An assessment of the surface longwave direct radiative effect of airborne Saharan dust during the NAMMA field campaign

In September 2006, NASA Goddard's mobile ground-based laboratories were deployed to Sal Island in Cape Verde (16.73°N, 22.93°W) to support the NASA African Monsoon Multidisciplinary Analysis (NAMMA) field study. The Atmospheric Emitted Radiance Interferometer (AERI), a key instrument for spectrally characterizing the thermal IR, was used to retrieve the dust IR aerosol optical depths (AOTs) in order to examine the diurnal variability of airborne dust with emphasis on three separate dust events. AERI retrievals of dust AOT are compared with those from the coincident/collocated multifilter rotating shadowband radiometer (MFRSR), micropulse lidar (MPL), and NASA Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) sensors. The retrieved AOTs are then inputted into the Fu-Liou 1D radiative transfer model to evaluate local instantaneous direct longwave radiative effects (DRE_LW) of dust at the surface in cloud-free atmospheres and its sensitivity to dust microphysical parameters. The top-of-atmosphere DRE_LW and longwave heating rate profiles are also evaluated. Instantaneous surface DRE_LW ranges from 2 to 10 W m^-2 and exhibits a strong linear dependence with dust AOT yielding a DRE_LW of 16 W m^-2 per unit dust AOT. The DRE_LW is estimated to be ~42% of the diurnally averaged direct shortwave radiative effect at the surface but of opposite sign, partly compensating for the shortwave losses. Certainly nonnegligible, the authors conclude that DRE_LW can significantly impact the atmospheric energetics, representing an important component in the study of regional climate variation.