Model description

Five different global models participated in the comparison of monthly averaged aerosol optical depth:

- ECHAM4, which originated at the Max-Planck-Institute for Meteorology [Hamburg, GER], is a global circulation model (Lohmann et al., 1999). Wind-, temperature- and pressure-fields are generated without a link to a particular year (3 year simulation).

MIRAGE, from PNNL [Richland, WA, USA], is a chemical transport model which is coupled on line with a global circulation model (R.Easter et al., 1999). To improve agreement on time-scales of days nudging can be (and in this case has been) applied. (Nudging towards analyzed winds, temperature and surface pressure is an elementary form of data assimilation, in order to reduce biases in simulated circulation and simulated winds). Results are based on ECMWF assimilated wind-, temperature-and pressure fields from June 1994 to May 1995. GOCART, from Georgia Institute of Technology and NASA-Goddard [Atlanta, GA / Greenbelt, MD, USA] is a chemical transport model (Chin et al. 2000) driven by assimilated meteorological fields from the GEOS DAS (Goddard Earth Observing System Data Assimilation System). Assimilated wind-, temperature- and pressure fields are based on 1990 data. In addition, simulations of 1996 data are presented, primarily to illustrate year-to-year variability from a modeling perspective.

- CCSR, from the Center for Climate System Research [Tokyo, JP], is a chemical transport model (Takemura et al., 2000) driven by NCEP / NCAR data (National Center for Environmental Predictions / National Center for Atmospheric Research). Wind-, temperature- and pressure fields are based on 1990 data.

- GISS, from NASA-GISS [New York, NY, USA] is a global circulation model (Tegen et al., 1997, Tegen et al., 2000; Koch et al. 1999). Wind-, temperature- and pressure-fields are generated without a link to a particular year (3 year simulation).

All models

- distinguish among five different aerosol components: (1) sulfate, (2) organic carbon, (3) black carbon, (4) soil dust and (5) sea-salt differentiate in aerosol emissions by type include sulfate chemistry (gas to particle conversions)

- simulate transport processes of advection, diffusion and convection and include aerosol processing.

The included aerosol processes are dry deposition (aerosol clustering from turbulent mixing) in-cloud scavenging (aerosol acting as cloud condensation nucleus or aerosol which diffuse in cloud drops with subsequent removal via precipitation)

below cloud scavenging (aerosol capture and removal by rain) aerosol re-emission by evaporation (of rain or cloud drops) and

- gravitational settling.

While a detailed model comparison goes beyond the scope of this paper a comparison of model resolution and references for aerosol emission by type are given in Table 6.

Table 6 Comparison of model resolution and (major) reference for emission source for aerosol components_

Model

ECHAM4

MIRAGE

GOCART

CCSR

GISS

M PI,Hamburg

PNNI., Rich).

NASA-godd.

NIES, Tokyo

NASA-giss

Hon.grid (lon/lat)

3.8*3.8 deg

2.8*2.8 deg

2.5* 2.0 deg

5.6*5.6 deg

4.0*5 .Odeg

Vertical layers

28

23

20

11

9

Simulation-period

3 years

6/94 - 5/95

1/90- 12/90

1/90- 12/90

3 years

Meteorology

generated

ECMWE*

GEOS/DAS

NCAR/NCEP

generated

Clouds

prognostic

prognostic

diagnostic

diagnostic

prognostic

Humidification

Koepke 97

Koehlcr-th.

Kiehl 00

Tang94,Ho97

Emis: dust

Ginoux 00

Gilcttc 88

Ginoux 00

Gil cue 78

Tc gen/Fung95

Emis: org. carbon

Liousse 96

Liousse 96

Cooke 96,99

L96, Guent.95

Liousse 96

Emis: black carb.

Cooke 96

Cooke 96

Cooke 96,99

Cooke 96

Cooke 96

Emis: sea salt

Monohan86**

O'Dowd 97

MonahanSú**

Erickson 86

Monahan86**

Emis: suiïur

Benkowitz 96

Benkowitz 96

Olivier 97

Benkowitz 96

Benkowitz 96

Sulfur - oxidants

imported

CO-CH4 ch..

imported

imported

imported

* MIRAGE: uses E-winds, calculates water-uptake from E- rel.humidity, applies nudging towards E-temp.

** GOCART uses SSM/I based winds, ECHAM4 and GISS are based on CanadianGCM winds (Gong 97)

* MIRAGE: uses E-winds, calculates water-uptake from E- rel.humidity, applies nudging towards E-temp.

** GOCART uses SSM/I based winds, ECHAM4 and GISS are based on CanadianGCM winds (Gong 97)

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