Retrieval of cloud properties from spectral zenith radiances observed by sky radiometers
Abstract. An optimal estimation algorithm to retrieve the cloud optical depth
(COD) and cloud particle effective radius (CER) from spectral zenith
radiances observed by narrow field-of-view (FOV) ground-based sky
radiometers was developed. To further address the filter degradation problem
while analyzing long-term observation data, an on-site calibration procedure
is proposed, which has good accuracy compared with the standard calibration
transfer method. An error evaluation study conducted by assuming errors in
observed transmittances and ancillary data for water vapor concentration and
surface albedo suggests that the errors in input data affect retrieved CER
more than COD. Except for some narrow domains that fall within a COD of < 15, the retrieval errors are small for both COD and CER. The retrieved cloud
properties reproduce the broadband radiances observed by a narrow FOV
radiometer more precisely than broadband irradiances observed by a wide-FOV
pyranometer, justifying the quality of the retrieved product (at least of COD)
and indicating the important effect of the instrument FOV in cloud remote
sensing. Furthermore, CODs (CERs) from sky radiometer and satellite
observations show good (poor) agreement.