Motivations: One of the most important tasks in the study of physical and socioeconomic aspects of climate change is the simulation of polar climate and how it changes in response to global warming. One import component of the polar energy budget (and thus of polar climate) is the radiative interaction between surface and clouds. As of today, the vast majority of state-of-the-art global climate models (GCMs), including all three US flagship GCMs (NCAR CESM, GFDL CM3, and GISS E2-H/E2-R), still assume constant surface emissivity over all spectral bands in their longwave radiation treatment. Similarly, a majority of state-of-the-art GCMs assume non-scattering clouds in the longwave spectrum, including both the NCAR CESM and GFDL CM3. The issues of ignoring spectral variation in surface emissivity and scattering by clouds manifest themselves in the far-infrared (IR) over the polar continents because (1) there is so little water vapor in such areas that surface far-IR emission can reach clouds; and (2) the imaginary part of the refractive index of ice has a local minimum over 350-550 cm-1, which implies possible strong scattering effects over this spectral region both within and between the snow surface and ice clouds. Our sensitivity studies using CloudSat-retrieved hydrometeor profiles over the high-elevation Antarctic continent show that, in winter, an appropriate inclusion of snow surface spectral emissivity and ice cloud scattering in radiative transfer calculations can noticeably reduce the monthly-mean surface net downward far-IR flux and net atmospheric far-IR emission over the entire region. For the far-IR alone, the magnitude of these effects is ~1Wm-2 or even larger.
Projection Objectives: We propose to carry out the following studies: (1) Develop or update schemes that can be used in the CESM to compute the optical properties of the snow surface and ice clouds consistently across the longwave and the shortwave spectrum; (2) Modify the longwave radiation scheme in the CESM such that it can incorporate surface spectral emissivity and scattering of clouds; (3) Validate schemes developed for (1) and (2) against measurements or benchmark models; (4) Evaluate the effects of these new treatments on the simulated polar energy budget and climate by the CESM.
Technical Approaches:1.Use a detailed snowpack radiative transfer model to simulate spectrally resolved emissivity of snow surfaces as a function of snow effective radius. The model will be validated against available high spectral-resolution measurements of snow spectral emissivity (ASTER spectra library v2.0). Currently the CESM prognoses vertically-resolved snow effective radius, and snow shortwave albedo is parameterized based on snow effective radius. Our treatment of snow spectral emissivity here will be consistent with the treatment of snow albedo in the shortwave and seamlessly connected with the snow physical module in the CESM. 2. Update the parameterization of ice cloud optical properties in the CESM, for both longwave and shortwave, based on the latest ice-cloud scattering database developed by co-I Prof. Yang. 3. Modify RRTMG_LW, the current longwave radiation scheme in the CESM, to include the surface spectral emissivity and the scattering of clouds in the longwave calculation. Validate modified scheme against a line-by-line radiative transfer model with multiple scattering capability. 4. Implement products from Steps 1-3 into the CESM and conduct simulations to evaluate their effects on the simulated polar climate and energy budget. These runs will be compared to control runs and evaluated using routine model-observation diagnostics, including data from the ARM NSA (North Slope of Alaska) site. Moreover, a novel aspect in this step is that we will evaluate the simulated longwave band-by-band fluxes and cloud radiative effect against observational counterparts over both polar continents, using a product recently derived by the lead PI Prof. Huang from collocated AIRS and CERES satellite observations from 2002 to 2013.
Benefits and outcomes: This project will significantly improve the longwave radiation scheme in the CESM and its successors and provide more realistic treatment of radiative interactions between polar surfaces and clouds. In the future, it will also be straightforward to extend this framework to include spectral emissivity of all surface types. This is responsive to identified DoE Earth System Modeling Science Drivers to improve the performance of the fully coupled climate system modeling and to help address questions on how rapid changes in cryospheric systems interact with the climate system.