Global Climate Modeling

Climate modeling is a primary research activity of many CCSR scientists. Our climate modeling effort is unique among the three major US climate models: we examine not only Earth's past and future climates but also climates on other Earth-like exoplanets.

Along with GFDL (funded mainly by NOAA) and NCAR (NSF & DOE), CCSR science has been an important contributor to all of the IPCC Assessment Reports. GISS models have been part of almost all the Model Intercomparison Projects (MIPs). Outside of the formal intercomparisons, CCSR/GISS science has been at the forefront of many steps forward in understanding the climate system through framing the issue of radiative forcing, the exploration of new feedbacks and interactions, quantification of the drivers of climate change, out-of-sample evaluation of climate models and innovative comparisons to observations.

Convection and cloud research emphasizes the use of observations to further understanding of fundamental physical processes and their relationship to climate and the general circulation, and development and testing of improved parameterizations of those processes for use in the GISS global climate model. Data analysis makes use of satellite observations from the NASA TRMM, CloudSat/CALIPSO, and ISCCP programs, surface-based observations from the DOE ARM sites, and reanalyses. Modeling includes development using a Single Column Model to simulate real-world case studies and the 3-D GCM to simulate convection and cloud effects on climate and climate change.

The GISS Earth System Models are well-documented over a development timeframe, which spans more than three decades (original model paper, Hansen et al., 1983). The current physics and dynamics of the GISS ModelE2 are predominantly based on those of GISS ModelE (Schmidt et al., 2006) and GISS Model II’ before it (Hansen et al. 2002, and references therein). Principal prognostic variables in the atmosphere are potential temperature, the water vapor mixing ratio, as well as the horizontal wind components. Virtual potential temperature is used for all density/buoyancy-related calculations.

The model has a Cartesian gridpoint formulation for with a standard horizontal resolution of 2° X 2.5° latitude by longitude (although 4° X 5° and 8° X 10° resolutions are also available for historical comparison to previous model experiments and for educational uses). The effective nominal resolution for tracer transports is significantly greater, however, because nine higher-order moments are carried along with the mean tracer values in each grid cell. Velocity points in the atmosphere are on the Arakawa-B grid and sigma coordinates are used in the vertical up to 150 hPa, with constant pressure layers above. The standard vertical resolution has 40 layers and a model top at 0.1 hPa.

Four surface types are defined including, open water (lakes and oceans), ice-covered water, ground (bare soil and vegetation), and glaciers. The model uses a 30 minute time step for all physics calculations and the radiation code is called every five physics time steps. The radiation code accounts for the radiatively important trace gases, including CO2, CH4, N2O, CFCs, and ozone, all of which are prescribed, but which can be updated as the model runs to incorporate trends. The model also includes treatments for both tropospheric and stratospheric aerosols and can be run with a climate-influenced gravity wave drag scheme that separately calculates the effects of gravity waves arising from mountain drag, penetrating convection, shear, and deformation.

Land process research at CCSR covers how land surface hydrology and terrestrial ecosystems couple with the atmosphere through the exchange of energy, water vapor, carbon, and other elements. Current research includes the on-going development of a dynamic global vegetation model to couple with the GISS GCM ModelE, studies of the impacts of human activity like irrigation and land cover change on climate, and feedbacks between vegetation and climate.

Ocean Physics

At GISS we help develop, run and analyze standalone ocean model runs as well as coupled climate model runs to assess the impact and the feedbacks of ocean state and circulation to climate and climate change. Two different ocean models are being utilized, each of which represents a distinct class of ocean model discretizations and parameterizations, the Hybrid Coordinate Ocean Model and the Russell ocean model. Model simulations are evaluated against a wide range of observational datasets in order to reduce the model biases in the estimate of both the natural as well as the anthropogenic climate variability and its dependence on changes of the ocean thermohaline circulation and the ocean heat content. A suite of idealized and realistic tracers, such as ideal ocean age, ventilation, water mass tracers as well as CFC and carbon isotopes, have been in included in both ocean models to help elucidate model deficiencies and highlight processes that play a role in deep ocean mixing and eddy transport.

Ocean Carbon Cycle

At GISS we study the interactions of global biogeochemical cycles and the aquatic ecosystems, assess global environmental change and describe the implications for Earth’s climate, productivity, and natural resources. A research objective is to quantify global productivity, biomass, carbon fluxes and provide information about future changes in global carbon cycling in the aquatic ecosystems for use in ecological forecasting and as inputs for improved climate change projections. We specifically focus on biogeochemical modeling of the oceanic component of the carbon cycle and use satellite data both for model assessment and improvement. Carbon fluxes are prognostic and interactive with the rest of the coupled climate system, thereby enabling us to study climate interactions and feedbacks and the role of the ocean in natural as well anthropogenic climate change. The NASA Ocean Biogeochemical model (NBOM, Gregg et al, 2007; Romanou et al, 2013), which was developed at GSFC and is coupled with the GISS climate model, simulates the ocean carbon cycle using phytoplankton groups differentiation. NBOM has been recently retrofitted with capabilities such as alkalinity and nutrient and carbon riverine input, which will help us model ocean acidification and its impact on climate and ocean biodiversity and land-ocean exchanges.

Paleoclimate modeling at GISS is aimed at better understanding how the Earth’s climate system operates under conditions that are extreme compared to the environmental range of the 20th century. Spanning time periods that include the early evolution of the Earth’s atmosphere, oceans, biosphere and cryosphere to just before the development of modern meteorological instruments, this research both constrains and expands NASA’s modeling program. Paleoclimate studies allow us to examine the full range of variability throughout geologic time, the potential magnitude of climate change in Earth’s near future, and even the prospective habitability of extrasolar earth-like planets. By it’s nature, paleoclimate research brings together both data and modeling techniques to provide the broadest possible view of Earth’s climate history and it especially helps delineate the processes the operate when climates are either much warmer or colder than at present. In turn, advances in our ability to simulate high-magnitude, global-scale climate changes that actually occurred in the past strengthens our confidence in the conclusions drawn from simulations of future climate.

Regional climate models simulate atmospheric processes over selected geographical regions. The limited area domains allow a higher density of computational points, which improves the representation of many climate and weather features. The RM3 is a regional climate model developed and run at CCSR/GISS.

The radiation model has explicit multiple scattering calculations for solar radiation [shortwave (SW)] and explicit integrations over both the SW and thermal [longwave (LW)] spectral regions. Gaseous absorbers of SW radiation are H2O, CO2, O3, O2, and NO2. Size-dependent scattering properties of clouds and aerosols are computed from Mie scattering, ray tracing, and T- matrix theory (Mishchenko et al. 1996) to include non-spherical cirrus and dust particles. The k-distribution approach (Lacis and Oinas 1991) utilizes 15 noncontiguous spectral intervals to model overlapping cloud–aerosol and gaseous absorption. The surface albedo utilizes six spectral intervals and is solar zenith angle dependent for ocean, snow, and ice surfaces. The spectral albedo of vegetation is seasonally dependent. The radiation model generates spectrally dependent direct/ diffuse flux ratios for use in biosphere feedback interactions.

Longwave calculations for H2O, CO2, and O3 use the correlated k distribution with 33 intervals (Lacis and Oinas 1991; Oinas et al. 2001), designed to match line-by-line computed fluxes and cooling rates throughout the atmosphere to within about 1%. Weaker bands of H2O, CO2, and O3, as well as absorption by CH4, N2O, CFC-11, and CFC-12 are included in an approximate fashion as overlapping absorbers, but with coefficients tuned to reproduce line-by-line radiative forcing over a broad range of absorber amounts. The vertical profiles and latitudinal gradients of CH4, N2O, and CFCs are from Minschwaner et al. (1998). Longwave forcing by aerosols is also included (Tegen et al. 2000).