Improving boundary-layer turbulence and cloud processes in CAM with a higher-order turbulence closure scheme and ARM measurements

Description

A major difficulty in addressing cloud feedback and climate sensitivity stems from the inadequate representation of cloud physical processes in general circulation models (GCMs) with traditional cloud parameterizations. An alternative approach is to use a cloud-resolving model (CRM) to replace cloud parameterizations in every GCM grid box, i.e., the multiscale modeling framework (MMF), and implement a sophisticated higher-order turbulence closure (HOC) scheme in its CRM component to better represent turbulence and low clouds. This approach is shown to produce realistic global distribution of low clouds, but the computational cost increases by nearly a factor of two compared to that of the original MMF (with a first-order turbulence closure in its CRM component) and a factor of 400 compared to that of the CAM at T42 resolution. The improved low-cloud simulation motivates us to directly implement the HOC scheme in a GCM. The goal of this investigation is to deliver a physically based HOC parameterization package for the global modeling community through its implementation in CAM and other models, which requires simplifications and development of an optical approach from the HOC used in CRM that likely take years to complete. In this investigation, we propose to undertake three tasks that enable improvement of CAM5 and make it useful for the global modeling community for further refining other parts of model parameterizations such as a tight coupling between subgrid cloud distributions and radiative transfer. These tasks are

• simplifying the HOC turbulence/low-cloud parameterization to a diagnostic package as one of the options and developing an optimal approach, which will combine the implicit time differencing for the equations with time derivative terms with the diagnostic approach in the remaining equations. This will be done in a step-by-step fashion through extensive testing
• testing the simplified scheme(s) in a single-column model, a 2D CRM using the prototype boundary-layer cloud cases and ARM intensive operational period data, and in an MMF and CAPT for global simulations
• performing long-term climate simulations with the modified CAM and CAM5, comparing with the available global data sets and the Climate Modeling Best Estimate data at selected ARM sites, and evaluating the low-cloud feedback.

In this poster, some preliminary results to illustrate the approach adopted in the first two steps will be given, and future research will be discussed.