Mausam, (1987), 3R, I , 67-72

551.551. 8: 551. 508.55: 551.573

The estimation of surface fluxes from heat balance equati onfor the city of Delh i

MA NJ U KU MARI and O. P. SHARMA

Indian Institute of Technology, Nell' De/hi

(Receim/ 26 February /985)

'"'' - <f.R: ';r'fli fUT'! .ron l«litimvi\ <n: anurfut~, 'i-;;t'll, ~'<I 3;,,!' am: 'l"TlP' 'llIl =rr art1R!&'i(~) ~~ .. ""':~~ 'I,'i\ "" llliPT f"'IPflIT t ,~ ~ '1"r." 'I,'i\ it ,,,,,for,, 'ff<>r f.lKc<-lIT'l"'i'i~~ A:t(R'IT , t'I'!~~~ "" ...a t I it~m.f\~ it l('1' <rioil ~if "'IT': llfuf.IfiI l!{AI "f",! m ~ t , wro~ <n: 1f<~ afto: 'l'<I' '"""~ "" am. ilfu<I 'Il'il " lIP< ~ it"~ ~ t ,~~, ""l'i"&'" ii 'lfu<ff arfi<;r>rr WIf~'.jlITf~oro-it "'" '1""'" t f.I; ill"""" it rrrr~ Pr-! ii am.m 1f<~ ii I s- so lIftnI<r qfl:q;H "" """'" .,.. 'miT t ,

ABSTRACT. Simplified empirical formulae have been used to compute radiative. so il heat, latent heat amisensible surface heat fluxes based o n rou tine meteorological surface obse rvations. These empirical formulaeenable us to compute these flUXC5 without involving too many complicated input parameters. Computatio ns havebeen performed for the four representative months of the season of a year for the city of Delhi. On comparison,net radia tion and latent heat flux show good agreement with their observed values. Furthermore, a sensitivitytest performed by varying turbidity in the atmo sphere show s that a two-fold increase in turbidity may cause achange of 15~50% of the net radiation at various times of the day.

I, IntrodUC1Jon 2. Flux computations

The beat balance equation for the eart h and earth­atmosphere system can be written as

The surface lIuxes of soil. heat and water vapour areuseful for many purpose, viz., the estimation of stabilitynear the ground, the determination of evaporation rateuseful. in hydrology, agriculture and for various airpollution problems.

In India , though a good network of observat ionalstations exist , but the radiation measurements are doneonly at very few stations. These measurements areavailable fully or partly only at eighteen stations inIndia (Mani 1980). In the absence of such measure ­ments, estimati on of net radia tion and associated surfacelIuxes is based on some formu lae using many inputparameters which are usually not available. With thisobjective, the present study deals with the computationof net radiation , in addition to the G, LE and H,g bymaking a choice from the available simplified formula eusing routin e meteorological surface observations forfour representative month s of the season for the city ofDelhi. These formul ae have been verified against theobservations with highly sophisticated computationsby othe r authors.

List of symbols

Latitude o f the place under studyOeclinition to the sunOccadic absorption coefficient (dm

'mol'")

Emissivity constant• . 95Stefan-Bcltzman consta nt. 5.998 x 10-' wm~

Psychrometric co nstant, .66 mb k- 1

Density of airConstant used for latent heat flux, 20 wm~Accounts in fractio ns the mo isture co ntent in soil(.) .6S for dry periods, (b) .9S for nonnal period .Transmissivity coe fficienl in clear air including Rayleighand Mie scat tering.

Albedo o f the surface

Amount of energy depleted from NOt abso rption

Coefficient for low cloud layer. . 15

Coefficient for middle cloud layer. .50

Coefficient for high clo ud layer, . 70Spec ific heat capacity of dry air at constant pressure.IO05JKg- ' k-'Vapour pressure at the surface (mh)Vapour pressure defic itEva porat ion rateAn.ount of energy transmiued after NO, abso rptionSOil heal flux (wm - !)

r p

y

p

jl

•

G'

s,o

. 1

ABSNO!

('B'

cc:en:

( I)R - LE + H,g + G

( 67 )