AMIP I/AMIP II Model Differences: Model JMA GSM9603 (T63 L30) 1998

AMIP II Model Designation

Most Similar AMIP I Model

AMIP I/AMIP II Model Differences

AMIP II Model Designation

Most Similar AMIP I Model

AMIP I/AMIP II Model Differences

• Model Lineage
• Model Documentation
• Horizontal Resolution
• Vertical Domain
• Vertical Resolution
• Smoothing/Filling
• Chemistry
• Radiation
• Convection
• Cloud Formation
• Precipitation

The JMA Global Spectral Model used in AMIP II first became operational in March 1996 (hence its designation as GSM9603).  It is the latest version of a line of global spectral models first described by Kanamitsu et al. (1983) [9] that also includes the AMIP I model GSM 8911.

Model Documentation

Key documentation of the AMIP II model characteristics is provided by the JMA Numerical Prediction Division's 1997 Outline of the Operational Numerical Weather Prediction at the Japan Meteorological Agency [10], a different reference than for the AMIP I model.

Horizontal Resolution

The AMIP II model's horizontal resolution is spectral triangular 63 (T63), roughly equivalent to 1.875 x 1.875 degrees latitude-longitude, a finer resolution than the AMIP I model.

Vertical Domain

The AMIP II model domain is from the surface to 1 hPa, a higher top than for the AMIP I model.

Vertical Resolution

Smoothing/Filling

Chemistry

Radiation

• In the near-infrared, scattering/absorption are modeled by a two-stream formulation using the delta-Eddington approximation (Joseph et al. 1976 [21] and Coakley et al. 1983 [22]) which is applied in the 10 spectral intervals, as described by Briegleb (1992) [23]. These include 7 intervals between 0.70 and 5.00 microns to capture oxygen A-band and water vapor absorptions, and 3 intervals between 2.63 and 4.55 microns to capture carbon dioxide absorption.

• Shortwave scattering/absorption due to clouds are treated by the delta-Eddington approximation. Cloud optical properties (optical depth, single-scattering albedo and asymmetry factor) are linked to diagnostic cloud water and effective droplet radius (specified differently for liquid droplets (Slingo 1989 [28]) and for ice particles (Ebert and Curry1992 [29])). Clouds are treated as graybodies in the longwave, and cloud absorption is modeled by an emissivity formulation from cloud water path. For the radiation calculations, vertical overlap of clouds is assumed to be mixed (changing from full to random overlap with increasing vertical distance). See also Cloud Formation.

Convection

• The vertical profile of upward mass flux is assumed to be a linear function of height, as proposed by Moorthi and Suarez (1992) [31]. The mass flux at the cloud base (fixed near 950 hPa in the model) is determined by the prognostic equation of Randall and Pan (1993) [32].  The major contributions of the cumulus clouds are made through compensating downward motion, detrainment of mass and reevaporation of the convective rainfall. See also Precipitation.

• The effects of an ensemble of multiple-type cumuli are considered. Each type is defined by the level of the cloud top, where the updraft cloud mass loses its buoyancy and detrainment occurs. During the upward movement of the cloud air mass, entrainment of the environment air mass is considered. The lower limit of the entrainment rate is proposed by Tokioka et al. (1988) [33].

• The convective downdraft associated with the cumulus clouds affects the environment by decreasing the net upward mass flux and detrainment of downdraft in the subcloud layer. The downdraft mass flux at the cloud base is specified so that the reevaporation of rainwater moistens the downward flux below the cloud base to its saturation. In the extratropics, where moist convection does not always initiate at the surface, the mass flux is determined so that the large-scale moisture increase is spent by the convection. The effect of the cumulus convection on the large-scale tendency is calculated from a budget equation.

Cloud Formation

Precipitation

• The treatment of convective precipitation and reevaporation of precipitation is different from in the AMIP I model.  In the AMIP II model, all the condensed water in the convective updraft is assumed to be carried to the cloud top. Here part of the condensed water is converted from cloud droplets to raindrops, while the remainder is evaporated, moistening the environment through the detrainment process. See also Convection.

• Subsequent evaporation of falling large-scale and convective precipitation is simulated following the formula proposed by Ogura and Takahashi (1971) [36] in the AMIP II model. The effect of the wind shear, which is responsible for the vertical tilting of the cumulus, is taken into account for reevaporation process using the Fritsch and Chappel (1980) [37] relation between the shear and the precipitation efficiency.  The AMIP I model does not simulate reevaporation of precipitation.

References

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