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Kenneth Sassen
and
John D. Horel

Abstract

Over an unusually brief three-day period in early August 1989, spectacular twilight effects indicative of a stratospheric volcanic cloud were seen at Salt Lake City, Utah. Concurrent polarization lidar observations detected an aerosol layer at altitudes between 14 and 16 km in the vicinity of the tropopause. Trajectory analyses indicate that the aerosol source was the relatively minor 19 July volcanic eruption of Santiaguito in Guatemala. Materials injected into the lower stratosphere by this eruption were transported initially by tropical easterlies and then by a subtropical jet stream to the locale. The sulfuric acid droplet cloud that formed during transport was affected locally by tropopause folds that promoted stratospheric-tropospheric exchanges. Although lidar depolarization analysis suggests that the ensuing cloud microphysical processes were usually dominated by acid droplet crystallization effects caused by ammonia gas absorption (yielding 0.1–0.2 linear depolarization ratios), there is also evidence for ice crystals at the coldest temperatures (∼−64°C) and for homogeneous droplets. Cloud optical thickness estimates are 0.01–0.02. Mesoscale cloud bands were observed visually near sunset and also occasionally during daylight, another unusual characteristic for volcanic clouds.

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Neil P. Lareau
and
John D. Horel

Abstract

High-resolution idealized numerical simulations are used to examine the turbulent removal of cold-air pools commonly observed in mountain valleys and basins. A control simulation with winds aloft increasing from 0.5 to 20 m s−1 over 20 h combined with typical cold-air pool stratification illustrates the interplay over time of lowering of the top of the cold-air pool, spillover downstream of the valley from the upper reaches of the cold-air pool, wavelike undulations affecting the cold-air pool’s depth and stratification across the valley, and smaller temporal- and spatial-scale Kelvin–Helmholtz waves within the uppermost layers of the cold-air pool. The heat budget within the cold-air pool demonstrates the nearly compensating effects of vertical and horizontal advection combined with turbulent heating of the upper portion of the cold-air pool and cooling in the layers immediately above the cold-air pool. Sensitivities of turbulent mixing in cold-air pools to stratification and upstream terrain are examined. Although the characteristics of the turbulent mixing differ as the stratification and topography are modified, a bulk parameter [the cold-air pool Froude number (Fr)] characterizes the onset and amplification of turbulent mixing and the time of cold-air pool removal. When Fr > 1, Kelvin–Helmholtz waves and turbulent heat fluxes commence. Turbulent heat flux and wave activity increase until Fr = 2, after which the cold-air pool breaks down and is removed from the valley. The rate of cold-air pool removal is proportional to its strength; that is, a strong inversion generates larger heat fluxes once turbulent erosion is underway.

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Ernesto H. Berbery
,
Julia Nogués-Paegle
, and
John D. Horel

Abstract

The dynamical basis of intraseasonal oscillations of the Southern Hemisphere summer and winter seasons is studied with a combination of observed diagnostics and simplified prognostic models. High-frequency oscillations, zonal mean variations, and seasonal and interannual variabilities are removed from the six-year dataset in an effort to reduce the effect of high-frequency dynamical instabilities and long-period forced fluctuations. The diagnoses focus upon those processes that have most frequently been explained in terms of Rossby-wave propagation through atmospheres with variable refractive indices. It is useful to study both winter and summer seasons simultaneously because of the large changes in the seasonally averaged state and large consequent changes in atmospheric waveguides between these seasons. A nonlinear shallow-water model slowly relaxed toward the time-averaged winter and summer observed mean fields is used to describe the characteristics of wave propagation in a horizontally varying basic state. Perturbations are introduced in four different regions corresponding to points where observed atmospheric teleconnectivities are relatively large, and the signal propagation is analyzed using averaging procedures similar to those employed for the observational study. Furthermore, differences between stationary and nonstationary patterns are also discussed.

The four general regions selected for the observational study are Australia, New Zealand, South America, and the Atlantic Ocean. Differences from winter to summer are related to concomitant changes of the background latitudinal gradient of absolute vorticity. During winter and summer meridional propagation is toward the tropics. Winter wave patterns have mainly zonal paths and show a slow phase velocity on the order of 3 m s−1, while during summer, patterns tend to be geographically fixed. During winter, regions of imaginary refractive index flank the subtropical and polar jet streams. These jet streams seem to act as waveguides for disturbances emanating from the southern Indian Ocean and western Australia, where two wave trains exist. Wave activity flux vectors suggest that these disturbances originate in the subtropical southern Indian Ocean and that equatorward propagation prevails at the exit region of the subpolar jet stream and over South America and the Atlantic Ocean. During summer, observed wave patterns tend to have a more meridional component, again in agreement with the background latitudinal gradient of absolute vorticity.

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