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Clark J. Weaver
,
Anne R. Douglass
, and
Richard B. Rood

Abstract

Ozone simulations are performed in an attempt to simulate laminar events with the frequency observed in balloon ozone sondes. The winds are taken from the Goddard Earth Observing System Data Assimilation System (GEOS DAS); the importance of horizontal and vertical resolution to production of lamina are investigated. A simulation with a high horizontal reolution (grid spacing 1° latitude by 1.25° longitude) and high vertical resolution (∼300 m grid spacing) isentropic model produces lamination frequencies close to the balloon sonde climatology near the polar vortex edge but exhibits too much lamination in the subtropics. This indicates that the GEOS DAS winds contain the information to produce laminar events, although such small-scale features are not manifest in the more commonly used 2° latitude by 2.5° longitude transport model, which uses the hybrid sigma-pressure vertical coordinate. The zonal average ozone tendencies due to horizontal mixing in the lamina-producing models are similar to the tendencies in coarser resolution models that show no lamination, suggesting that it is not necessary to resolve laminar events to maintain a realistic ozone budget. The comparison of the modeled lamination frequency with the balloon sonde climatology indicates that the model horizontal mixing at the vortex edge is accurate but in the subtropics the mixing is excessive.

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Clark J. Weaver
,
Anne R. Douglass
, and
Richard B. Rood

Abstract

The NASA/Goddard three-dimensional chemistry and transport model is driven by winds from a stratospheric data assimilation system. Synoptic- and planetary-scale patterns, apparent in satellite observations of trace constituents, are successfully reproduced for seasonal integrations. As model integrations proceed, however, the quality of simulations decreases, and systematic differences between calculation and measurement appear. The differences are explained by examining the zonal-mean residual circulation. The vertical residual velocity * is calculated two ways: (i) from the diabatic heating rates and temperature tendency and (ii) from the Eulerian vertical velocity and the horizontal eddy heat flux convergence. The results from these calculations differ substantially. Periodic insertion of observational data during the assimilation process continually shocks the general circulation model and produces these differences, which leads to an overestimate of the mean vertical heat and constituent transport. Such differences are expected to be general to all data assimilation products. This interpretation is corroborated by two-dimensional (2D) model calculations. When * is calculated from (ii), the 2D ozone evolution is unrealistic and qualitatively similar to the 3D model simulation. The 2D ozone evolution is reasonable when * is calculated from (i).

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Clark J. Weaver
,
Paul Ginoux
,
N. Christina Hsu
,
Ming-Dah Chou
, and
Joanna Joiner

Abstract

This study uses information on Saharan aerosol from a dust transport model to calculate radiative forcing values. The transport model is driven by assimilated meteorological fields from the Goddard Earth Observing System Data Assimilation System. The model produces global three-dimensional dust spatial information for four different mineral aerosol sizes. These dust fields are input to an offline radiative transfer calculation to obtain the direct radiative forcing due to the dust fields. These estimates of the shortwave reduction of radiation at the top of the atmosphere (TOA) compare reasonably well with the TOA reductions derived from Earth Radiation Budget Experiment (ERBE) and Total Ozone Mapping Spectrometer (TOMS) satellite data. The longwave radiation also agrees with the observations; however, potential errors in the assimilated temperatures complicate the comparison. Depending on the assumptions used in the calculation and the dust loading, the summertime forcing ranges from 0 to −18 W m−2 over ocean and from 0 to +20 W m−2 over land.

Increments are terms in the assimilation general circulation model (GCM) equations that force the model toward observations. They are differences between the observed analyses and the GCM forecasts. Off west Africa the analysis temperature increments produced by the assimilation system show patterns that are consistent with the dust spatial distribution. It is not believed that radiative heating of dust is influencing the increments. Instead, it is suspected that dust is affecting the Television Infrared Observational Satellite (TIROS) Operational Vertical Sounder (TOVS) satellite temperature retrievals that provide the basis of the assimilated temperatures used by the model.

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