Model MRI1: Elaborations

Participation

Spinup/Initialization

• The atmospheric model was integrated for 2.5 years with observed AMIP SST/sea ice.

• The ocean model was integrated by the acceleration method of Bryan (1984) for 1000 years from an initial rest state with uniform potential temperature (1.5 deg C) and salinity (34.65 ppt).  The surface momentum, energy, and freshwater (P-E) fluxes obtained from the atmospheric model integration were used as forcing at the ocean's upper boundary.  In addition, the temperatures and salinities of the uppermost ocean layer were relaxed toward the SSTs used in the atmospheric run and towards the seasonally varying Levitus (1982) SSS data.

• In order to obtain more realistic sea ice extents, the integration of the ocean model was extended an additional 500 years using the same atmospheric forcing, but with relaxation of the temperature and salinity of the upper ocean (upper 130 m southward of 70 N and upper 30 m northward of 70 N) to modified climatological Levitus (1982) SST and SSS data.  The modification involved resetting the ocean mixed-layer temperature in the regions of sea ice  to the freezing temperature calculated from salinity.

• The atmosphere and ocean models then were coupled and integrated for 30 years with sea ice initial conditions obtained from the 1973-1990 Joint Ice Center sea-ice concentration data.  (The initial sea ice thicknesses were estimated as linear functions of the concentration, with maximum of 3 m in the Northern Hemisphere and 0.5 m in the Southern Hemisphere.)  During this coupled integration, the temperature and salinity of the uppermost ocean layer were relaxed to the modified Levitus (1982) climatological SST and SSS data described above.  The monthly means of the relaxation terms over the last 10 years of the coupled integration were stored as heat and freshwater flux adjustment terms for subsequent coupled integration.

• The coupled model was integrated for an additional 70 years with these flux adjustments which mostly kept the annual mean SST close to Levitus (1982) values.  The greatest deficiency was a weak thermohaline circulation in the Northern Hemisphere ocean, related to the different ocean depths used for the relaxations above.  (The positive heat flux adjustments required to maintain the observed SST in the North Atlantic may excessively stabilize the very thin ocean surface layer in the final stage of ocean spinup, resulting in weak overturning.)

Land Surface Processes

• Land surface parameterizations follow Katayama (1978), with subsequent modifications in the treatment of soil moisture described by Kitoh et al. (1988). Soil heat and moisture move along the gradients of temperature and wetness, respectively. The thermodynamic and hydrologic effects of a vegetation canopy are not explicitly modeled, however.

• The temperature of bare and snow-covered land is predicted in four layers, and that of glacial ice in a single-layer. The upper boundary condition is the net balance of surface energy fluxes, while there is zero heat transfer at the lower boundary. Heat exchange associated with the freezing or melting of soil moisture and interstitial ice is taken into account. Snow cover and soil moisture/ice also affect the heat capacity/conductivity of the surface, but the heat capacity of snow is assumed to be independent of its depth.

• Soil moisture (in both liquid and frozen form) is predicted in four layers (with bottom boundaries at 0.10, 0.50, 1.50, and 10.0 m below the surface); it is augmented by precipitation and snow/ice melt, and is depleted by surface evaporation and runoff. The evapotranspiration efficiency beta is set to unity if the fractional soil moisture (the ratio of soil moisture to the field capacity, assumed to be 20 percent of the volume of a soil column) is at least 0.5; beta is set to twice the fractional soil moisture otherwise. When all of the moisture in a soil layer is completely frozen (freezing begins when the layer temperature falls below 0 degrees C), no deeper penetration of moisture is allowed. Runoff occurs in any layer that is either saturated or completely frozen.

• Runoff from the land is distributed to the ocean by means of a simple river routing model. Surface runoff is assumed to instantaneously reach the outlet of each river basin, which are determined at 4x5-degree resolution from the drainage basin map of Walter (1957).

Sea Ice

• Prognostic variables include ice thickness, concentration, velocity, and internal energy (cf. Mellor and Kantha 1989 ).  The salt content of the ice is concentrated in brine pockets, with the average salinity prescribed as a vertical constant.

• The treatment of thermodynamics is similar to that of Semtner (1976), but with inclusion of an ice concentration parameter A in each grid box.  Surface energy fluxes are calculated separately for ice and leads, weighted according to their respective fractional areas. Snow is represented by a top layer and ice by a bottom layer. Snow acts as a heat insulator and an absorber of incoming radiation during melting; its sensible heat capacity is neglected. Temperatures are predicted at the snow surface, the snow-ice interface, the interior ice interface, and the ice bottom.  Melting and freezing rates are calculated at the ice-atmosphere, ice-ocean, and atmosphere-ocean interfaces.

• The ice is advected via reduced (by a constant fraction) surface ocean currents.

References

Kitoh, A., K. Yamazaki, and T. Tokioka, 1988: Influence of soil moisture and surface albedo changes over the African tropical rain forest on summer climate investigated with the MRI GCM-I. J. Meteor. Soc. Japan, 66, 65-86.

Levitus, S., 1982: Climatological atlas of the world's oceans. NOAA Professional Paper 13, 173 pp.

Mellor, G.L. and L. Kantha, 1989: An ice-ocean coupled model. J. Geophys. Res., 94, 10,937-10,954.

Semtner, A.J., 1976: A model for the thermodynamic growth of sea ice in numerical investigations of climate.  J. Phys. Oceanogr., 6, 379-389.

Tokioka, T., A. Noda, A. Kitoh, Y. Nikaidou, S. Nakagawa, T. Motoi, S. Yukimoto, and K. Takata, 1996: A transient CO₂ experiment with the MRI CGCM: Annual Mean Response.  CGER Supercomputer Monograph Report Vol. 2, CGER-I022-'96, Center for Global Environmental Research, National Institute for Environmental Studies, Environment Agency of Japan, ISSN 1341-4356, 86 pp.

Walter, Y. (ed.), 1957: Encyclopedia Britanica World Atlas. Encyclopedia Britanica, Inc.
 

Top

CMIP Documentation Directory

Last update 15 May, 2002.  This page is maintained by Tom Phillips (phillips@pcmdi.llnl.gov).
 

LLNL Disclaimers

UCRL-MI-126431