IMausam. ( 198 5), 36) 2. 127·1.14

551.509.327 : 551.509.313

Certain aspect s on the rea lization of pr im it ive equationsforecasting model in the low lat itudes

U. C. MO HANTY and FLT. LT. S. C. MADA •

Centre f or Atmospheric & Fluids Sciences,IlIdi(1II institute of Technology, Nell' Delhi

(Receirecl27 JOIIIIGry 1983)

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AUSTRACf. A number of numeri cal experiments are curried out with a barotropic primi tiveequat ions (PE) model for forecasting the tropical systems. A systematic approach has been mode tosolve some of the importan t problems arising in the process of integrat ing such a simple PH modelin the low latitudes. Emph asis ba.. been Riven 10 the initial data problem in the tropics. In order 10

study the influence of dynamical initiali zat ion and filterin g techniques on the forecasting model andto study their smoothing characteristics. the spectral decompos ition of kinetic energy of initial ,initiali zed, forecast and actual wind field.., is curried out. Further. the improvement in the effic iencyof the model b)' incorpo r ating these techniques is demonstrated. Finally, this model is applied toforecast the movement of tropical cyclones ami a monsoon depression in the Bay of Bengal andinspire of the co nstraints of the model, the results arc quite encouraging.

L Introduction

Most or the numerical predict ion models (filtereda nd PE model s) de veloped for the mid-lat itude. ma keuse of the observed geop otential field as initia l data .because th is field i comparatively smoother. more ac­curate in measurement in mid-lat itudes an d easier toanalyse than the wind field. The velocity field is ob­ta ined from geopotential field b) using geostrophicbala nce . H owever. the real izat ion or init ial dat a b)th is proces~ in the tropics is more difficult a s :

(I) Nearly 2/3 of the tro pica l belt is covered withdata sparse ocea nic region.

t li) In the tropics, the gradient o r geo pot entia lis very small and a lso the day to day va ria­bility is of t he same order as the inherentobservat ional erro r. Further. the geostro­ph ic relati on does not hold good and thesolution of the ba lance equat ion suffersfrom ellipticity problems.

Fro m the above it is clear t hat the use or geopo­tentia l height as the basic input to the prediction modelsin the tr opics. has certai n serio us pract ical a nd theoret i­cal limitat ions. Whereas, the wind field. as fo und from

theoretical a nd model consideration s. seems to bethe better source or data in the tropics (Obukhov 1949.Winn inehotT 1968. Houghton and Washington 1969,Gordon' et al. 1972), it su tTers from spa rsity of observa­t ional netw ork. Efforts are continuing to increasethe den sity or observations in the tropics. One orthe ma in purpose of First GAR P Global Experiment(FGGE) was to evaluate the performance of the modelswith the increase in density of ob servations in the tro­pics. With the introduction or geo stationary satelli teswith meteorological payloads, the wind field ca n bederi ved fro m the cloud mo vement more accurately forthe tropical regions than fOI the polar latitudes.

ln spite or enhancement of wind observa tio ns bygeostatio nary sa tellites. choice of wind field as basicdata suffers from the problem or objective a nalysi satregular grid po int s a nd est imation or gcopotentialhe ight by solving non-linear balance eq uations. which

. require the knowledge of stream func tion s ( Mohantyand Madan 1983).

T herefo re so far, there is no ' sing le and uniqueopinion in favour or either wind or geo potc ntia l heightdata. fOI thei r use as the initial data in t he tropisc.

Directorat e of Meteoro logy, Air HQ s, vayu Bhavan , New Delhi·tlOOll( 127 )