm?
    3 years ago

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