Hw. Barker et al., BROAD-BAND SOLAR FLUXES AND HEATING RATES FOR ATMOSPHERES WITH 3D BROKEN CLOUDS, Quarterly Journal of the Royal Meteorological Society, 124(548), 1998, pp. 1245-1271
A 3D Monte Carlo photon transport algorithm is presented that computes
broadband solar fluxes and heating rates. It treats attenuation by cl
oud droplets and gases separately and can produce 3D distributions of
constituent absorptances. Underlying surfaces are accounted for and di
urnal-mean calculations can be achieved in the same time as typical si
ngle-zenith-angle experiments. Domain-averaged fluxes and heating rate
profiles are presented for two very different 3D cloud fields: (i) sc
attered, shallow cumuli inferred from Landsat imagery; and (ii) toweri
ng clouds simulated by a cloud-resolving model. Plane-parallel, homoge
neous (PPH), independent column approximation (ICA), and clear-sky ver
sions of the 3D fields were generated and used as well. For both cloud
fields, total atmospheric absorptance depends very weakly on cloud ge
ometry. Cloud geometry does, however, invoke major differences in surf
ace absorptance and, hence, reflectance to space. At high sun, albedos
for 3D clouds are less than corresponding PPH values, but are in almo
st perfect agreement with ICA estimates. This indicates that simple ho
rizontal variability of cloud optical depth outweighs the impact of cl
oud sides. At very low sun 3D fields reflect most because of intercept
ion of radiation by cloud sides, while PPH and ICA albedos come into b
etter agreement. For the towering cloud field, radiative fluxes are de
termined largely by clouds below 6 km, despite some clouds reaching 12
km. Heating rate profiles are also affected by cloud geometry. For mo
st sun angles, PPH clouds exhibit anomalously large heating near cloud
tops and anomalously small heating beneath clouds. On the other hand,
profiles for 3D and ICA fields are very similar and depend much less
on altitude; partly because of side illumination but also because the
dense cores of inhomogeneous clouds are often radiatively-shielded (un
like their PPH counterparts). Finally, regular arrays of idealized clo
ud forms are used to demonstrate the potential ambiguity of using clou
d radiative forcing ratios, R, as proxy measures for the impact of clo
uds on atmospheric absorptance. In essence, R depends not only on how
clouds influence atmospheric al,sorption, but also on how they partiti
on radiation between albedo and transmittance.