Earth Observation Data Group

Abstract:

Seasonal integrations of a three-dimensional fully coupled model of the radiation, dynamics and chemistry of the stratosphere and mesosphere are presented for the southern hemisphere spring. Included in the model are heterogeneous reactions which take place in sulphuric acid aerosol droplets as well as on the surface of Polar Stratospheric Clouds (PSCs). Calculations are performed for background levels of stratospheric aerosol and for conditions following the eruption of Mt. Pinatubo. For the volcanic case, surface area densities are derived from Improved Stratospheric and Mesospheric Sounder data. For background aerosol loadings there are significant increases in HOx and ClOx, as well as reductions in NOx. These effects are enhanced following the Pinatubo eruption but saturate at relatively low aerosol levels and hence can persist in the stratosphere for several years. Where pSCs are predicted to form, the sulphate aerosol chemistry does not operate in the model since the aerosols are incorporated as nuclei within the PSCs. In contrast, on the edge of the ozone hole, where temperatures are only just too high for the formation of PSCs, destruction by aerosol can result in further total ozone depletion of order 20 Dobson Units. In addition, the size and duration of the ozone hole are both increased by the presence of volcanic aerosols. The results support previous suggestions that the eruption of Mt. Pinatubo resulted in a temporary increase in ozone depletion over Antarctica and elsewhere.

Abstract:

Volcanic sulfate aerosols in the stratosphere produce significant long-term solar and infrared radiative perturbations in the Earth's atmosphere and at the surface, which cause a response of the climate system. Here we study the fundamental process of the development of this volcanic radiative forcing, focusing on the eruption of Mount Pinatubo in the Philippines on June 15, 1991. We develop a spectral-, space-, and time-dependent set of aerosol parameters for 2 years after the Pinatubo eruption using a combination of SAGE II aerosol extinctions and UARS-retrieved effective radii, supported by SAM II, AVHRR, lidar and balloon observations. Using these data, we calculate the aerosol radiative forcing with the ECHAM4 general circulation model (GCM) for cases with climatological and observed sea surface temperature (SST), as well as with and without climate response. We find that the aerosol radiative forcing is not sensitive to the climate variations caused by SST or the atmospheric response to the aerosols, except in regions with varying dense cloudiness. The solar forcing in the near infrared contributes substantially to the total stratospheric heating. A complete formulation of radiative forcing should include not only changes of net fluxes at the tropopause but also the vertical distribution of atmospheric heating rates and the change of downward thermal and net solar radiative fluxes at the surface. These forcing and aerosol data are available for GCM experiments with any spatial and spectral resolution. Copyright 1998 by the American Geophysical Union.

Abstract:

The isentropic transport of stratospheric air from the tropics into northern hemisphere mid-latitudes is examined using Mt. Pinatubo aerosol observed by the Improved Stratospheric and Mesospheric Sounder (ISAMS). The technique of contour advection is used to produce high-resolution aerosol fields which are used to help diagnose transport processes. By sampling the high-resolution contour-advection fields at the satellite measurement points, we show that the small-scale information present in the contour-advection calculation is consistent with the ISAMS measurements. Results of this study indicate that transport from the tropics into mid-latitudes is strongly dependent on altitude. In the lowest regions of the stratosphere, the upward influence of the subtropical jet and the synoptic-scale waves that form on the storm tracks are important in transporting air from the tropics and in the distribution of aerosol in mid-latitudes. Synoptic-scale spirals form in the aerosol field on the 500 K isentrope at the end of storm tracks associated with LC1 baroclinic life-cycle behaviour. At altitudes near the 600 K isentrope, direct tropospheric effects diminish and intrusions of air from the tropics to mid-latitudes weaken. At altitudes approaching the middle stratosphere, planetary-wave activity becomes stronger and the associated displacements of the polar vortex to low latitudes produce transport from the tropics, particularly during mid-winter warming events.

Abstract:

Stratospheric aerosol loading is reviewed in the context of the parameters necessary to describe stratospheric aerosol in chemical and radiative studies. The large spatial and temporal variability of sulphate aerosol loading makes a single reference atmosphere impractical and it is suggested that the liquid content of sulphate aerosols be used in the construction of future reference atmospheres as this parameter is invariant to temperature changes. The construction of an a posteriori climatology is recommended (C) 1998 COSPAR. Published by Elsevier Science Ltd.