Towards Quantifying Global Aerosol Radiative Effects Using Lidar

Current satellite observations can only provide relative weak constraints on the direct aerosol radiative forcing. The gap in our knowledge of the climate system was reflected by the 2017 NRC Decadal Survey that identified as ``most important'' the objective of reducing the IPCC AR5 total aerosol radiative forcing uncertainty by a factor of 2. In this work, we focus on the simplest of aerosol-radiation interactions: the aerosol direct radiative effect (DRE), i.e. the radiative effect of all aerosols both natural and anthropogenic. Several recent estimates of the aerosol DRE have been made using observations the CALIPSO lidar, which has the potential to provide better global all-sky estimates than passive sensor-derived
estimates. However, comparisons to more advanced ground-based and airborne lidars show that CALIPSO does not detect all radiatively-significant aerosol, i.e. aerosol that directly modifies the Earth's radiation budget. We estimated that CALIPSO's lack of sensitivity results in an underestimate of the magnitude of the global mean aerosol DRE by up to 54%. Additionally, the CATS lidar on-board the ISS is shown to have a poorer sensitivity than CALIPSO.

To achieve the goal of reducing aerosol radiative forcing, the most basic requirement is a more sensitive lidar. To this end, high-accuracy ground-based and airborne lidar datasets have been used to compute the detection sensitivity required to accurately resolve the aerosol DRE. Preliminary simulations of the MESCAL high spectral resolution lidar (HSRL) mission concept shows a detection sensitivity that could nearly eliminate the current aerosol DRE bias.

Also surveyed is the inherent detection advantages of a HSRL and other benefits relevant for aerosol-radiation studies. Multiwavelength HSRL measurements also can retrieve vertically-resolved aerosol optical properties needed for radiative transfer calculations which are not provided by current satellite observations. Current satellite observations also do not provide all the quantities needed to compute the aerosol direct radiative forcing, i.e. the radiative effect of just anthropogenic aerosols. A multiwavelength HSRL allows for a more refined aerosol classification to be made enabling both calculations of anthropogenic aerosol radiative effects and better constraints on global models.