'I'metnet.imd.gov.in/mausamdocs/53344.pdfa tentative model of Andhi (the convectiveduststorm of...
Transcript of 'I'metnet.imd.gov.in/mausamdocs/53344.pdfa tentative model of Andhi (the convectiveduststorm of...
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Mal/sam, (1982), 33, 4, 417·422
551.5 15.3
A tentative model of AI/dbi
P. V. JOSE PH
M eteorological 0 ffice, PlIne
(Receil'cd IS August 1980)
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ABSTRACT. Based on the thunderstorm downdra ft model de....eloped by Charba (1974) and Goff ((976),a tentative model of Andhi (the convective duststorm of northwest lndia) has been suggested. This model Is able10explain qualitatively the visibility variation.. in a typical type- I Amllri(Tho details of visibility variations in thefour typesof AndMhave beengiven by Joseph et al. 1980). The slower visibility improvementand theconsequentlonger duration of typc-2 (night time) Am/hialso gets a logical explana tion, if one assumes that the Am/hi 'head'moves forward more slowly at night. when the ambient air-density is higher. This assumption has theoretical support from studies on the propagation or 'gravily current' heads.
I . Introduction
The convective type of duststorm occurringover northwest India during the pre-monsoonseason Apri l 10 June is called Andhi. Joseph ct al.(1980) made a study of 40 cases of Andh! thatoccurred at Delhi airport during the period1972 to 1977, using available meteorologicalrecords. From the nature or variations orhorizontal visibility and wind speed near the groundlevel associated with these duststorrn s, it wasfound that 4 types of Andhi occur. From radarphotographs of the cumulonimbus cloud orsquall line associated with A",11ri it was foun dthat the dista nce between the cb cloud or squa llline (as seen in the radar) and the associated Andhidust-wall on the ground is as large as 30 km.
Study of the downdraft from severe thunderstorms is considered very important in view orthe role of such downdrafts in causing a ircra ftaccidents. " The large wind-shears that are characteristic of the gust-front occurs mainly in thelowest 1-2 km of the atmosphere and, therefore.are a particular haza rd to aircraft flying at lowlevels. In fact, low level wind shear in and aroundthunderstorms has recently been labelled as theprimary probable cause in severa l major airlinecrashes" (Mitchell and Hovermale 1977).
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2. Cha rba-Goff model of thunderstorm downdraft
Charba (1974) studied the low level wind andtherm al structure of one intense Oklaharna gustfront using data upto a height of 444 m aboveground from an instrumented tower and alsodata from a surface meso-network . both operatedby the Nationa l Severe Storms Laboratory (NSSL)of USA. Goff (1976) also using the NSSL tower .studied time-height sections of the wind andthermal patierns of 20 d ifferent gust-front cases.Because of the dynamic similarity between thegust-front and experimental gravity currents (Simpson 1969), laboratory gravity current studieshave enabled deduction of the gust-fro ntstructure above the tower o bservations, especiallyin regard to the frontal boundary shape and internal circulat ion in the cold outflow (Charba 1974and Goff 1976). Informat ion on these werealso deduced from the observed profiles of thedust-walls of duststorrns (Lawson 1971 and Idsoet al. 1972) .
Essential detail s of a model of thundersto rmdowndraft as obtained from the studies of Cha rba(1974) and Goff (1976) are given in Fig I. Someof the terminology used in describing importantfeat ures of the downdraft tt hunderstor m outflow)arc the following: