AMIP I/AMIP II Model Differences: Model MRI MRI/JMA98
(T42 L30) 1998
AMIP II Model Designation
Most Similar AMIP I Model
AMIP I/AMIP II Model Differences
AMIP II Model
Designation
MRI MRI/JMA98 (T42 L30) 1998
Most Similar AMIP
I Model
JMA GSM8911
(T42 L21) 1993
AMIP I/AMIP II Model Differences
Model Lineage
The most similar AMIP I model to MRI MRI/JMA98
(T42 L30) 1998 is JMA
GSM8911(T42 L21) 1993 , and these inter-model differences are described
here.
(See also the AMIP II model JMA GSM9603
(T63 L30) 1998 which shares a common antecedent with MRI
MRI/JMA98 (T42 L30) 1998, but differs in several
respects .)
Model Documentation
Key documentation of the AMIP II model characteristics is provided by
Shibata
et al. (1999), a different reference than for the AMIP
I model.
Vertical Domain
The AMIP II model domain is from the surface to 0.4 hPa, a higher top
than for the AMIP
I model.
Vertical Resolution
For the AMIP II model, there are 30 unevenly spaced hybrid levels, a substantial
increase of the vertical resolution of the AMIP
I model. For a surface pressure of 1000 hPa, 6 levels are below 800
hPa and 16 levels are above 200 hPa.
Gravity Wave Drag
In addition to the orographic gravity wave drag parameterization used in
the AMIP
I model, Rayleigh friction, which follows a hyperbolic tangent vertical
profile, also contributes to this drag.
Chemistry
The recommended monthly climatological zonal mean ozone profiles of Wang
et al. (1995) [1] are linearly interpolated
to obtain intermediate daily values in the AMIP II model, while the AMIP
I model's ozone distributions are specified from another data set.
Radiation
The radiation scheme is different from that of the AMIP
I model:
-
A delta-two-stream approximation is used to represent short wave radiation.
Spectral intervals, the data for k-distribution and absorption cross section
and optical parameters for clouds are taken from Briegleb
(1992). The absorption gases and bands are ozone (O3) ultraviolet
and visible region (8 intervals in the 0.2-0.7 micron band), water vapor
near-infrared region (7 intervals in the 0.5-5.0 micron band), carbon dioxide
(CO2, 2.7 and 4.3 micron) and O2 (A and B bands).
Delta-two-stream calculation for transmission and reflection are implemented
by the discrete ordinate method (Shibata
and Uchiyama, 1992).
-
The multi-parameter random model (Shibata
and Aoki, 1989) is used to represent long-wave radiation. Four spectral
intervals (20-550, 550-800, 800-1200 and 1200-2200 cm-1) and
five gases are treated. Water vapor is treated in all the intervals with
continuum absorption, while carbon dioxide (CO2,15 micron) and
ozone (O3, 9.6 micron) absorption are included in the second
and third spectral intervals, respectively. In addition nitrous oxide (N2O,
7.8 micron) and methane (CH4,7.6 micron) absorption are incorporated
in the fourth spectral interval. Full- and half-level temperatures are
calculated from the prognostic layer-mean temperature profile (cf. Shibata
and Uchiyama, 1994) and a two-grid noise suppression scheme is included
in the integration of transmission function (Shibata,
1989). In the longwave, cloud is treated as a gray body, with optical
depth proportional to the cloud water path.
Convection
The AMIP II model uses an economical version of the Arakawa-Schubert (1974)[30]
scheme to simulate penetrative (deep) convection, in place of the Kuo parameterization
in the AMIP
I model.
-
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
In the AMIP II model, cloud water content (not included in the AMIP
I model) is parameterized as a function of temperature after
Heymsfield (1977) [35]. All clouds form
completely as liquid clouds if the temperature is higher than 273.15 K,
and completely as ice clouds if the temperature is lower than 233.15 K.
In between, liquid/ice phase ratio of cloud water is linearly interpolated
by temperature. The effective radius of cloud liquid droplet is fixed at
15 microns, while the radius of an ice particle varies in the range 20-50
microns, according to the temperature.
Precipitation
In the AMIP II model, convective precipitation and the reevaporation of
precipitation are treated differently than in the AMIP
I 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]. 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.
Land Surface Processes
In the AMIP II model, the land-surface scheme of the AMIP
I model is modified to predict temperatures in 3 (instead of 4) soil
layers and to allow both diffusion of frozen soil water and phase change
of soil water/ice.
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Last update August 10, 1999. For questions or comments, contact
Tom Phillips (phillips@pcmdi.llnl.gov).
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