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Giovanni Pitari

Abstract

A spectral three-dimensional model of the stratosphere is used to investigate the possible dynamical effects in the stratosphere of the aerosol cloud formed after the eruption of Mt. Pinatubo. A non-negligible perturbation of the diabatic circulation is found, with a substantial upwelling in the tropical latitudes generated by the aerosol-induced diabatic heating. This effect has been recently shown to have also occurred in coincidence with the eruption of El Chichón. In this work a substantial anomaly in the planetary wave activity is also found, with some remarkable similarities to what is observed for the easterly phase of the quasi-biennal oscillation. A parameterization of horizontal eddy mixing for use in photochemical two-dimensional models is attempted: the modeled Kyy anomaly forced by the aerosols during the Northern Hemisphere winter is such that the northward ozone transport toward midhigh latitudes is enhanced. It is shown that this effect coupled with the additional subsidence related to the diabatic circulation anomaly produces a total ozone increase of about 5% at the northern midhigh latitudes during the winter months. This purely dynamical perturbation should be taken into account in ozone assessments following the eruption, along with heterogeneous chemical depletion and changes in the photodissociation.

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Giovanni Pitari
and
Guido Visconti

Abstract

A simple scheme is proposed to account for the Rayleigh scattering effects on photodissociation rates. The method is based on combining the reflectivity and transmission effects of two layers—one above and one below the height considered. Reflectivities are computed using existing data and tables on Rayleigh scattering. Computations using this method are reported and compared with other results obtained with a very detailed radiative scheme. The accuracy of the results and the short computing time make the method suitable for chemical-dynamical models of the atmosphere.

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Guido Visconti
,
Marco Verdecchia
, and
Giovanni Pitari

Abstract

A two-dimensional model has been integrated for two years to study the evolution of the El Chichón aerosol cloud in the stratosphere, starting about three months after the eruption. Initial conditions for the backscattering ratios are taken from airborne lidar measurements, while observations taken at Mauna Loa are used to estimate the initial size distribution for the aerosols. Aerosols have been treated as a passive tracer, because the small changes in the stratospheric dynamics due to the aerosol interaction with solar and longwave radiation can only induce marginal effects on large scale transport. We have also completely neglected microphysical processes like coagulation as well as photochemical effects on chemically reacting species. Results are shown for aerosol extinction mixing ratios, optical thickness, mass column density and size distribution. Extensive comparison with available lidar measurements are also presented and discussed. Major discrepancies are noted between the measured optical thickness and the one predicted by the model with the former being systematically higher. Neglect of coagulation and nucleation prevents formation of large particles at high altitude which are depleted in the simulation by sedimentation. The interhemispheric asymmetry is overestimated by the model with much more aerosol being transported in the Northern Hemisphere than in the Southern Hemisphere. Other differences are found in the sudden changes in the aerosol distribution. It is argued that two dimensional models are not suitable to simulate sporadic events and that microphysics should be taken into account even several months after the eruption.

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Joyce E. Penner
,
Sophia Y. Zhang
,
Mian Chin
,
Catherine C. Chuang
,
Johann Feichter
,
Yan Feng
,
Igor V. Geogdzhayev
,
Paul Ginoux
,
Michael Herzog
,
Akiko Higurashi
,
Dorothy Koch
,
Christine Land
,
Ulrike Lohmann
,
Michael Mishchenko
,
Teruyuki Nakajima
,
Giovanni Pitari
,
Brian Soden
,
Ina Tegen
, and
Lawrence Stowe

Abstract

The determination of an accurate quantitative understanding of the role of tropospheric aerosols in the earth's radiation budget is extremely important because forcing by anthropogenic aerosols presently represents one of the most uncertain aspects of climate models. Here the authors present a systematic comparison of three different analyses of satellite-retrieved aerosol optical depth based on the Advanced Very High Resolution Radiometer (AVHRR)-measured radiances with optical depths derived from six different models. Also compared are the model-derived clear-sky reflected shortwave radiation with satellite-measured reflectivities derived from the Earth Radiation Budget Experiment (ERBE) satellite.

The three different satellite-derived optical depths differ by between −0.10 and 0.07 optical depth units in comparison to the average of the three analyses depending on latitude and month, but the general features of the retrievals are similar. The models differ by between −0.09 and +0.16 optical depth units from the average of the models. Differences between the average of the models and the average of the satellite analyses range over −0.11 to +0.05 optical depth units. These differences are significant since the annual average clear-sky radiative forcing associated with the difference between the average of the models and the average of the satellite analyses ranges between −3.9 and 0.7 W m−2 depending on latitude and is −1.7 W m−2 on a global average annual basis. Variations in the source strengths of dimethylsulfide-derived aerosols and sea salt aerosols can explain differences between the models, and between the models and satellite retrievals of up to 0.2 optical depth units.

The comparison of model-generated reflected shortwave radiation and ERBE-measured shortwave radiation is similar in character as a function of latitude to the analysis of modeled and satellite-retrieved optical depths, but the differences between the modeled clear-sky reflected flux and the ERBE clear-sky reflected flux is generally larger than that inferred from the difference between the models and the AVHRR optical depths, especially at high latitudes. The difference between the mean of the models and the ERBE-analyzed clear-sky flux is 1.6 W m−2.

The overall comparison indicates that the model-generated aerosol optical depth is systematically lower than that inferred from measurements between the latitudes of 10° and 30°S. It is not likely that the shortfall is due to small values of the sea salt optical depth because increases in this component would create modeled optical depths that are larger than those from satellites in the region north of 30°N and near 50°S. Instead, the source strengths for DMS and biomass aerosols in the models may be too low. Firm conclusions, however, will require better retrieval procedures for the satellites, including better cloud screening procedures, further improvement of the model's treatment of aerosol transport and removal, and a better determination of aerosol source strengths.

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