To compute the solar radiation flux density at the surface we need to know effects of atmosphere in...
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Transcript of To compute the solar radiation flux density at the surface we need to know effects of atmosphere in...
To compute the solar radiation flux density at the surface we need to know effects of atmosphere in filtering and depleting the beam from the top of the atmosphere to the ground
Absorption and scattering by the atmosphere
Scattering•Rayleigh, for molecules and tiny particles
•Mie for larger particles
41
1
Rayleigh, blue sky
Mie, with large particulate matter. Whitish sky
Absorption, especially due to O3, H2O,CO2
Combined in a simple slab approach, scattering and absorption reduce transmissivity, so for cloudless atmosphere:
mao ZcosIK
ma Depends on turbidity of the air (scattering +
absorption) and path length or optical air mass (m)
Z Zcosdistance zenith
path slanthm
1
Typically ma varies from about 0.9 (clean) to 0.6
(dirty), typical 0.84
Physically based models
Attempt to account for all physical processes in the chain
terrain sloping
K
surface horizontal cloudy
K
surface horizontal cloudless
KExKoI
Some calculate components of direct (S) and diffuse (D) radiation
DSK
Absorption
Scattering
Example: Davies et al. 1975
Assumptions:•Absorption occurs before scattering•Half of dust deplection is due to absorption•Scattering occurs equally in forward and backward direction•Absorption by ozone neglected
Cloudless sky
dsrswsdawaoo ZcosIS
2
1 )(ZcosID dsrsws
dawaoo
scattering erosola
scattering ayleighR
scattering vapor water
absorption erosola
absorption vapor water
ds
rs
ws
da
wa
2
1 )(ZcosIDSK dsrsws
dawaoooo
data. alexperiment on fitted Curves
m. and ble water)(precipita
w of functionare
Cloudy skyDavies et al. 1975 (continue)
cloud of amount totalC
groundthe of albedo
base cloud of albedo
fraction) cloud andtype cloudthe of (function
i layer of ontransmissi cloud
CKK
c
c
Ci
gc
n
iCio
11
Cloud layers
Comparison with measurements
Physical models are capable of approaching accuracy of measurements, especially in cloudless case and for daily averages
Won (1977)
Absorption + Scattering
•It uses hourly reported meteorological parameters
Law) s(Beer' form onentialexpT
dust todue g scatterinT
vapor waterof absorption and scatteringT
airdry of scatteringT
TTTT
equation)quadratic (using cloudinessC
CTKK
dw,p,
d
w
p
dwpt
t
ttEx
In the computation of the Tp,w,d functions, empirical coefficients are used. It may be place specific
Beer’s Law (Monteith p. 32-35)
It describes the attenuation of flux density of a parallel beam of monochromatic radiation through an homogeneous medium
dxx
)x( dx)x(k)x(
dx layerthe in absorption is )x(d
dx)x(k)x(d
Integrating
medium.the ofnature the
to related )(m tcoefficien extinction is k
e)()x(1-
kx 0
(0).value initialthe from
(x)distance the lly withexponentia deplets
It has been found that the very restrictive assumption about single wave length and homogeneity of the medium can be relaxed or modified. So the Beer’s Law can be applied to:
Air (Won, 1977 model), •k= atmospheric extinction due to turbidity•x=path length
And also in water, snow, ice, soil, vegetation canopy