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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 Meteorologi cal 0 ffice, PlIne (Receil'cd IS August 1980) lIT< - lROl (191 ') a'I1 '"'" (1 91 s) "' 'r"T'I .. " 'If' 'fIR '" mUTful.,;!it "' . fi;t ('Fl'<- 'lfRq- '"'" iT <rM ) 'f.T'fJlR fro '!'IT I ll'fT':- I iT <rM iT ?"'II"' "" """'"" "'" lr _ "" ."" if 'I'l'J . ('flU lI'r.T< iT .,;!it iT 'i""" iT <m'lT WI 1980 if iT I ll'fT':-2 (mit """ ) iT <rM '1""""',;rq '.!"'l': 1I'fT ...r' 1l'fi'I """, .ro" >it , 'If. 'l11 'If'll >rr>J f", ";"T 'f.T>M uf;< if 'I'" it d\li\ 'Ifu lr ":."':' f", ""'''lIN'' ;{1'1l ij ",!'lA " 1'1' oM "' H'm' "' -.. "' 'm 'iii >it 'if'"""" I ABSTRACT. Based on the thunders torm d owndra 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 able 10explain qualitatively the visibility variation..in a typical type- I Amllri (Tho details of visibility variati ons in the four types of AndMhave beengiven by Joseph et al. 1980). The slowervisibility improveme ntand the consequent longer dura tion of typc-2 (night time) Am/hi also gets a logical explanation, 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 sup- port from studies on the propagation or 'gravily current' heads. I. Introduction The convective type of duststorm occurring over northwest India during the pre-monsoon season Apri l 10 June is called Andhi. Joseph ct al. (1980) made a study of 40 cases of Andh! that occurred at Delhi airport dur ing the period 1972 to 1977, using available meteorological records. From the nature or variations or horizontal visibility and wind speed near the ground level associated with these du ststorrn s, it was found that 4 types of Andhi occur. From radar photographs of the cumulonimbus cloud or squall line associated with A",11ri it was foun d that the dista nce between the cb cloud or squa ll line (as seen in the radar) and the associated Andhi dust-wall on the ground is as large as 30 km. Study of the downdraft from severe thunder- storms is considered very important in view or the role of such downdrafts in causing aircraft accidents. " The large wind-shears that are charac- teristic of the gust-front occurs mainly in the lowest 1-2 km of the atmosphere and, therefore. are a par ticular hazard to aircraft flying at low levels. In fact, low level wind shear in and around thunderstorms has recently been labelled as the primary probable cause in severa l major airline crashes" (Mitchell and Hovermale 1977). (411) 2. Charba-Goff model of thunderstorm down draft Ch arba (1974) studied the low level wind and thermal structure of one intense Oklaharna gust- front using data upto a height of 444 m above ground from an instrumented tower and also data from a surface meso-network . both operated by the Nationa l Severe Storms Lab oratory (NSSL) of USA. Goff (1976) also using the NSSL tower. studied time-height sections of the wind and thermal patierns of 20 different gust-front cases. Because of the dynamic similarity between the gust-front and experimental gravity currents (Simp- son 1969), la borat ory gravity current studies have enabled deduction of the gust-fro nt structure above the tower observations, especially in regard to the frontal boundary shape and inter- nal circulat ion in the cold outflow (Charba 1974 and G off 1976). Information on these were also dedu ced from the observed profiles of t he dust-walls of duststorrns (Lawson 1971 and Idso et al. 1972). Essential detail s of a model of thunderstorm downdraft as obtained from the studies of Cha rba (1974) and Goff (1976) are given in Fig I. Some of the terminology used in describing important feat ures of the downdraft tthunderstorm outflow) arc the followi ng:
