The radiative energy budget in the Multi-scale Modeling Framework: A comparison to ARM observations

The amount of radiation absorbed in the atmospheric column and its vertical distribution is an important driver of the atmospheric circulation. In order to produce realistic representations of cloud and water vapor feedbacks, climate models must produce not only accurate surface and top-of-atmosphere energy budgets, but accurate vertical distribution of clouds, water vapor, and their associated radiative heating. One of the difficulties in producing accurate cloud and heating rate profiles within a large-scale general circulation model (GCM) is the sub-grid scale nature of cloud dynamical processes and their interactions with radiation, which requires many cloud processes to be parameterized. A new approach to climate modeling, the Multi-Scale Modeling Framework (MMF), reduces the need for cloud parameterizations by coupling cloud-scale dynamics with the larger scale dynamics of the GCM. In the MMF, cloud processes are treated more explicitly by replacing the cloud parameterizations of the GCM with a 2-D cloud resolving model (CRM) embedded in each GCM gridbox.

While we have good knowledge of radiative fluxes at the top of the atmosphere and at specific surface sites, observations of atmospheric profiles of radiative heating, particularly in cloudy sky conditions, have been largely unavailable. Current estimates of cloudy sky radiative heating in the tropics are typically based on model simulations, residuals from total heat and moisture budgets, or satellite observations. The remote sensing observations taken at the Department of Energy's Atmospheric Radiation Measurement program's sites on the islands of Nauru and Manus in the tropical western Pacific region provide a more direct method of calculating all sky heating rate profiles with high vertical and temporal resolution. We have calculated radiative heating rate profiles for several months at each of these sites using observed and retrieved inputs of water vapor, condensed water, phase, particle size, and mass.

In this paper, we present analysis of the components of the radiative energy budget at the ARM sites from the observed surface radiative fluxes, top of atmosphere fluxes from the Geostationary Meteorological Satellite (GMS), and the calculated vertical distribution of heating rates. We compare this radiative energy budget dataset to model output from the NCAR Community Atmosphere Model (CAM 3.0), and the Multi-Scale Modeling Framework (MMF). The models are run using observed sea surface temperature for this comparison. We find that the CAM generally has better agreement with the surface and top-of-atmosphere energy budgets, while the MMF shows better agreement with the vertical distribution of heating in the atmosphere.