Radiative Interactions of Clouds Embedded in wildfire Smoke using Multiangular Airborne observations

Abstract:

Presence of absorbing aerosols above clouds causes its apparent darkening, observed at Top of Atmosphere. Owing to the large radiative warming and potential impacts on climate, above clouds aerosols (ACA) have recently been characterized in multiple satellite-based studies. While satellite data have particularly been useful in demonstrating the climate impact of ACA, uncertainties remain in the indepth understanding of aerosol-cloud radiative interactions. Recent studies also indicate large uncertainties in satellite retrievals of ACA optical depth and Single Scattering Albedo (SSA), which are among the most important parameters in their radiative impact.

Here, we present observations of clouds embedded in vertically extended and optically thick smoke, from NASA/GSFC’s airborne Cloud Absorption Radiometer. We focus on smoke plumes associated with pyroconvective activity from wildfires in Canada, during the ARCTAS campaign. Our key findings include characterization of clouds embedded in smoke, associated with a strong positive reflectance (R) gradient in the UV-VIS-NIR spectrum, 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 vertical distribution of smoke and clouds (within 0.5-5km). Specifically, the largest cloud darkening was found in the UV/VIS, with significant reduction at R_0.34μm in contrast to the NIR wavelengths (e.g. 1.27μm). The observed darkening was found to be associated with large optical depths (0.5 - 3) and moderately low SSA (0.85–0.93 at 530nm), resulting in significantly large forcing efficiency of 250Wm^-2τ^-1 (using coincident sunphotometer, insitu and irradiance measurements). Our observations of smoke-cloud radiative interactions were found to be physically consistent with theoretical 1D and 3D radiative transfer calculations, capturing the observed gradient across UV-VIS-NIR. This study offers new insights in aerosol-cloud radiative interactions, which also can help in better constraining satellite-retrieval algorithms.

http://car.gsfc.nasa.gov/