Paul E. Meade

Stratospheric effects of Mount Pinatubo aerosol studied with a coupled two‐dimensional model

A new interactive radiative-dynamical-chemical zonally averaged two-dimensional model has been developed at Goddard Space Flight Center. The model includes a linear planetary wave parameterization featuring wave-mean flow interaction and the direct calculation of eddy mixing from planetary wave dissipation. It utilizes family gas phase chemistry approximations and includes heterogeneous chemistry on the surfaces of both stratospheric sulfate aerosols and polar stratospheric clouds. This model has been used to study the effects of the sulfate aerosol cloud formed by the eruption of Mount Pinatubo in June 1991 on stratospheric temperatures, dynamics, and chemistry. Aerosol extinctions and surface area densities were constrained by satellite observations and were used to compute the aerosol effects on radiative heating rates, photolysis rates, and heterogeneous chemistry. The net predicted perturbations to the column ozone amount were low-latitude depletions of 2-3% and northern and southern high-latitude depletions of 10-12%, in good agreement with observations. In the low latitudes a depletion of roughly 1-2% was due to the altered circulation (increased upwelling) resulting from the perturbation of the heating rates, with the heterogeneous chemistry and photolysis rate perturbations contributing roughly 0.5% each. In the high latitudes the computed ozone column depletions were mainly a result of heterogeneous chemistry occurring on the surfaces of the volcanic aerosol. Temperature anomalies predicted were a low-latitude warming peaking at 2.5 K in mid-1992 and high-latitude coolings of 1-2 K which were associated with the high-latitude ozone reductions. The sensitivity of the predicted perturbations to changes in the specification of the planetary wave forcings was examined. The maximum globally averaged column ozone depletions ranged from 2 to 4% for the cases studied.

Interhemispheric asymmetry in the 1 mbar O3 trend : An analysis using an interactive zonal mean model and UARS data

Trends in O3 calculated from solar backscattered ultraviolet (SBUV) observations near 1 mbar are more negative at high latitudes in the southern hemisphere than in the northern hemisphere [Hood et al., 1993]. A mechanism is presented that produces an interhemispheric O3 trend asymmetry similar to the observed asymmetry in the Goddard Space Flight Center dynamically interactive zonal mean model. Upper Atmosphere Research Satellite (UARS) data are then examined for evidence that the atmospheric trend asymmetry is produced by a similar mechanism. The model O3 trend asymmetry is mainly due to interhemispheric differences in odd chlorine (Cly) partitioning. The asymmetry in Cly partitioning is caused primarily by lower amounts of CH4 and NO in the southern hemisphere than the northern hemisphere high latitudes, due to differences in the dynamical behavior of the model hemispheres. Symmetric increases in Cly are accompanied by a southern hemisphere increase in ClO that is larger than in the northern hemisphere. Concentrations of CH4 and N2O retrieved by the cryogenic limb array etalon spectrometer aboard UARS during 1992 are lower in the southern hemisphere fall and winter seasons at high latitudes in the upper stratosphere than in the northern hemisphere, favoring higher southern hemisphere ClO values. However, observations of ClO by the microwave limb sounder on UARS do not show consistently higher values in the southern hemisphere compared with the northern hemisphere in 1992, 1993, or 1994. The UARS data therefore do not confirm that a mechanism similar to the model mechanism occurs in the real atmosphere and is the cause of the SBUV O3 trend asymmetry.

THERMALLY DRIVEN DIFFUSION OF SO2 WITHIN THE SURFACE OF IO

The presence of sulfur dioxide (SO2) on Io, together with the fact that the surface layer of Io has extremely high porosity, suggests the possibility of diffusion of this volatile within the surface, as well as exchange between the surface and an atmosphere. We investigate the former possibility through the development of a surface layer thermal model and subsequent calculations of the thermally driven diffusion flux of SO2 within the layer. The major factors affecting the diffusion process are the temperature and temperature gradient in the surface layer throughout the day (which are results of the thermal model), and the porosity and grain size in the surface layer. Our results indicate that the net transport of SO2 in the near-surface region is downward into the subsurface, causing near-surface depletion of SO2-Near-surface depletion would result in a layer of reduced thermal inertia overlying the bulk of the surface, consistent with thermal eclipse observations of Io. For our nominal model with 10-μm grains and a porosity of 85%, the peak net diurnal downward flux reaches nearly 8×10−3 g cm−2 period−1.