Cloud radiative forcing at the top of the
atmosphere during FIRE ACE derived from AVHRR data
D.R. Doelling, P. Minnis, D.A. Spangenberg,
V. Chakrapani, A. Mahesh, S.K. Pope, and F.P.J. Valero
J. Geophys. Res., 106(D14),
15279-15296, 27 July 2001.
Cloud radiative forcing at the top of the atmosphere is derived from
narrowband visible and infrared radiances from NOAA-12 and 14 Advanced
Very High Resolution Radiometer (AVHRR) data taken over the Arctic Ocean
during the First ISCCP Regional Experiment Arctic Cloud Experiment
(FIRE ACE) during spring and summer of 1998. Shortwave and longwave
fluxes at the top of the atmosphere (TOA) were computed using
narrowband-to-broadband conversion formulae based on coincident Earth
Radiation Budget Experiment (ERBE) broadband and AVHRR narrowband radiances.
The NOAA-12/14 broadband data were validated using model calculations and
coincident broadband flux radiometer data from the Surface Heat Budget
of the Arctic Ocean experiment and from aircraft data. The AVHRR TOA albedos
agreed with the surface- and aircraft-based albedos to within one standard
deviation of + 0.029 on an instantaneous basis. Mean differences ranged from
-0.012 to 0.023 depending on the radiometer and platform. AVHRR-derived
longwave fluxes differed from the model calculations using aircraft- and
surface-based fluxes by -0.2 to -0.3 Wm-2 , on average, when the atmospheric
profiles were adjusted to force agreement between the observed and calculated
downwelling fluxes. The standard deviations of the differences were less
than 2%. Mean total TOA albedo for the domain between 72N and 80N and
between 150W and 180W changed from 0.695 in May to 0.509 during July, while
the longwave flux increased from 217 to 228 Wm-2. Net radiation increased
from -89 to -2 Wm-2 for the same period. Net cloud forcing varied from
-15 Wm-2 in May to -31 Wm-2 during July, while longwave cloud forcing was
nearly constant at ~8 Wm-2. Shortwave cloud forcing dominated the cloud
effect, ranging from -22 Wm-2 during May to -40 Wm-2 in July. The mean
albedos and fluxes are consistent with previous measurements from the ERBE,
except during May when the albedo and longwave flux were greater than the
maximum ERBE values. The cloud forcing results, while similar to some earlier
estimates are the most accurate values hitherto obtained for regions in the
Arctic. When no significant melting was present, the clear-sky longwave flux
showed a diurnal variation similar to that over land under clear skies.
These data should be valuable for understanding the Arctic energy budget and
for constraining models of atmosphere and ocean processes in the Arctic.