Comparison of Marine Boundary Layer Cloud Properties From CERES-MODIS Edition 4 and DOE ARM AMF Measurements at the Azores

Abstract:

Marine Boundary Layer (MBL) cloud properties derived for the NASA CERES Project using Terra and Aqua MODIS data are compared with observations taken at DOE ARM Mobile Facility at the Azores site from Jun. 2009 to Dec. 2010. Cloud properties derived from ARM ground-based observations were averaged over a 1-hour interval centered at the satellite overpass time, while the CERES-MODIS (CM) results were averaged within a 30×30 km² 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 and satellite observations. The CM cloud-top/base heights (H_top/H_base) were determined from cloud-top/base temperatures (T_top/T_base) using a regional boundary-layer lapse rate method. For daytime comparisons, the CM-derived H_top (H_base), on average, is 0.063 km (0.068 km) higher (lower) than its ARM radar-lidar observed counterpart, and the CM-derived T_top and T_base are 0.9 K less and 2.5 K greater than the surface values with high correlations (R²=0.82 and 0.84, respectively). In general, the cloud-top comparisons agree better than cloud-base comparisons because the CM T_base and H_base are secondary product determined from T_top and H_top. No significant day-night difference was found in the analyses.

The comparisons of microphysical properties reveal that, when averaged over a 30x30 km² area, the CM-retrieved cloud-droplet effective radius (r_e) is 1.3 µm larger than that from the ARM retrievals (12.8 µm). While the CM-retrieved cloud liquid water path (LWP) is 13.5 gm^-2 less than its ARM counterpart (114.2 gm^-2) due to its small optical depth (τ, 9.6 vs. 13.7). The differences are reduced by 50% when the CM averages are computed only using the MODIS pixel nearest the AMF site. Using effective radius retrieved at 2.1-µm channel to calculate LWP can reduce the difference between the CM and ARM from -13.7 to 2.1 gm^-2. The 10% differences between the ARM and CM LWP and r_e retrievals are within the uncertainties of the ARM LWP (~ 20 gm^-2) and r_e (~ 10%) retrievals, however, the 30% difference in τ is significant. Possible reasons contributed to this discrepancy increased sensitivities in τ from both surface retrievals when τ ~ 10 and topography. The τ differences vary with wind direction and are consistent with the island orography.