Diagnosis of the Marine Low Cloud Simulation in the NCAR Community Earth System Model (CESM) and the NCEP Global Forecast System (GFS)
Abstract
We present a diagnostic analysis of the marine low cloud climatology simulated by two state-of-the-art coupled atmosphere-ocean models: the NCAR Community Earth System Model (CESM) and the NCEP Global Forecasting System (GFS). In both models, the shallow convection and boundary layer turbulence parameterizations have been recently updated: both models now use a mass-flux scheme for the parameterization of shallow convection, and a turbulence parameterization capable of handling Stratocumulus (Sc)-topped Planetary Boundary Layers (PBLs). For shallow convection, both models employ a convective trigger function based on the concept of convective inhibition and both include explicit convective overshooting/penetrative entrainment formulation. For Sc-topped PBL, both models treat explicitly turbulence mixing and cloud-top entrainment driven by cloud-top radiative cooling. Our focus is on the climatological transition from Sc to shallow Cumulus (Cu)-topped PBL in the subtropical eastern oceans. We show that in the CESM the coastal Sc-topped PBLs in the subtropical Eastern Pacific are well-simulated but the climatological transition from Sc to shallow Cu is too abrupt and happens too close to the coast. By contrast, in the GFS coupled simulation the coastal Sc amount and PBL depth are severely underestimated while the transition from Sc to shallow-Cu is "delayed" and offshore Sc cover is too extensive in the subtropical Eastern Pacific. Preliminary analysis suggests that despite similar frameworks for representing shallow convection and boundary layer turbulence in the two models, differences in the formulations of key physical processes, for example, in the formulations of convective overshooting, lateral entrainment rate, convective precipitation efficiency, and shallow convection triggering, could contribute to the differences in the simulated marine low cloud distribution between the two models. Further testing of the sensitivity of marine low cloud simulation to the parametric choices in the shallow convection and boundary layer turbulence parameterizations with both single column and global model setups is recommended.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFMOS44C..04X
- Keywords:
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- 3307 ATMOSPHERIC PROCESSES / Boundary layer processes;
- 3310 ATMOSPHERIC PROCESSES / Clouds and cloud feedbacks;
- 3314 ATMOSPHERIC PROCESSES / Convective processes;
- 3337 ATMOSPHERIC PROCESSES / Global climate models