An assessment of the surface longwave direct radiative effect of airborne dust in Zhangye, China, during the Asian Monsoon Years field experiment (2008)

Abstract

In April–June 2008, NASA Goddard's ground‐based mobile laboratories (SMART‐COMMIT) were deployed to Zhangye China (39.0°N; 101°W) to support the Asian Monsoon Years field experiment and the East Asian Study of Tropospheric Aerosols and Impact on Regional Climate. One of the primary objectives at Zhangye, a semi‐arid region located between the Taklimakan and Gobi Deserts, was to capture and characterize dust aerosols near the source and to quantify their direct radiative effects (DRE). A regional dust optical model was constructed by combining previously measured soil mineralogy data at Zhangye with COMMIT's particle microphysical measurements. During a 2‐week period of heightened dust activity, retrieved longwave (LW) aerosol optical thickness ( τ ) from SMART's Atmospheric Emitted Radiance Interferometer was used in the Fu‐Liou radiative transfer model to derive LW instantaneous DRE (DRE LW ) at the surface, top of atmosphere, and heating rate profiles for cloud‐free conditions. Conservatively, surface instantaneous DRE LW and LW forcing efficiency range from about 2–20 Wm −2 and 31–35 Wm −2 τ −1 (0 ≤ τ ≤ 0.83), respectively. The significance of DRE LW relative to its shortwave counterpart was estimated to be between 51 and 58%, but of opposite sign, partly compensating shortwave surface cooling. Compared to Saharan dust observed during the NAMMA‐2006 field experiment at Cape Verde, dust LW forcing efficiency for this study was found to be a factor of two larger stemming from differences in environmental and surface conditions, aerosol absorption, and Zhangye's close proximity to major desert sources. Relative to observed and modeled ranges in surface DRE LW for clouds (∼30–80 Wm −2 ) and greenhouse gases (∼2 Wm −2 ), this study's upper range in DRE LW represents a significant perturbation to the climate system with important implications for better understanding regional changes in surface temperatures and moisture budgets.