BULLETIN OF IMSP - (IMD), Pune

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How the Palghat Gap can influence the north-south rainfalldistribution across its axis?S. Dutta, P.P.Toro and S. Das GuptaOffice of the DDGM(WF), IMD, Pune.Palghat Gap (PG), about 40 km wide & 80 km long, is a gap in the Western Ghats(WG). Climatologically normal rainfall pattern during the Southwest monsoon seasons(SWMS) over south peninsular India shows an area of rainfall discontinuity near PG(fig.1). Divergence caused due to Bernoulli’s effect in the gap flow is generally attributedto this effect.Observed downwind increase of wind speed transiting the PG at its eastern endmade by Ramachandran et al. (1980) was attributed to divergence in the lower levels atthe gap exit. The study also suggested a dynamical study, using a dynamical model, toconfirm their ideas.Asnani (1993) explained the above divergence using the familiar analogy of theventuri effect. But venturi effect is not always the correct explanation for gap winds.Because, unlike the tube or nozzle of engineer’s venturi, the mountain gap is not a fullyenclosed space but a channel with an open top, that is, no rigid upper lid restricts the flowto three-dimensions.To explore for a plausible dynamical explanation for the observed downwindincrease of winds transiting the PG, as mentioned above, a dynamical model (Dutta 2005)for meso-scale airflow across a meso-scale 3-D orographic barrier has been used. Themountain gap has been synthesized by two-meso-scale three-dimensional ellipticalbarriers and is shown schematically in fig.2. Vertical profile of perturbation verticalvelocity ( w′) at different points at the exit (eastern end) of the gap, computed using theabove model, are shown in fig 3(a) and 3(b).From the figures it is clear that at x=35km and x = 40km, w′ decreases with height.Hence there is perturbation horizontal divergence at the gap exit. This divergence maycause a cyclonic / anti cyclonic turning of the streamlines to the north/ south of the gapaxis near its exit. Further details of this numerical experiment may be found in Dutta et al.(2006).Thus it appears that low level perturbation horizontal divergence at the gap exit iscaused due to decrease of perturbation vertical velocity with height. This divergence inturnmay cause a cyclonic/anti-cyclonic turning of the streamlines to the north/south ofthe gap axis at the exit, which again may be attributed for the observed difference in themean seasonal rainfall during SWMS to the north and south of the PG.REFERENCES:Asnani.G.C, 1993, “Tropical Meteorology”, Volumes 1 & 2, IITM, Pune-411008, India.Dutta, Somenath., 2005: Effect of static stability on the pattern of three dimensionalbaroclinic lee wave across a mesoscale elliptical barrier. Meteorology andAtmospheric Physics, 90, 139-152.12
Dutta, Somenath., Mukherjee, A.K. and Singh, A.K., 2006, Effect Of Palghat Gap OnThe Rainfall Pattern To The North & South Of Its Axis, Mausam,57,4, 675-679.Ramachandran, G., Rao K.V, Krishna K.,.1980,”Observational study of Boundary layerwinds in the exit region of a mountain gap”, J. of Appl. Meteorlology., 19, 281- 288.800PALGHATJULYRAINFALL(mm)600400SOUTH STATIONSNORTH STATIONSJUNEAUGUST200SEPTEMBER1030 1035 1040 1045 1050 1055 1060LATITUDEFig.1: Variation of rainfall with latitude atdifferent stations during SWMSFIG.2:A SCHEMATIC REPRESENTATION OF AGAP BETWEEN TWO MESO-SCALE 3-D ELLIPTICAL BARRIERS10108866Z (km)4Z (Km)422000.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50w' (m/s)FIG.4(A):VERTICAL PROFILE OF w' AT X=35KM ALONG GAP AXIS0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40w' (m/s)FIG.4(B):VERTICAL PROFILE OF w' AT X=40KM ALONG GAP AXIS13
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