In this practical we compute the heat budget using measured quantities. It is ma

Question

In this practical we compute the heat budget using measured quantities. It is made up of incoming short wave and outgoing longwave radiation, sensible heat and evaporation. The total budget can be used to estimate changes in soil temperature. Field data for the calculation are given below Variable Value Wind speed Label 10 m Air Temperature Surface Temperature 10 m Specific Humidity Surface Spec Humidity 22 C 26 C 15 g/kg 20 g/kg qs 1. Calculate the solar insolation at 9h00 assuming the season is equinox eg. 21 March and the latitude is Puerto Rico 18°N, with the equation Qs - (sin(d)(S)(insolation fraction) where s is the solar constant 1370 W m-2 d is the sun angle = (90 deg-latitude -season), and dividing for time of day, the fraction - 0.6 insolation

Explanation / Answer

(1)

Qs=(sin(d))*(S)*(insolation factor)

d=(90 deg - lattitude - season) =90-18-0=72 deg [ the seasonal angle is zero since the season is equinox and sun rays are perpendicular to earth's axis at equator in the noon ]

S=1370 Wm-2 and insolation fraction = 0.6 ( given)

therefore,

Qs=sin(72 deg)*(1370)*(0.6) = (0.9510)*(1370)*(0.6) =781.768 Wm-2

(2)

The longwave radiation emitted upwards is given by QL=(d)*(273 K+Ts)4(Greenhouse fraction)

d=5.67*10-8 W m-2 K-4 , Greenhouse fraction=0.4

therefore QL =5.67*10-8(273+26)4 *(0.4) =181.27 W-m-2

(3)

The sensible heat flux , Qh = U * (Ts-Ta)

U= 5 ms-1 , Ta=22 0C and Ts=26 0C

therefor, Qh =5*(26-22)=20 W-m-2

(4)

The evaporation is given by Qe = 2.5*U*(qs-qa)

qs= 20 g/Kg , qa=15 g/Kg , U=5 m/s

Therefore, Qe =2.5*5*(20-15) = 62.5 W-m-2

(5)

QT=Qs-(QL+Qe+Qs)= 781.768-(181.27+20+62.25)=518.245 W-m-2

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