Professor Roy Grainger

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.

Abstract:

The simultaneous measurements of temperature, aerosol extinction, and concentrations of radiatively active gases by several instruments aboard the Upper Atmosphere Research Satellite permit an assessment of the uncertainties in the diagnosed stratospheric heating rates and in the resulting residual circulation. In this paper, measurements taken by the Cryogenic Limb Array Etalon Spectrometer (CLAES) are used to compute the circulation and to compare it against values obtained previously from the measurements obtained by the Microwave Limb Sounder (MLS). There is a broad agreement between the two sets of calculations and known biases in either CLAES or MLS ozone and temperature measurements are found to be responsible for the areas of disagreement. The inclusion of aerosols has improved the estimates of the residual circulation in the lower stratosphere during the 1992-93 period covered by CLAES. Present estimates of the aerosol heating are significantly different from those found in other studies, probably as a result of differences in the treatment of tropospheric clouds and in the adopted vertical profiles of aerosol extinction. Moreover, a large uncertainty in these estimates is caused by the uncertainties in the assumed refractive indices for sulfuric acid solutions.