Wednesday, 9 July 2014
Marine Boundary Layer (MBL) cloud properties derived for the Clouds and the Earth's Radiant Energy System (CERES) Project using Terra and Aqua Moderate Resolution Imaging Spectroradiometer (MODIS) data are compared with observations taken at the Atmospheric Radiation Measurement (ARM) Mobile Facility at the Azores (AMF-Azores) site from June 2009 through December 2010. Cloud properties derived from ARM ground-based observations were averaged over a 1-hour interval centered at the time of satellite overpass and the CERES-MODIS (CM) results were averaged within a 30 km × 30 km grid box centered over the Azores site. A total of 63 daytime and 92 nighttime single-layered overcast MBL cloud cases were selected from 19 months of ARM radar-lidar observations. The CM cloud-top/base heights (Htop/Hbase) were determined from cloud-top/base temperatures (Ttop/Tbase) using a regional boundary-layer lapse rate method. For daytime comparisons, the CM-derived Htop (Hbase), on average, are 0.063 km (0.068 km) higher (lower) than the ARM radar-lidar observed counterpart(s), and the CM-derived Ttop and Tbase are 0.9 K less and 2.5 K greater than the surface values with correlations of 0.90 and 0.92, respectively. In general, the cloud-top comparisons agree better than cloud-base comparisons because the CM cloud-base temperatures and heights are partially determined from cloud-top temperatures and heights. No significant day-night difference was found in the analyses.
The comparisons in MBL microphysical properties reveal that the averaged CM-retrieved MBL cloud-droplet effective radius (re) is 1.3 µm larger than that from the ARM retrievals (12.8 µm). In addition, the retrieved LWP values are comparable (114.2 vs. 100.5 gm-2) with a relatively large difference in optical depth (13.7 vs. 9.6). Using effective radius retrieved at 2.1 µm to calculate LWP can reduce the difference between the CM and ARM microwave radiometer retrievals from -13.7 to 2.1 g m-2. The 10% differences between the ARM and CERES-MODIS LWP and re retrievals are within the uncertainties of the ARM LWP (~ 20 g m-2) and re (~ 10%) retrievals, however, the 30% difference in optical depth is significant. Possible reasons contributed to this discrepancy include the degrading of MODIS visible observations and increased sensitivities in optical depth from both surface and satellite retrievals when τ ~ 10.
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