To those who are interested in weather phenomena, the meteorological term "squall line" will not be unfamiliar. Regular readers of the Observatorys educational resources may recall several online articles on related topics, e.g.:
- Northwest "Shi Hu Feng" and Northeast "Shi Hu Feng";
- Thunderstorms and squalls;
- Squall lines and "Shi Hu Feng" - what you want to know about the violent squalls hitting Hong Kong on 9 May 2005.
The theme common to all these articles is the damaging squalls associated a squall line. While squalls mean the severe gust produced by a thunderstorm, a squall line refers to a chain of active thunderstorms organized in a curvilinear fashion with multiple number of rain cores. Besides a squall line, another type of severe thunderstorm called supercell can also produce damaging squalls. Different from a squall line, a supercell consists of a single rain core only. Unlike a heat thunderstorm, which usually also has a single cell in structure but is short-lived, a supercell distinguishes itself by having a rotating updraft and its capability to persist for several hours.
So, how can these severe thunderstorms produce damaging squalls? A common mechanism is the formation of a "downburst" in the rain core of a thunderstorm. When evaporation of raindrops or melting of hailstones occurs in the upper air, the temperature of the concerned air parcel (an imaginary volume of air in the atmosphere) will be significantly lowered. Both microphysical processes demand a lot of energy from the surrounding air and the concerned air parcel is therefore cooled. The evaporative cooling process can become very efficient and rapid if the middle layers of the atmosphere are very dry, arising from, say, the intrusion of a dry northeast monsoon to the south China coastal areas in late spring or early summer. As it cools, the air parcel will become significantly denser than its surroundings and lose its buoyancy. As a result, the parcel will burst downward onto the earth surface and hence the formation of a downburst. When the downward impinging air stream hits the ground, the energy and momentum carried with it will be forced to propagate horizontally. The outflow so generated will spread out in all directions producing strong gusty winds along its swath. In some sense, the effect resembles the spectacular splash resulted from a bombing of water from mid air onto the ground.
The leading edge of an outflow is known as gust front. Close to the front, the region of the maximum wind speed will usually be located. Because of its higher density when compared with the surrounding air, the outflow and the associated gust front can propagate even faster than its originating thunderstorm. To a ground observer at a distance downstream of an outflow, he or she will feel the strong gusts brought about by the gust front before getting wet by the showers that follow. Moreover, the gusty winds will appear transient in time, lasting for a few minutes only, as the gust front will usually pass over quickly.
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