CALIPSO-inferred aerosol direct radiative effects: Bias estimates using ground-based Raman lidars

Abstract:

Observational constraints on the change in radiative energy budget
caused by the presence of aerosols, i.e. the aerosol direct radiative
effect (DRE), have recently been made using observations from the
Cloud-Aerosol Lidar and Infrared Pathfinder Satellite (CALIPSO). CALIPSO
observations have the potential to provide improved global estimates of
aerosol DRE compared to passive sensor-derived estimates due to CALIPSO's
ability to perform vertically-resolved aerosol retrievals over all surface
types and over cloud. In this study we estimate the uncertainties in
CALIPSO-inferred aerosol DRE using multiple years of observations from the
Atmospheric Radiation Measurement (ARM) program's Raman lidars (RL) at
midlatitude and tropical sites. Examined are assumptions about the ratio of
extinction-to-backscatter (i.e. the lidar ratio) made by the CALIPSO
retrievals, which are needed to retrieve the aerosol extinction
profile. The lidar ratio is shown to introduce minimal error in the mean
aerosol DRE at the top-of-atmosphere and surface. It is also shown that
CALIPSO is unable to detection all radiatively-significant aerosol,
resulting in an underestimate in the magnitude of the aerosol DRE. Therefore, global estimates of the aerosol DRE inferred from
CALIPSO are likely too weak.