Search Results

You are looking at 1 - 3 of 3 items for

  • Author or Editor: Charles T. Gordon x
  • Refine by Access: All Content x
Clear All Modify Search
Charles T. Gordon
and
William P. Stern

Abstract

A preliminary evaluation is made of the medium range predictive capability of a GFDL global spectral model of the atmosphere, based upon three winter blocking cases. Analogous forecasts by a GFDL global grid point model provide a background standard of comparison. The spectral model is rhomboidally truncated at wavenumber 30, has 9 sigma levels, incorporates sub-grid scale physical processes commonly associated with general circulation models and employs semi-implicit time differencing. The grid point model has somewhat finer horizontal resolution and fairly similar sub-grid scale physical processes, and employs explicit time differencing. The spectral model is up to 6 times more economical.

The level of forecast skill for the 5 to 15 day range is generally less than practically useful and is more case-dependent than spectral versus grid point model-dependent. In the most successful case, i.e., 16 January 1979, an observed Atlantic blocking ridge is simulated quite well, especially by the spectral model. The predicted Atlantic ridges tend to retard approaching upstream transient disturbances. A zonal bias of the midlatitude circulation, which develops in all three spectral and grid point model predictions is most pronounced in the spectral model forecast from 1 January 1977.

Results of a diffusion sensitivity experiment and other evidence suggest that insufficient frictional dissipation may have enhanced the zonal bias of the above forecast. The bias diminishes, consistent with a redistribution of spectral kinetic energy among zonal wavenumbers 0, 1 and 2, if a static stability-dependent pararmeterization of vertical mixing or stronger ∇4 horizontal diffusion are used. Also, the predicted-enstrophy spectrum at midlatitudes steepens, given the stronger ∇4 horizontal diffusion.

Full access
Charles T. Gordon
and
William F. Stern

Abstract

A multi-level, global, spectral transform model of the atmosphere, based upon spherical harmonies, has been developed at GFDL. The basic model has nine sigma levels in the vertical and rhomboidal spectral truncation at wavenumber 30. However, finer spectral or vertical resolution versions are available as well. The model's efficient semi-implicit time differencing scheme does not appear to adversely affect medium range predictions. The model has physical processes commonly associated with grid point GCM'S. Two unique features are a linearized virtual temperature correction and an optional, spectrally-computed non-linear horizontal diffusion scheme. A parameterization of vertical mixing based upon the turbulent closure method is also optional.

The GFDL spectral model has been widely utilized at GFDL for extended range weather prediction experiments. In addition, it has been adapted and applied to climate studies, four-dimensional data assimilation experiments and even to the atmosphere of Venus. These applications are briefly reviewed.

Full access
Leo J. Donner
,
Bruce L. Wyman
,
Richard S. Hemler
,
Larry W. Horowitz
,
Yi Ming
,
Ming Zhao
,
Jean-Christophe Golaz
,
Paul Ginoux
,
S.-J. Lin
,
M. Daniel Schwarzkopf
,
John Austin
,
Ghassan Alaka
,
William F. Cooke
,
Thomas L. Delworth
,
Stuart M. Freidenreich
,
C. T. Gordon
,
Stephen M. Griffies
,
Isaac M. Held
,
William J. Hurlin
,
Stephen A. Klein
,
Thomas R. Knutson
,
Amy R. Langenhorst
,
Hyun-Chul Lee
,
Yanluan Lin
,
Brian I. Magi
,
Sergey L. Malyshev
,
P. C. D. Milly
,
Vaishali Naik
,
Mary J. Nath
,
Robert Pincus
,
Jeffrey J. Ploshay
,
V. Ramaswamy
,
Charles J. Seman
,
Elena Shevliakova
,
Joseph J. Sirutis
,
William F. Stern
,
Ronald J. Stouffer
,
R. John Wilson
,
Michael Winton
,
Andrew T. Wittenberg
, and
Fanrong Zeng

Abstract

The Geophysical Fluid Dynamics Laboratory (GFDL) has developed a coupled general circulation model (CM3) for the atmosphere, oceans, land, and sea ice. The goal of CM3 is to address emerging issues in climate change, including aerosol–cloud interactions, chemistry–climate interactions, and coupling between the troposphere and stratosphere. The model is also designed to serve as the physical system component of earth system models and models for decadal prediction in the near-term future—for example, through improved simulations in tropical land precipitation relative to earlier-generation GFDL models. This paper describes the dynamical core, physical parameterizations, and basic simulation characteristics of the atmospheric component (AM3) of this model. Relative to GFDL AM2, AM3 includes new treatments of deep and shallow cumulus convection, cloud droplet activation by aerosols, subgrid variability of stratiform vertical velocities for droplet activation, and atmospheric chemistry driven by emissions with advective, convective, and turbulent transport. AM3 employs a cubed-sphere implementation of a finite-volume dynamical core and is coupled to LM3, a new land model with ecosystem dynamics and hydrology. Its horizontal resolution is approximately 200 km, and its vertical resolution ranges approximately from 70 m near the earth’s surface to 1 to 1.5 km near the tropopause and 3 to 4 km in much of the stratosphere. Most basic circulation features in AM3 are simulated as realistically, or more so, as in AM2. In particular, dry biases have been reduced over South America. In coupled mode, the simulation of Arctic sea ice concentration has improved. AM3 aerosol optical depths, scattering properties, and surface clear-sky downward shortwave radiation are more realistic than in AM2. The simulation of marine stratocumulus decks remains problematic, as in AM2. The most intense 0.2% of precipitation rates occur less frequently in AM3 than observed. The last two decades of the twentieth century warm in CM3 by 0.32°C relative to 1881–1920. The Climate Research Unit (CRU) and Goddard Institute for Space Studies analyses of observations show warming of 0.56° and 0.52°C, respectively, over this period. CM3 includes anthropogenic cooling by aerosol–cloud interactions, and its warming by the late twentieth century is somewhat less realistic than in CM2.1, which warmed 0.66°C but did not include aerosol–cloud interactions. The improved simulation of the direct aerosol effect (apparent in surface clear-sky downward radiation) in CM3 evidently acts in concert with its simulation of cloud–aerosol interactions to limit greenhouse gas warming.

Full access