Evaluating the Representation of Low Clouds in CAM5 Based on ARM Observations from the Azores

Although several studies have found that the annual mean properties of modeled low clouds has been improved due to more realistic representation of cloud processes in the new generation of climate models, the shorter time scale properties such as the seasonal variability, precipitation, and the diurnal cycle of low clouds still need to be evaluated with observations to reduce the uncertainties in low clouds modeling.

The Department of Energy’s Atmospheric Radiation Measurements (ARM) Mobile Facility (AMF) was on Graciosa Island in the Azores from June 2009 to December 2010 as part of the Clouds, Aerosol, and Precipitation in the Marine Boundary Layer (CAP-MBL) field campaign. CAP-MBL obtained 19 months of intensive cloud and radiation observations on Graciosa Island in the Azores, which offered us a good opportunity to evaluate the modeled low clouds with observations.

To spot and address issues in the simulation of marine low clouds in CAM5, this study compared the hourly CAM5 output at the grid column (39.1N, 27.5W), which is the nearest grid to Graciosa Island (39.1N, 28.0W), with 19-month cloud and radiation observations in the Azores. The representation of low clouds and their radiative impacts in CAM5 will be evaluated in different cloud regimes (i.e. cumulus vs. stratocumulus) and boundary layer (BL) structures (i.e. well-mixed BL vs. decoupled BL). The origin of the model bias for different cloud regimes will also be analyzed.

Acknowledgement: This work is supported by the ASR Program for the Office of Science of the U.S. DOE. This work was performed under the auspices of the U.S. DOE by LLNL under contract DE-AC52-07NA27344. LLNL-ABS-658041.