PNNL

Abstract

From April 2009 to December 2010, the Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) program carried out an observational field campaign on the Graciosa Island, targeting on the marine boundary layer (MBL) clouds over the Azores region. In this paper, we present an inter-comparison of the MBL cloud properties, namely, cloud liquid water path (LWP), cloud optical thickness (COT) and cloud effective radius (CER), between the retrievals from ARM mobile facility (AMF) during this campaign and two satellite remote sensing products based on NASA’s Moderate Resolution Spectroradiometer (MODIS) observations. From the comparison of instantaneous retrievals, we found that the two MODIS cloud products show excellent agreement with correlation coefficient exceeding 0.95. However, we noted in both products significant differences between retrievals using the 2.1 µm MODIS band and those using the 3.7µm band. When closely collocated, the MODIS and ground-based cloud property retrievals from ARM AMF agree reasonably well on LWP. However, MODIS retrieved COT and CER tend to be smaller and larger, respectively, than ground-based retrievals. We found similar results in the comparison of monthly mean cloud properties. In comparison with observations, the MBL clouds in the Azores region simulated in the NCAR Community Atmosphere Model are too bright and their droplet sizes are too small. Our study demonstrates that the ground-based retrievals from the ARM Azores site provide an independent and critical data set on the MBL cloud properties that are highly useful for evaluating the MBL cloud simulations in climate models.