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- Author or Editor: David Crisp x
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Abstract
The 1982 eruptions of the El Chichón volcano injected large quantities of sulfur dioxide gas and silicate ash into the stratosphere. Several studies have shown that the long-lived sulfuric acid aerosols derived from these volcanic effluents produced measurable changes in the radiative heating rates and the global circulation. The radiative and dynamical perturbations associated with the short-lived but more strongly absorbing sulfur dioxide and ash clouds have received much 1ess attention. The authors therefore used an atmospheric radiative transfer model and observations collected by satellites, aircraft, and ground-based observers to estimate the amplitudes of the stratospheric radiative heating rate perturbations produced by each of these components during the first few weeks after the El Chichón eruption. One week after the 4 April 1982 eruption, net radiative heating rate perturbations exceeding 20 K per day were found at altitudes near 26 km. The absorption of sunlight by the silicate ash accounts for the majority of this heating. The sulfur dioxide gas and sulfuric acid aerosols each produced net heating perturbations that never exceeded 3 K per day. In spite of the intense heating by the ash, observations indicate that stratospheric temperatures never increased by more than a few degrees Kelvin. The authors therefore concluded that this radiative heating was largely balanced by upwelling and adiabatic cooling. The amplitude and spatial extent of this upwelling was estimated with a diagnostic, two-dimensional dynamical model. The ash heating rates may have been balanced by a global enhancement in the stratospheric meridional circulation, with zonally averaged upward velocities of about 1 cm sec−1 near the latitude of the plume. This enhanced stratospheric circulation persisted only for a few weeks but it may have played a major role in the vertical and horizontal dispersal of the plume. The vertical transport needed to balance the heating by sulfur dioxide gas was only 5%–10% as large, but this perturbation may have produced a 2-km increase in the altitude of the plume. These results suggest that the radiative forcing by the ash and the sulfur dioxide gas should be included in more comprehensive models of the plume evolution. They also suggest that particle size distributions inferred from ash fallout rates could be wrong if the upwelling associated with this radiative heating is not considered.
Abstract
The 1982 eruptions of the El Chichón volcano injected large quantities of sulfur dioxide gas and silicate ash into the stratosphere. Several studies have shown that the long-lived sulfuric acid aerosols derived from these volcanic effluents produced measurable changes in the radiative heating rates and the global circulation. The radiative and dynamical perturbations associated with the short-lived but more strongly absorbing sulfur dioxide and ash clouds have received much 1ess attention. The authors therefore used an atmospheric radiative transfer model and observations collected by satellites, aircraft, and ground-based observers to estimate the amplitudes of the stratospheric radiative heating rate perturbations produced by each of these components during the first few weeks after the El Chichón eruption. One week after the 4 April 1982 eruption, net radiative heating rate perturbations exceeding 20 K per day were found at altitudes near 26 km. The absorption of sunlight by the silicate ash accounts for the majority of this heating. The sulfur dioxide gas and sulfuric acid aerosols each produced net heating perturbations that never exceeded 3 K per day. In spite of the intense heating by the ash, observations indicate that stratospheric temperatures never increased by more than a few degrees Kelvin. The authors therefore concluded that this radiative heating was largely balanced by upwelling and adiabatic cooling. The amplitude and spatial extent of this upwelling was estimated with a diagnostic, two-dimensional dynamical model. The ash heating rates may have been balanced by a global enhancement in the stratospheric meridional circulation, with zonally averaged upward velocities of about 1 cm sec−1 near the latitude of the plume. This enhanced stratospheric circulation persisted only for a few weeks but it may have played a major role in the vertical and horizontal dispersal of the plume. The vertical transport needed to balance the heating by sulfur dioxide gas was only 5%–10% as large, but this perturbation may have produced a 2-km increase in the altitude of the plume. These results suggest that the radiative forcing by the ash and the sulfur dioxide gas should be included in more comprehensive models of the plume evolution. They also suggest that particle size distributions inferred from ash fallout rates could be wrong if the upwelling associated with this radiative heating is not considered.
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.
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.
Abstract
Results for the residual circulation in the stratosphere and lower mesosphere between September 1991 and August 1994 are reported. This circulation is diagnosed by applying an accurate radiative transfer code to temperature, ozone, and water vapor measurements acquired by the Microwave Limb Sounder (MLS) onboard the Upper Atmosphere Research Satellite (UARS), augmented by climatological distributions of methane, nitrous oxide, nitrogen dioxide, surface albedo, and cloud cover. The sensitivity of the computed heating rates to the presence of Mt. Pinatubo aerosols is explored by utilizing aerosol properties derived from the measurements obtained by the Improved Stratospheric and Mesospheric Sounder instrument, also onboard UARS. The computed vertical velocities exhibit a Semiannual oscillation (SAO) around the tropical stratopause, with the region of downward velocities reaching maximum spatial extent in February and August. This behavior reflects the semiannual oscillation in temperature and ozone and mimics that seen in past studies of the October 1978–May 1979 period based on data from the Limb Infrared Monitor of the Stratosphere onboard the Nimbus 7 satellite. The SAO vertical velocities are stronger during the northern winter phase, as expected if planetary waves from the winter hemisphere are involved in driving the SAO. A possible quasi-biennial oscillation (QBO) signal extending from the middle into the upper stratosphere is also hinted at, with the equatorial vertical velocities in the region 10–1 hPa significantly smaller (or even negative) in 1993/94 than in 1992/93. Despite the short data record, the authors believe that this pattern reflects a QBO signal rather than a coincidental interannual variability, since the time–height section of vertical velocity at the equator resembles that of the zonal wind. Wintertime high-latitude descent rates are usually greater in the Northern Hemisphere, but they also exhibit significant variability there. In the three northern winters analyzed in this study, strong downward velocities are diagnosed in the lower stratosphere during stratospheric warmings and are associated with enhanced wave forcing (computed as the momentum residual) in the mid- and upper stratosphere. The implications of the computed circulation for the distribution of tracers are illustrated by the example of the “double-peaked” structure in the water vapor distribution measured by MLS.
Abstract
Results for the residual circulation in the stratosphere and lower mesosphere between September 1991 and August 1994 are reported. This circulation is diagnosed by applying an accurate radiative transfer code to temperature, ozone, and water vapor measurements acquired by the Microwave Limb Sounder (MLS) onboard the Upper Atmosphere Research Satellite (UARS), augmented by climatological distributions of methane, nitrous oxide, nitrogen dioxide, surface albedo, and cloud cover. The sensitivity of the computed heating rates to the presence of Mt. Pinatubo aerosols is explored by utilizing aerosol properties derived from the measurements obtained by the Improved Stratospheric and Mesospheric Sounder instrument, also onboard UARS. The computed vertical velocities exhibit a Semiannual oscillation (SAO) around the tropical stratopause, with the region of downward velocities reaching maximum spatial extent in February and August. This behavior reflects the semiannual oscillation in temperature and ozone and mimics that seen in past studies of the October 1978–May 1979 period based on data from the Limb Infrared Monitor of the Stratosphere onboard the Nimbus 7 satellite. The SAO vertical velocities are stronger during the northern winter phase, as expected if planetary waves from the winter hemisphere are involved in driving the SAO. A possible quasi-biennial oscillation (QBO) signal extending from the middle into the upper stratosphere is also hinted at, with the equatorial vertical velocities in the region 10–1 hPa significantly smaller (or even negative) in 1993/94 than in 1992/93. Despite the short data record, the authors believe that this pattern reflects a QBO signal rather than a coincidental interannual variability, since the time–height section of vertical velocity at the equator resembles that of the zonal wind. Wintertime high-latitude descent rates are usually greater in the Northern Hemisphere, but they also exhibit significant variability there. In the three northern winters analyzed in this study, strong downward velocities are diagnosed in the lower stratosphere during stratospheric warmings and are associated with enhanced wave forcing (computed as the momentum residual) in the mid- and upper stratosphere. The implications of the computed circulation for the distribution of tracers are illustrated by the example of the “double-peaked” structure in the water vapor distribution measured by MLS.
