q f, if ,,,'!WImetnet.imd.gov.in/mausamdocs/13621_F.pdf · I Mausam. (1985), 36) 2. 127·1.14...
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I Mausam. (1985), 36) 2. 127·1.14 551.509 .327 : 551.509 .313 Certa in aspec ts on the rea liz ation of p r im it ive equations forecasting mo del in th e low l at i tudes U. C. MO HANTY and FLT. LT. S. C. MADA • Centre for Atmospheric & Fluids Sci ences, IlIdi(1II institute of Technology, N ell' Delhi (Receirecl27 JOIIIIGry 1983) "'" - _ m...rTll ""TTP,,;j .. i f"" """" ..m i "t >t orif "'" q I f;:rq )fe n," q i 'I'f' I Ji"crf_;rein """'" 'TT '>If'n. m f"", 'TllI " "," ""R '" ,,"'iT fima->:" "" otm ;;,,"'. "" """ if"" " >t t '" '" ""''''' '" f"" {f.!fuo""' J:n. q'lf.\- "'" ... <t\ .,-f'f' :>:'!j . " 'fRt'l'lP! fof""" ,, f...iT 'TllI """'"" i iT..trn <t\ "",,""'", '!'JT>:'lir..mt >rA""''r{'f 'lft! "'" 'I'f' t, , ,, '!WI <t\!fl"I >ii ", m , AUSTRACf. A number of numeri cal experiments are curried out with a barotropic primi tive equ at ions (PE) model for forec asting the tropical systems. A systematic approach has been mode to solve some of the important problems arising in the process of integrat ing such a simple PH model in 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 forec asting model and to study their smoothing characteristics. the spectral decompos ition of kinetic energy of initial, initialized, forecast and actual wind fiel d.., is curried out. Further. the improvement in the e ffic iency of the model b)' incorpo r ating these techniques is demonstrated. Finally, this model is applied to forecast the movement of tropical cyclones ami a monsoon depression in the Bay of Bengal and inspire of the constraints of the model, the results arc quite encouraging. L Introduction Most or the numer ical prediction models (filtered a nd P E model s) de vel ope d for the mid-lat itude. make use of the observed ge op otent ial field as initial data. because this field i c om paratively smo other. more ac- curate in measurement in mid-lat itudes and easier to analyse than the wind field. The velocity field is ob- ta ined from ge opoten tial field b) using geos tro phic bala nce . However. the real izat ion or initial data b) th is in the trop ics is more difficult as : (I) Nearly 2/3 of the t ro pica l belt is covered with data sparse oceanic region. t li) In the trop ics, the grad ient or geopotential is very small and a lso the day to day varia- bility is of the same order as the inherent obser vational error. Further . the geostro- ph ic relati on does not hold good and t he solution of the balance equ at ion suffers from ellipticity problems. From the abo ve it is clear t hat the use or geopo- tentia l height as the basic input to the prediction models in the tr op ics. has certain serio us practical and theoreti- cal limitations. Whereas, the wind field. as found from the oretical and model consideration s. seems to be the better source or data in the trop ics (Obukhov 1949. Winn inehotT 1968. Hought on and Wash ington 1969, Gordon ' et al. 1972), it su tT ers from sparsity of observa- tional netw ork . Eff orts are continuin g to increase the den sit y or observations in the trop ics. One or the ma in purpose of First GA RP Global Experiment (FGGE) was to evaluate the perf ormance of the models with the increase in density of observations in the tro- pics. With the introduction or geo stationa ry satellites with meteorological payloads, the wind field ca n be deri ved fro m the cloud movement more accurately for the tropical regions than fOI the polar latitudes. ln sp ite or enhancement of wind observa tio ns by geostationary sa tellites. choice of wind field as basic d ata suffers from the prob lem or objective analysi sa t regular grid po int s a nd est imation or gcopote ntial height by solving non-linear balance equations. which . require the knowledge of stream functions ( Moha nty and Mad an 1983). T he refo re so far, there is no ' sing le and unique opinion in favour or either wind or geopotcntial height data. fOI their use as the initial da ta in the trop isc. Directorat e of Meteoro logy, Air HQ s, va yu Bhavan , New Delhi·tlOOll ( 127 )
Transcript of q f, if ,,,'!WImetnet.imd.gov.in/mausamdocs/13621_F.pdf · I Mausam. (1985), 36) 2. 127·1.14...
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)
"'" - _ m...rTll ""TTP,,;j .. Hi~'fT'ii i f"" ~'" ./~ """" 'l.t~~ ..m i "t>t~ orif"'"~I't f~ ~q I f;:rq )fen," lli!q qTUf7Gfq;,,,qoIriR'ir~lJ1R1i-~"'fTq*Tt~~~~ i qml1T",if'i'i~WITJf f~
'I'f' ~ I 'If~) Ji"crf_;rein """'" 'TT '>If'n. m f"", 'TllI ~ " ","""R ~w.r '"m~,', f" , ,, '~ ,,"'iT fima->:"""otm ;;,,"'. "" ""'.~ """ if"" " >tt fif,~ 'f,,;iT~t '" 'I'ffil~~~i '" ""''''' '" f""~, {f.!fuo""'J:n. q'lf.\'fTf~ "'" ~r"lf... ",,~..ron <t\ .,-f'f' :>:'!j . " 'fRt'l'lP! fof"" " ,, f...iT 'TllI ~ , """'"" ~ ,,~ i :;"llil~ iT..trn <t\"",,""'", ii"T~ '!'JT>:'lir..mt 'l'ft ~, >rA""''r{'f ~mro'To7rr'l<t\'fI~)ii'f.f~~<t\'lft! "'" "l~ ~t~"''l.fI1"",''f,,~~ml'f f.,.., 'I'f' t , ,,, '!WI <t\!fl"I>ii ", ~'''''':: m Tf'<;orrq'f.l'il~j",~ ,
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 )
