Earth Observation Data Group

Abstract:

The retrieval and validation of the infrared measurements of the stratospheric aerosol layer derived from the improved stratospheric and mesospheric sounder (ISAMS) on board the Upper Atmsophere Research Satellite (UARS) are discussed in detail. The retrieval method is presented and an error analysis and sensitivity study are used to provide an error budget. The validation involves internal consistency checks, comparisons with coincident aerosol observations from other satellite-based instruments (cryogenic limb array etalon spectrometer (CLAES) and Stratospheric Aerosol and Gas Experiment II (SAGE II)) and in situ particle counters. The internal comparisons show that the precision of the 6.20-mu m retrievals are better than 30% for the data on the 32-hPa and 22-hPa pressure surfaces and that the 12.11-mu m retrievals are better than 20% and 25% for the data on pressure surfaces at 46 hPa and 32 hPa, respectively. Comparison of the ISAMS aerosol extinctions with the simultaneous CLAES measurements show systematic biases for both the 6-mu m and the 12-mu m retrievals with ISAMS generally measuring lower extinction values than CLAES. The comparison with calculated extinctions from balloon dustsonde data at 41 degrees N shows that the aerosol extinctions are systematically too high for pressures less than 40 hPa.

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 (SAG) 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 SAG. 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:

This paper introduces a series of papers describing the validation of data products from the improved stratospheric and mesospheric sounder (ISAMS) on the Upper Atmosphere Research Satellite. ISAMS is a limb-sounding infrared gas-correlation radiometer, measuring thermal emission from a range of constituents. The constituents measured are ozone, water vapor, methane, nitrous oxide, nitric oxide, nitrogen dioxide, nitrogen pentoxide, nitric acid, carbon monoxide, and aerosol. Atmospheric temperature and composition data were obtained for approximately 180 days between September 26, 1991, and July 29, 1992, with near-global coverage. The instrument and the retrieval process are briefly described, together with aspects of the validation process relevant to all data products, including the radiometric calibration and the analysis of the calibrated radiances to produce data on a standard time-altitude grid. Copyright 1996 by the American Geophysical Union.

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. © 1996 American Meteorological Society.

Abstract:

The ambient water vapour and temperature conditions of stratospheric sulphate aerosol particles govern their composition and thereby influence their infrared extinction properties. This causes problems in the use of the infrared aerosol extinction as a tracer because variations in the aerosol composition modify the changes in extinction that may arise from the transport of aerosols. An improved tracer which can be derived from measurements of the infrared aerosol extinction, temperature and water vapour abundance, is the H2SO4 component of aerosols. The application of this tracer to studies of stratospheric transport is demonstrated using data from instruments on the Upper Atmosphere Research Satellite and the results are compared to a contour advection calculation. Copyright 1996 by the American Geophysical Union.