On the Angular Effect of Undetected Clouds in Infrared Window Radiance Observations: Aircraft Experimental Analyses

This presentation furthers previous investigations into the zenith angular effect of cloud and/or aerosol contamination within infrared (IR) radiance observations (obs) commonly used in the retrieval of environmental data records (EDRs) from hyperspectral sounders such as the Joint Polar Satellite System (JPSS) Cross-track Infrared Sounder (CrIS). In this paper, we corroborate two previous related papers (Nalli et al. 2012, 2013) by performing analyses of hyperspectral IR microwindow observations against forward calculations (calc - obs) taken from the 2007 Joint Airborne IASI Validation Experiment (JAIVEX). High resolution IR spectra obtained from the aircraft-based NPOESS Aircraft Sounder Testbed Interferometer (NAST-I) are compared against forward calculations based upon collocated dropsondes. We utilize aerosol optical depth (AOD) EDR derived from NOAA Geostationary Operational Environmental Satellite (GOES) to determine the approximate locations of cloud contaminated NAST-I fields-of-view (FOV). To estimate the angular variation of clouds, we derive expressions for estimating cloud aspect ratio from visible imagery where cloud shadows can be measured relative to the cloud horizontal diameter. In agreement with the sensitivity calculations in Nalli et al. (2012), it is found that a small cloud fraction within sensor FOV can have a measurable concave-up impact on the angular agreement with clear-sky calculations on the order hundreds of millikelvins. It is also found that sun-glint can have a measurable impact on agreement between calc – obs in longwave IR microwindow channels (in aircraft experimental data) on the same order of magnitude as residual cloud/aerosol contamination. Our results are germane to inversion (retrieval) schemes that depend upon cloud-cleared or cloud-masked IR radiance data, as well as in hemispheric radiative flux calculations involving randomly distributed broken cloud fields.