Validation of Aerosol Optical Depth Retrieved from CALIPSO Lidar Ocean Surface Backscatter

Extinction retrieved from the CALIOP lidar on the CALIPSO satellite is known
to underestimate the aerosol optical depth (AOD) which has hampered its use
in understanding aerosol radiative impacts. This low bias in AOD has been
attributed to undetected aerosols since the "standard" retrieval approach is
to only estimate optical depth in regions of the column where aerosols are
directly detected. As part of a recent CALIPSO v4.5 data release, the Ocean
Derived Column Optical Depth (ODCOD) is a new retrieval for estimating total
column particulate optical depths from the ocean surface backscatter
returns. ODCOD provides an estimate of the AOD that is largely independent
from the standard CALIPSO retrieval. In this study, we perform the obligatory
validation of the ODCOD AOD using the ground-based AERONET sun
photometers. The agreement between AERONET and the ODCOD AOD is contrasted
with that from the standard CALIPSO retrievals. The quality of the AOD
uncertainty estimates provided in the CALIPSO data products is assessed using
the diagnostic AERONET-based error estimates. Also discussed are the
potential uses of ODCOD as a compliment to existing CALIOP retrievals and for
improving estimates of aerosol extinction and radiative impacts.