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Estimation of divergen t wind from OLRdat a for use in objective analysisover the Indian region

P. L. KULKARN I. D. R. TALWALKAR. S. NAIR " ",I S. RAJA:\lA NI

Indian Institute 0/ Tropical ,Heleo/'olog l', PUfI('

(R eceived JJ November 1991. Modifi('d 15 J UII (' 1992)

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A HST RAC r . A ...chcr m i... fo rmu lated for the use of Ol R data in the estimat ion or ver tical velocity. tli­vergence and then the divergent pa n of the wind ove r India n region. In thi... schem e, ascendi ng mot ion o verclo udy region i" est ima ted fro m an emp irica l r ela tion bet ween the cloud top te mpe rature and descending 1110­

rion over cloud -fre e region is estimat ed from the t hermody namic ene rgy equ a tion an d both arc blended . Fr \1I11thi s blended ver tica l velocit y field, ui vcrgencc. velocity po tent ial an d d ivergent wind" a t a ll standa rd levcl-, frum.t to ,.. J u ! ~ 1979 a t OJ U Te a rc com put ed . Th ese fields a rc co mpa red with sat ellite cloud pict ures , ra infa ll CICand they arc fo und to be rea list ic in de picti ng the syno ptic cond itions . To tal wind is computed a'i l h~ sum ofthe evtirnated divergent compone nt and rotational co mponen; computed from obse rved wind field. Fo r asses­SOlen! o f the scheme. thi ... total wind field at 850 hPa is used as ini tial ~ UNS field in univaria te optimu m inter ­polation scheme and a nalyses were m3(] C fo r th e period 4 to 8 July 1979, R esults sho w that scheme i" a ble (0produce reali stic analyses whic h included dive rgent pa rt of the wind .

2. 1. D('linealion of cloud)' alld clolld-frc(, regiol/

2. :\ Iethodolog)-

K"sahara (' I 01. (1988) h" d sugges ted a scheme inwhich the ascending motion was mor e dir ectly computedfrom OLR dat a and the dzscend ing motion fromth e thermodynamic en-:rgy equation. Following th issc heme, in the pre sent study we have computl.:d th '': ve rt i­c,,1 moti on "nd subseq uently estimaled t he divergentwind over the Ind i" n region. Thi s diverg,nt wind hasbeen included in the "nalysis thus improving the initialspecificat ion of wind field for NWP. In this scheme, thevertic" lIy integmted divergence is m"de zero with lheconstmint that vert ical velocity is zero "t 1000 " nd100 hPa .

Mord.. - Dive rgent winds, OLR, Objective analysis, NWP. Equivalent blad bod y lemp:ratur( ,

W" S sep" ratcly computed the requirement tha I thevertically integrated divergence is zero. was not ensuredin this study .

1. In lrodaction

Initi al specification of divergence is important fo rreali stic forecast of precipitati on r"te particularly inthe tr opics. Due to non-divergent in iti,,1 conditionsprecipitation mt es "re und erforecast in the first fewhours of forecas t bec"use mod el requires several hoursto develop a consistent vertical velocity field . Thi s'spin up' pr ob lem in forecast mod els m"v be due toinaccurate specificaion of divergence, mo~isture andth erm al fields elc (K"saha m ('I 01. 1988). Many studiesha ve been m"de 10 incorporate the diverge nt wind in"naly sed wind s so th at specific" tion of divergence inthe initial co ndition is impro ved in order to reducethe spin up pr obl em. Sumi el al. (1979), Sumi (i981),Tarbell ('I 01. (1981), Juli "n (1984), Krishnamurti el 01.(1984), Kri shn"murt i & Low-N" m (1986) and S" lmon &Thomas (1986) h" ve " tle mpted to specify the divergentpart by different methods. Most of these studies haveused OLR d"ta to es timate d ivergent p" rt of the wind.In our earlier study (Kulk" rn i ('I 01. 1992) also usingregression rel"t ion between OLR data and the divergence,lhe J"tler w"s estimated. Then the divergent pa rt of wind The cloudy region is " region of ascend ing motionwas obtain'd "nd included in the initial guess field of the and the rest of the cloud- free region is likely to be ofanalysis sc heme. Since the divergence at every level descending mot ion . In this scheme the descending

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