Abstract
Abstract. We present the validation analysis of above-cloud aerosol optical depth (ACAOD) retrieved from the “color ratio” method applied to MODIS cloudy-sky reflectance measurements using the limited direct measurements made by NASA's airborne Ames Airborne Tracking Sunphotometer (AATS) and Spectrometer for Sky-Scanning, Sun-Tracking Atmospheric Research (4STAR) sensors. A thorough search of the airborne database collection revealed a total of five significant events in which an airborne sun photometer, coincident with the MODIS overpass, observed partially absorbing aerosols emitted from agricultural biomass burning, dust, and wildfires over a low-level cloud deck during SAFARI-2000, ACE-ASIA 2001, and SEAC4RS 2013 campaigns, respectively. The co-located satellite-airborne matchups revealed a good agreement (root-mean-square difference < 0.1), with most matchups falling within the estimated uncertainties associated the MODIS retrievals (about −10 to +50 %). The co-retrieved cloud optical depth was comparable to that of the MODIS operational cloud product for ACE-ASIA and SEAC4RS, however, higher by 30–50 % for the SAFARI-2000 case study. The reason for this discrepancy could be attributed to the distinct aerosol optical properties encountered during respective campaigns. A brief discussion on the sources of uncertainty in the satellite-based ACAOD retrieval and co-location procedure is presented. Field experiments dedicated to making direct measurements of aerosols above cloud are needed for the extensive validation of satellite-based retrievals.
Highlights
Aerosol–cloud interaction continues to be one of the leading uncertain components of climate models, primarily due to the lack of an adequate knowledge of the complex microphysical and radiative processes associated with the aerosol–cloud system (Stocker et al, 2013)
We find that the cloud optical depth (COD) retrieved from the color ratio” (CR) algorithm are consistently higher by 30–50 % than those retrieved from MODIS C006 for the SAFARI-2000 case
The satellite retrieval of above-cloud aerosol optical depth (ACAOD) is found to be in good agreement with airborne measurements, some discrepancies remain
Summary
Aerosol–cloud interaction continues to be one of the leading uncertain components of climate models, primarily due to the lack of an adequate knowledge of the complex microphysical and radiative processes associated with the aerosol–cloud system (Stocker et al, 2013). In the past few years, the development of several independent algorithms that quantify aerosol loading above cloud from satellite-based active as well passive measurements has been a major breakthrough in the fields of aerosol and cloud remote sensing These algorithms have shown the potential to retrieve quantitative information on aerosol loading above cloud using measurements from different A-train sensors including CALIOP/CALIPSO (Hu et al, 2007; Chand et al, 2008), Parasol/POLDER (Waquet et al, 2009, 2013; Peers et al, 2015), Aura/OMI (Torres et al, 2012), and Terra-Aqua/MODIS (Jethva et al, 2013; Meyer et al, 2015; Sayer et al, 2016).
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