On the angular effect of residual clouds and aerosols in clear‐sky infrared window radiance observations 2. Satellite experimental analyses

Abstract

AbstractThis paper continues an investigation into the zenith angular effect of cloud‐contamination within “clear‐sky” infrared (IR) radiance observations commonly used in the retrieval of environmental data records (EDRs), which include “cloud‐cleared radiances” (as is typical from hyper/ultra spectral IR sounders), as well as “cloud‐masked” data (as is typical from imagers). The simple probability of clear line of sight (PCLoS) models and sensitivity studies of Part 1 (Nalli et al., 2012a) are corroborated with experimental analyses of environmental satellite data products as functions of sensor zenith angle, including sounder cloud‐cleared radiances (CCRs) and retrieved effective cloud fraction, as well as narrowband imager cloud masking. Analyses of hyperspectral microwindow calc − obs are performed using MetOp‐A Infrared Atmospheric Sounding Interferometer (IASI) CCR observations matched to dedicated radiosonde observations (RAOBs) during intensive validation field campaigns. The IASI calc − obs are found to exhibit a systematic positive bias with a strong concave‐up variation with satellite zenith angle (i.e., an increasing positive bias symmetric over the scanning range) on the order of 1–3 K in magnitude, a signal associated with both residual clouds and dust aerosols. This is corroborated by analysis of the IASI retrieved effective cloud fraction product compared to the expected angular variations predicted by the PCLoS models, which show that the observed concave‐up calc − obs variation may be the result of contamination by mid‐to‐upper tropospheric clouds. Finally, a corollary global analysis of the MetOp‐A Advanced Very High Resolution Radiometer (AVHRR) cloud‐mask shows concave‐up variation that may be underestimating the angular variation for global ensembles containing clouds with vertical development (i.e., aspect ratios >0.5). The results presented in this work thus support the sensitivity studies of Part 1, indicating that contamination by residual clouds and/or aerosols within clear‐sky observations can have a measurable concave‐up impact on the angular agreement of observations with calculations.