Examining the impact of overlying aerosols on the retrieval of cloud optical properties from passive remote sensing

Abstract

[1] Haywood et al. (2004) show that an aerosol layer above a cloud can cause a bias in the retrieved cloud optical thickness and effective radius. Monitoring for this potential bias is difficult because space-based passive remote sensing cannot unambiguously detect or characterize aerosol above cloud. We show that cloud retrievals from aircraft measurements above cloud and below an overlying aerosol layer are a means to test this bias. The data were collected during the Intercontinental Chemical Transport Experiment (INTEX-A) study based out of Portsmouth, New Hampshire, United States, above extensive, marine stratus cloud banks affected by industrial outflow. Solar Spectral Flux Radiometer (SSFR) irradiance measurements taken along a lower level flight leg above cloud and below aerosol were unaffected by the overlying aerosol. Along upper level flight legs, the irradiance reflected from cloud top was transmitted through an aerosol layer. We compare SSFR cloud retrievals from below-aerosol legs to satellite retrievals from the Moderate Resolution Imaging Spectroradiometer (MODIS) in order to detect an aerosol-induced bias. In regions of small variation in cloud properties, we find that SSFR and MODIS-retrieved cloud optical thickness compares within the uncertainty range for each instrument while SSFR effective radius tend to be smaller than MODIS values (by 1-2 mm) and at the low end of MODIS uncertainty estimates. In regions of large variation in cloud properties, differences in SSFR and MODIS-retrieved cloud optical thickness and effective radius can reach values of 10 and 10 mm, respectively. We include aerosols in forward modeling to test the sensitivity of SSFR cloud retrievals to overlying aerosol layers. We find an overlying absorbing aerosol layer biases SSFR cloud retrievals to smaller effective radii and optical thickness while nonabsorbing aerosols had no impact.

Investigating potential biases in observed and modeled metrics of aerosol-cloud-precipitation interactions

“…Haywood et al, 2004;Cattani et al, 2006;Bennartz and Harshvardhan, 2007;Wilcox et al, 2009;Coddington et al, 2010). In support of the presented analysis, potential biases in r e have been shown to be less than 1 µm for retrievals where the visible reflectance is matched with the 3.7 µm or 2.13 µm reflectance.…”

Retrieval of aerosol microphysical and optical properties above liquid clouds from POLDER/PARASOL polarization measurements

“…The transport of Saharan mineral dust is also frequent across the Atlantic, and dust plumes are regularly transported above low-level clouds off the coasts of Senegal . Other types of aerosols such as volcanic dust particles and pollutant particles (Hsu et al, 2003;Coddington et al, 2010) were also observed above clouds in other regions of the world. The most commonly observed situation corresponds to an elevated aerosol layer suspended above a low-level liquid water cloud, but properties of aerosol above clouds (AAC) have not been extensively studied yet.…”

Impact of cloud horizontal inhomogeneity and directional sampling on the retrieval of cloud droplet size by the POLDER instrument

“…It is found that the vertical structure induced by drizzle and 3-D radiative effects operate together to cause dramatic differences between the 1.6, 2.1, and 3.7 µm retrievals (Zhang et al, 2012;Zhang, 2013;Nakajima et al, 2010a, b;Nagao et al, 2013). In addition, the water vapor absorption within a cloud and the presence of an absorbing aerosol layer above a cloud leads to a positive bias in the retrieval (Alexandrov et al, 2012a;Coddington et al, 2010;Haywood et al, 2004).…”

“…The CDR values of the clouds ranged from 5 to 20 µm in 1 µm increments, and the EV values were 0.01, 0.02, and 0.05. Three COT values (1, 5, and 10) were considered in this study, and the contributions from the underlying surface and the aerosol and molecule layers are negligible (Coddington et al, 2010;Goloub et al, 2000). The Rayleigh optical thickness for different wavelengths was set according to the results of Bodhaine et al (1999).…”