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JOURNAL OF THE ATMOSPHERIC SCIENCES, 61, 2951-2956, 2004
Assessing a cloud optical depth retrieval algorithm with model-generated data and the frozen turbulence assumption
H. W. Barker, C. F. Pavloski, M. Ovtchinnikov, E. E. Clothiaux
Abstract
A cloud optical depth retrieval algorithm that utilizes time series of solar irradiance and zenith downwelling radiance data
collected at a fixed surface site is assessed using model-generated cloud fields and simulated radiation measurements.
To date, the retrieval algorithm has only been assessed using instantaneous cloud fields in which time series were mimicked
via the frozen turbulence assumption. In this study, time series of radiation data are generated for use by the algorithm from
a series of snapshots of an evolving and advecting cloud field, with values of optical depth retrieved for clouds occurring at
the midpoint of the time series. This approach resembles conditions encountered in the field much better than those arising
from the convenient frozen turbulence assumption. Values of optical depth are also retrieved for the same cloud field by
employing the frozen turbulence approach. For the field of broken, shallow cumulus considered here, differences between
the two sets of retrievals are small. This suggests that the encouraging results obtained thus far for this retrieval
algorithm have not been secured falsely by the frozen turbulence assumption.
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