Evaluation of Clouds and Precipitation in the ECHAM5 General Circulation Model UsingCALIPSOandCloudSatSatellite Data

• Published in: Journal of climate Vol. 25; no. 14; pp. 4975 - 4992
• Main Authors: Nam, Christine C. W., Quaas, Johannes
• Format: Journal Article
• Language: English
• Published: American Meteorological Society 15.07.2012
• Subjects: Clouds; Convection clouds; Cumulus clouds; Hydrometeorology; Precipitation; Radar; Radar echoes; Reflectance; Simulations; Stratocumulus clouds
• ISSN: 0894-8755; 1520-0442

Observations fromCloud–Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO)andCloudSatsatellites are used to evaluate clouds and precipitation in the ECHAM5 general circulation model. Active lidar and radar instruments on boardCALIPSOandCloudSatallow the vertical distribution of clouds and their optical properties to be studied on a global scale. To evaluate the clouds modeled by ECHAM5 withCALIPSOandCloudSat, the lidar and radar satellite simulators of the Cloud Feedback Model Intercomparison Project’s Observation Simulator Package are used. Comparison of ECHAM5 withCALIPSOandCloudSatfound large-scale features resolved by the model, such as the Hadley circulation, are captured well. The lidar simulator demonstrated ECHAM5 overestimates the amount of high-level clouds, particularly optically thin clouds. High-altitude clouds in ECHAM5 consistently produced greater lidar scattering ratios compared withCALIPSO. Consequently, the lidar signal in ECHAM5 frequently attenuated high in the atmosphere. The large scattering ratios were due to an underestimation of effective ice crystal radii in ECHAM5. Doubling the effective ice crystal radii improved the scattering ratios and frequency of attenuation. Additionally, doubling the effective ice crystal radii improved the detection of ECHAM5’s highest-level clouds by the radar simulator, in better agreement withCloudSat. ECHAM5 was also shown to significantly underestimate midlevel clouds and (sub)tropical low-level clouds. The low-level clouds produced were consistently perceived by the lidar simulator as too optically thick. The radar simulator demonstrated ECHAM5 overestimates the frequency of precipitation, yet underestimates its intensity compared withCloudSatobservations. These findings imply compensating mechanisms in ECHAM5 balance out the radiative imbalance caused by incorrect optical properties of clouds and consistently large hydrometeors in the atmosphere.