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551.509.3 : 551.578 .1(548) Forecasting of pentad rainfall character in Kerala coast during July D. A. )lOOr,EY Institute of Tropical Jleteor olog y, I' OOIiU ( Received 9 August l fJGG) A6ST I'tACT. Utilising the 700·mb lev el, 6-day moan and 1.day mean (i.e., moan of 00 and 12 GMT) charts p-eccding tho occurrence of extreme Abnormal end extreme subnormal rahr ..U in Kerela, lfignil1ca.l1t parameters hav e beea obtalncd. These pl "l\ '1u te rll have boon used in evolving lob" 1I0l10 :1l:) bu."t 0I co.u.ingeney technique for Io-ecaering pa.uad rainfall eao -nvly in Komia. I I thle '1 ::1'3.3 n '. Io r th a Il[)s: t a-ll:.y patio:! id leeue.I after cvury two or th roo days, dcpondlng on lo bo period. of the meen 700.mb conto ur charta for ove rla p ping pentad e. • • 1. Introduelion )l eth04..ls utilising contingency technique have been developed hy a number of workers for fore- casting rainfall during short. and medium periods. J a!, 'llnnathan and Hnmamurthi (1961) developed a method fur foreca sting 5-dn.y rainfall anomaly over Bombay. Working on similar lines, Sajnani (1964) obtained a method for forecaating 5-day rainfall for Calcutta. In this meth od, suitable upper air parameters like 5-day mean or l -day contour heights at iOO and flOO-lIlh levels at sig- nificant stations, nrc obtained after s t. udying the cont rast ing upper nir feat ures associated with abnormal and suhnormn l rainfall at tho station. These parameters, hereafter called the predictors, were tested for statistical significance ami pairs of these were graphically correlated with the next pentad rainfall anoma ly by means of scatter diagram . The point s on the scntter d iagrnm are divided into three classes which IDay be res- pectively associat ed with three classes of th e predictand, l'iz.) abnorma l, normal and subnormal. Contingency tables arc prepared and finally after applying contingency tec hniques as given by " ·a h1 et " I. and Lnnd nnd Wahl (1955), prcdict and class (i .e., rainfall anomaly) is obtained. The main requ irements of t he appl ication of cont ingency techniques of foreca, st.ing: arc - (a) no class should linvc a frequency of less than o per cent of the total frequency and (b) stat istical stability of th e significance of t he predict and s should be maintained over long periods of time. In the present study, it is proposed to apply the contingency technique to the forecasting of next pentad rainfall character in Kerala. The o-day moan iOO-mb cha rts considered arc those for overlapping pentads, tho overlap being 2 or 3 days , for example, 3 -7 Jnly and 0-9 July, 15-19 Jul y and July. The rainf all anomaly considered is for tho subsequent 5-day period. In tho scheme to bo evolved forecast, of pentad rainfall character in Kurala coast would be issued every 2 or :}days, depending on tho periods of the pentads for which mean 700 4mb charts arc nvaila- blo. Tho area for which rainfall anomaly is to be forecast is shown in F ig, 1. 2. Classlneatlon or pentad ralnrall anomaly Rainfall anomaly for each of the pentads is available for the stations, Trivnud rum , Cochin and Kozhikocl o in Korula. It hus been o bta ined on th o basis of ,Iat" for period As mentioned by Pan t (1961), tho limits of rainfall for classification of rainfall anomaly are fixed in such a W il Y that each character of the pentad rai nfa ll anomaly of every sta tiou, (li z., abnormal, normal and subnormal, has equal probability of one-third. From the pentad rainfall anomaly at the thr ee stations , tho same for Kerala was obtained as follows. When all stations had the same character, e.g., a bnormal then that character, viz., abnormal, was taken for rainfall anomaly of Kerala. In all other cases, tho ra infal l anoma ly was ob tai ned by obta ining the total pentad rain fall of th e t hree stat ions and finding out whether this total was above tho 811 m of t he limits for the three stations for abnormal rainfall, for subncrmnl rainfall or bet ween t he two. In the firat cuse, the rainfall anomaly for the area has been taken as abnormal; in the second case, subnormal and in the last case, normal. Suppose, RT, Re. Il K arc pent ad ra infall for the three stat ions Tri van drum, Cochin and Kozltikode, RTA, IleA, Il KA the limits of abnormal ra infa ll a nd Rrs, Rrn, a KS the limits of subnorma l rainf all for these stations respectively. Then, if (RT + Re + RK) is equal to or gre ate r than (RTA + Rc.\ + RKA) , pentad rainfall for Kerala is abnorma l, if less th an or equal to (Rrs + Rcs + IlKS) , rai nfall is subnormal ami if it is le..s t.han (RTA + Rm + RKA ) but greater than(RTS + Rcs+ R KS) ra infall is normal. In t his way, rai nfall anomaly for Kerala for all the pentad in Jnl y for the yo ars 1957 to 1964 was classified as abnormal, normal and subnormal.
