Lidar-aided Estimates of the Vertical Structure of the Direct Shortwave Aerosol Radiative Forcing of Climate

Lidar is a powerful tool to characterize the vertical structure of aerosol optical properties. In this paper we will present a technique to estimate the effective aerosol complex index of refraction from a combination of lidar-derived aerosol backscatter, sunphotometer-derived aerosol optical depth and in situ particle size distribution data.

An application of this technique to two case studies during the Tropospheric Aerosol Radiative Forcing Observational Experiment (TARFOX) yields aerosol refractive indices with real parts between 1.33 and 1.45, and imaginary parts between 0.001 and 0.008. Sensitivity studies with our refractive index estimation technique show a 5% error in the derived real parts as a consequence of a 30% random error in the lidar-derived aerosol backscatter.

The refractive indices thus derived can be used to calculate the vertical structure of aerosol optical properties relevant for the height-resolved simulation of the instantaneous, direct shortwave aerosol radiative forcing. Model calculations with a broadband radiative transfer model indicate substantial differences in the aerosol forcings at the top of the atmosphere (~ -36 W m^-2) from those at the surface (~ -56 W m^-2).

Importantly, our simulations suggest that, for the two TARFOX case studies presented here, the aerosol radiative forcing results are not sensitive to the exact values of the particle size distributions as long as the combination of the aerosol size distributions and the aerosol refractive indices reproduces the lidar-derived aerosol backscatter profiles accurately.