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Radiative characteristics of clouds embedded in smoke derived from airborne multiangular measurements
Author(s) -
Gautam Ritesh,
Gatebe Charles K.,
Singh Manoj K.,
Várnai Tamás,
Poudyal Rajesh
Publication year - 2016
Publication title -
journal of geophysical research: atmospheres
Language(s) - English
Resource type - Journals
eISSN - 2169-8996
pISSN - 2169-897X
DOI - 10.1002/2016jd025309
Subject(s) - radiative transfer , environmental science , smoke , atmospheric sciences , single scattering albedo , cloud top , satellite , aerosol , remote sensing , atmospheric radiative transfer codes , troposphere , cloud albedo , optical depth , cirrus , cloud cover , albedo (alchemy) , radiometer , meteorology , cloud computing , physics , geology , astronomy , optics , art , performance art , computer science , art history , operating system
Clouds in the presence of absorbing aerosols result in their apparent darkening, observed at the top of atmosphere (TOA), which is associated with the radiative effects of aerosol absorption. Owing to the large radiative effect and potential impacts on regional climate, above‐cloud aerosols have recently been characterized in multiple satellite‐based studies. While satellite data are particularly useful in showing the radiative impact of above‐cloud aerosols at the TOA, recent literature indicates large uncertainties in satellite retrievals of above‐cloud aerosol optical depth (AOD) and single scattering albedo (SSA), which are among the most important parameters in the assessment of associated radiative effects. In this study, we analyze radiative characteristics of clouds in the presence of wildfire smoke using airborne data primarily from NASA's Cloud Absorption Radiometer, collected during the Arctic Research of the Composition of the Troposphere from Aircraft and Satellites campaign in Canada during the 2008 summer season. We found a strong positive reflectance ( R ) gradient in the UV‐visible (VIS)‐near infrared (NIR) spectrum for clouds embedded in dense smoke, as opposed to an (expected) negative gradient for cloud‐free smoke and a flat spectrum for smoke‐free cloud cover. Several cases of clouds embedded in thick smoke were found, when the aircraft made circular/spiral measurements, which not only allowed the complete characterization of angular distribution of smoke scattering but also provided the vertical distribution of smoke and clouds (within 0.5–5 km). Specifically, the largest darkening by smoke was found in the UV/VIS, with R 0.34μm reducing to 0.2 (or 20%), in contrast to 0.8 at NIR wavelengths (e.g., 1.27 µm). The observed darkening is associated with large AODs (0.5–3.0) and moderately low SSA (0.85–0.93 at 0.53 µm), resulting in a significantly large instantaneous aerosol forcing efficiency of 254 ± 47 W m −2 τ −1 . Our observations of smoke‐cloud radiative interactions were found to be physically consistent with theoretical plane‐parallel 1‐D and Monte Carlo 3‐D radiative transfer calculations, capturing the observed gradient across UV‐VIS‐NIR. Results from this study offer insights into aerosol‐cloud radiative interactions and may help in better constraining satellite retrieval algorithms.