Charles R. Trepte

The poleward dispersal of Mount Pinatubo volcanic aerosol

Using the SAGE II 1-μm stratospheric aerosol extinction ratio observations, the dispersal of Mount Pinatubo aerosol within two transport regimes during the first 10 months after the eruption is displayed in meridional cross sections. Maximum aerosol extinction ratio values were contained in a tropical reservoir bounded by strong gradients in the subtropics. The detrainment of aerosol from the equatorial reservoir occurred in episodic synoptic scale events and two examples are presented: (1) transport into the boreal summer hemisphere in a lower regime just above the tropopause associated with upwardly decaying tropospheric disturbances and (2) dispersal into the austral winter hemisphere in an upper transport regime near 30 hPa associated with planetary wave activity in the southern subtropics.

A climatology of stratospheric aerosol

A global climatology of stratospheric aerosol is created by combining nearly a decade (1979–1981 and 1984–1990) of contemporaneous observations from the Stratospheric Aerosol and Gas Experiment (SAGE I and II) and Stratospheric Aerosol Measurement (SAM II) instruments. One goal of this work is to provide a representative distribution of the aerosol layer for use in radiative and chemical modeling. A table of decadal average 1μm extinction values is included, extending from the tropopause to 35 km and 80°S to 85°N, which allows estimation of surface area density. We find that the aerosol layer is distinctly volcanic in nature and suggest that the decadal average is a more useful estimate of future aerosol loading than a “background” loading, which is never clearly achieved during the data record. This climatology lends insight into the general circulation of the stratosphere. Latitude - altitude sections of extinction ratio at 1 μm are shown, averaged by decade, season, and phase of the quasi-biennial oscillation (QBO). A tropical reservoir region is diagnosed, with an “upper” and a “lower” transport regime. In the tropics above 22 km (upper regime), enhanced lofting occurs in the summer, with suppressed lofting or eddy dilution in the winter. In the extratropics within two scale heights of the tropopause (lower regime), poleward and downward transport is most robust during winter, especially in the northern hemisphere. The transport patterns persist into the subsequent equinoctial season. Ascent associated with QBO easterly shear favors detrainment in the upper regime, while relative descent and poleward spreading during QBO westerly shear favors detrainment in the lower regime. Extinction ratio differences between the winter-spring and summer-fall hemispheres, and differences between the two phases of the QBO, are typically 20–50%. Dynamical implications of the aerosol distributions are explored, with focus on interhemispheric differences, strong subtropical gradients, and the pronounced annual cycle.

An empirical model study of the tropospheric meridional circulation based on SAGE II observations

This study investigates the tropospheric mean meridional circulation important to the development of opaque clouds and the measurement opportunity of the 1.02-μm channel of the Stratospheric Aerosol and Gas Experiment (SAGE) II in the troposphere. A simple empirical model is formulated to derive the mean meridional circulation from the 6-year (1985-1990) statistics of the SAGE II tropospheric measurement frequency. The vertical circulation of the model is assumed to be related to the departure field of the zonally averaged SAGE II measurement frequency from the corresponding global mean in a linear fashion. The proportional constant is calibrated with the observed upwelling circulation statistics in the tropics. The obtained model vertical circulation is then used to determine the distribution of meridional velocity according to the continuity equation. The derived model mean circulation features the influence from both the diabatic circulation and the eddy quasi-isentropic transport, with a distinct pattern of material advection into the upper troposphere from both the lower troposphere and the stratosphere. Most significantly, the model circulation is shown to be highly consistent with the observed free tropospheric aerosol and ozone distributions, particularly with their seasonal variations given the aerosol and ozone source regions. This high degree of consistency illustrates the intimate relationship between the large-scale circulation, cloudiness, and the SAGE II tropospheric measurement frequency, and the robust nature of the empirical model despite the models simplicity. The discussion in relating the model circulation to the conventional Eulerian circulation and the Lagrangian transport, based on isentropic consideration. is also provided.

Stratospheric aerosol measurements by the Lidar in Space Technology Experiment

The Lidar in Space Technology Experiment (LITE) is a three-wavelength backscatter lidar developed by NASA Langley Research Center to demonstrate and explore the capabilities of space lidar. LITE was flown on space shuttle Discovery in September 1994. Among the primary experimental objectives of LITE was the measurement of stratospheric aerosols. High-quality stratospheric aerosol measurements at 532 nm and 355 nm were obtained during nighttime, high-gain operation. These LITE data provide a detailed global view of the vertical structure and optical properties of the stratospheric aerosols. The data are also used to study the transport processes influencing the aerosol spatial distribution. LITE data compare well with measurements made by the Stratospheric Aerosol and Gas Experiment (SAGE) II. Individual profile comparisons and comparisons of more global features reinforce and extend the validation of the LITE stratospheric data. LITE demonstrates that a spaceborne lidar, with its high vertical resolution and global coverage, is a powerful tool for tracing atmospheric transport.

Circulation Deduced from Aerosol Data Averaged by Season and Phase of the Quasibiennial Oscillation

Aerosol measurements obtained from the Stratospheric Aerosol and Gas Experiments (SAGE I and II), for the periods 1979–81 and 1984–91, are averaged by season and by the phase of the quasibiennial oscillation (QBO). Largest values of aerosol extinction ratio are found in the tropical lower stratosphere. Latitude-altitude distributions suggest the existence of an upper and a lower transport regime out of this tropical reservoir. Above 25 km upward detrainment occurs more readily in the subtropics during summer and fall. Detrainment pole-ward and downward occurs within ~5 kilometers above the tropopause most readily during the austral winter and spring and during the boreal fall, winter and spring. Ascent associated with QBO easterly shear favors detrainment in the upper regime, while relative descent and poleward spreading during QBO westerly shear favors detrainment in the lower regime.