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George S. Young

Abstract

Profiles of turbulence statistics from aircraft observations of the Phoenix 78 convective boundary layer experiment are compared with those from previous observational and modeling studies. The sources and degree of variability of the normalized results, both within and between experiments, are discussed. The intercomparison provides evidence that moderately rolling terrain does not bias convective boundary layer turbulence structure away from that observed over more uniform terrain. The manner in which cross inversion entrainment affects turbulence in the atmosphere and in models is also discussed.

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George S. Young

Abstract

A conditional sampling technique based upon the mixed layer spectra of vertical velocity and temperature is developed. This technique is used to analyze the turbulence data obtained by aircraft during the Phoenix 78 convective boundary layer experiment. Observations of the size, spacing and structure of thermals as well as their contribution to mixed layer processes are presents. Implications of these results for pollution dispersion are discussed. The observed scale dependence is also used to estimate what fraction of a turbulence statistic must be accounted for by the subgrid parameterizations of large eddy simulations.

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George S. Young

Abstract

The dynamics of thermal updrafts and compensating environmental downdrafts in the convective boundary layer are examined using observations from the Phoenix 78 field experiment. Separate vertical velocity budgets are presented for thermal updrafts and environmental downdrafts. These two budgets show the existence of qualitative differences in the forcing of the two leg of the convective circulations.

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George S. Young and Jonathan Zawislak

Abstract

Vortex streets are a frequent occurrence in stratocumulus-topped flow downwind of mountainous islands. Theoretical studies dating back to von Kármán, supported by laboratory and numerical studies, have yielded similarity theories for the size and spacing of these vortices behind bluff bodies. Despite dynamical differences between such two-dimensional flows and the three-dimensional flow past isolated islands, satellite case studies suggest these geometric similarities may also hold for the atmospheric case. In this study, two of the resulting dimensionless ratios are measured using satellite imagery. One is the aspect ratio between cross-street and along-street spacing of the vortices. The second is the ratio of the cross-street spacing to the crosswind width of the island. A 30-image sample from the Aqua and Terra Moderate Resolution Imaging Spectroradiometer satellites is analyzed to obtain these ratios. The resulting set of values for the two dimensionless ratios is tested against the values found in bluff body studies. The aspect ratio is tested against the value of 0.28 resulting from von Kármán’s inviscid theory, and the dimensionless width ratio is tested against the value of 1.2 from Tyler’s laboratory study of flow around a bluff body. It is found that atmospheric vortex streets do indeed follow the geometric similarity theories, but with different values for the two ratios than those predicted by von Kármán and Tyler. The aspect ratio is larger than predicted as is the dimensionless width ratio. Both differences are consistent with the turbulent diffusion of vorticity in the wake of the island. The vortex streets more closely follow inviscid theory close to the island, with vortex expansion taking place a few vortex diameters downwind of the island.

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Nathaniel S. Winstead and George S. Young

Abstract

Synthetic aperture radar has shown great promise in detecting surface roughness patterns generated by atmospheric and oceanic features. Those roughness patterns that are the result of sea surface wind stress may be analyzed and related to characteristics of the atmospheric boundary layer. Previously reported examples of detectable atmospheric signatures include gravity waves and Rayleigh–Benard convection in cold-air outbreaks. In this paper, the results from an analysis of an image that contains the signatures of nocturnal-drainage-flow-forced exit jets along the western shore of Chesapeake Bay is presented. A regression analysis is performed that links the length of the surface stress patterns associated with these exit jets to the geometry of their source basins. This analysis differs from previous drainage-flow studies in that a population of drainage flows of varying sizes is studied under identical synoptic conditions. This large sample size provides a unique opportunity to examine the role that topography plays in forcing this kind of flow.

To complement the observational study, a two-dimensional, shallow-fluid model is developed to simulate the drainage-flow exit jets once they leave their source basins. This model allows simulation of the behavior of these flows over the entire range of forcing values observed in the image. This kind of analysis provides physical insight into the dynamics of these hybrid flows and a basis for the development of a similarity theory that relates the physically significant forcing parameters to the characteristic length and speed scales of this phenomenon. The lack of in situ observations unfortunately prevents a direct comparison between model results and observations; however, the model is shown to give characteristic jet length scales that are in reasonable agreement with values obtained from the image analysis.

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George S. Young and John W. Winchester

Abstract

Peak concentrations of aerosol sulfur in Tampa, Florida may be the result of either regional-scale transformation and transport processes or local-scale transport from nearby air pollution sources. The existence of the latter has been demonstrated in Tampa through correspondence of sulfur with sea breeze circulation patterns and the resulting chloride concentration maxima (which serve as indicators of the marine aerosol), vanadium concentration maxima (which indicate times of high concentrations of certain plume constituents), and the locations of sources favorable for high concentrations of air pollution-derived sulfate during occurrences of the sea breeze. The analysis indicates that locally derived sulfate in the Tampa atmosphere, which may be less abundant than sulfate due to regional-scale processes, can be identified by the use of combined meteorological and chemical tracer interpretation.

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George S. Young and Todd D. Sikora

Abstract

Examination of visible and infrared imagery from geosynchronous and polar orbiter satellites reveals the occasional existence of mesoscale cloud bands of unusual width and area, originating over the open northwest Atlantic Ocean during cold-air outbreaks. This phenomenon is of both dynamic and synoptic interest. As a dynamic phenomenon, it represents a mesoscale flow that is driven by transient surface features, which are meanders in the Gulf Stream. The forcing geometry and the resulting cloud pattern are similar in many respects to the anomalous cloud lines observed downwind of Chesapeake and Delaware Bays in similar conditions. These open ocean cloud bands are often of a larger scale, however, because the Gulf Stream meanders represent the largest-scale high-amplitude “coastal features” in the western North Atlantic. These cloud bands are of synoptic interest because, when present, they play a major role in determining the cloud pattern over much of this oceanic region.

Examination of surface and 850-hPa analyses demonstrates that these open ocean cloud bands occur during cold-air outbreaks and that they align approximately with the boundary layer wind. Comparison of visible and infrared satellite imagery with contemporaneous sea surface temperature analyses derived from infrared polar orbiter satellite imagery reveals that the open ocean cloud bands originate at the upwind end of Gulf Stream meanders. Climatological data and synoptic observations from land and sea indicate that these events occur only during that part of the spring season in which coastal temperature differences are small but cold-air outbreaks continue to reach the Gulf Stream. Examination of this observational evidence suggests that these open ocean cloud bands result from mesoscale solenoidal circulations driven by the horizontal gradients in sea surface temperature caused by Gulf Stream meanders.

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Jeffrey J. Nucciarone and George S. Young

Abstract

Mixed-layer scaling was successfully applied to the velocity, temperature, and moisture spectra of the marine stratocumulus-topped mixed layers observed during the First ISCCP Regional Experiment (FIRE). These turbulence spectra provide considerable insight into the physical phenomena that affect this kind of boundary layer. At least four of these phenomena contribute to the turbulence spectra: eddies that result directly from shear and buoyant production of turbulent kinetic energy (the microscale energy production subrange); smaller eddies that result from the inertial cascade of energy (the inertial subrange); quasi-two-dimensional mesoscale variations; and gravity waves. The velocity, temperature, and moisture spectra are affected differently by each of these phenomena. The turbulence spectra highlight the differences between the cold-current marine stratocumulus-topped boundary layer and the overland convective boundary layer.

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William M. Frank and George S. Young

Abstract

This paper examines the interannual variability of tropical cyclones in each of the earth’s cyclone basins using data from 1985 to 2003. The data are first analyzed using a Monte Carlo technique to investigate the long-standing myth that the global number of tropical cyclones is less variable than would be expected from examination of the variability in each basin. This belief is found to be false. Variations in the global number of all tropical cyclones are indistinguishable from those that would be expected if each basin was examined independently of the others. Furthermore, the global number of the most intense storms (Saffir–Simpson categories 4–5) is actually more variable than would be expected because of an observed tendency for storm activity to be correlated between basins, and this raises important questions as to how and why these correlations arise. Interbasin correlations and factor analysis of patterns of tropical cyclone activity reveal that there are several significant modes of variability. The largest three factors together explain about 70% of the variance, and each of these factors shows significant correlation with ENSO, the North Atlantic Oscillation (NAO), or both, with ENSO producing the largest effects. The results suggest that patterns of tropical cyclone variability are strongly affected by large-scale modes of interannual variability. The temporal and spatial variations in storm activity are quite different for weaker tropical cyclones (tropical storm through category 2 strength) than for stronger storms (categories 3–5). The stronger storms tend to show stronger interbasin correlations and stronger relationships to ENSO and the NAO than do the weaker storms. This suggests that the factors that control tropical cyclone formation differ in important ways from those that ultimately determine storm intensity.

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Richard H. Johnson and George S. Young

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An analysis of the heat and moisture budgets of tropical mesoscale anvil clouds has been carried out. Mesoscale anvils, defined as widespread (∼100 km) cloud systems extending from near the freezing level to the high troposphere, are characterized by light, stratiform precipitation. These cloud features, which are preceded by and generally merged with cumulonimbus clouds, are prevalent throughout the tropics and summertime midlatitudes and may account for an important fraction of the total tropical rainfall.

Sounding data from the December 1978 field phase of the International Winter Monsoon Experiment (Winter MONEX) are used to determine heat and moisture (Q 1t and Q 2) budgets for a number of mesoscale anvil cloud systems. The composite heating (Q 1) profile shows a warming peak in the upper troposphere near 350 hPa or 8–9 km that can be attributed to condensation and freezing in the anvil and a cooling peak in the lower troposphere near 700 hPa or 3 km due to rainfall evaporation and melting. The moisture (Q 2) budget shows a drying maximum in the upper troposphere coincident with the warming peak and a moistening maximum in the lower troposphere near 800 hPa or 2 km.

The heat budget is compared with that determined recently for mesoscale anvils by Houze (1982), who has used an independent and different approach, and good agreement between the two heating distributions is found. The heating and moistening profiles for mesoscale anvils are considerably different from those determined by large-scale budget studies of entire tropical cloud clusters, which contain both cumulonimbus and mesoscale anvil cloud effects. In particular, the heating profiles diagnosed here show an upward shift in the level of maximum heating, and cooling, instead of heating in the lower troposphere, extending from near the freezing level to the surface.

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