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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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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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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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Stephen D. Nicholls
and
George S. Young

Abstract

An observational analysis of the structure and synoptic setting of tropical dendritic cumulus formations was undertaken using 30 months of global data from the Moderate Resolution Imaging Spectroradiometer aboard the National Aeronautics and Space Administration Terra satellite, the Quick Scatterometer aboard the SeaWinds satellite, and the National Centers for Environmental Prediction global reanalysis. This analysis yielded 1216 cases of tropical dendritic cumulus formations of which 61 were randomly selected for quantitative study. From these sample cases, it was found that dendritic patterns in shallow cumulus occurred over warm tropical oceans in response to cold air advection. They typically dissipate downstream in regions of cooler water, neutral to warm advection, or deep convection. Moreover, shallow cumulus formations take on a dendritic pattern only in areas where the background wind velocity is between 1.5 and 13 m s−1 in the surface to the 850-mb layer and a shallow layer of conditional instability is present. Individual cumulus clouds in these dendritic formations are arranged in a compound, hierarchical branching pattern in which each element of the pattern takes the form of a Y-shaped cloud line. Analysis of the cloud pattern observations in conjunction with the scatterometer-derived surface winds and the lower-tropospheric wind profiles from reanalysis data reveals that the individual Y elements are aligned closely with the surface wind direction, as linear cloud streets would be. These Y elements are oriented so that their forked end aligns as closely as possible with the surface-to-850-mb shear vector, even when this conflicts with the surface wind direction. A formation mechanism is hypothesized by which the secondary circulation of a towering cumulus line modifies the shear and stability profiles in the adjacent areas to favor shallower cumulus lines oriented at an angle to itself, thus forming a hierarchical branching structure. This hypothesis is supported by stability profiles from the reanalysis data.

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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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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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M. Sean Sublette
and
George S. Young

Abstract

The meteorological regime off the coast of North Carolina exhibits little synoptic-scale baroclinity during the summer months. As a result, the large-scale atmospheric forcing in this region is frequently weak. Given this weak synoptic forcing, mesoscale boundary layer circulations are dominant. One such circulation develops in response to the sea surface temperature discontinuity between the Gulf Stream and the relatively cooler water of the Continental Shelf. When synoptic conditions are favorable, differences in surface fluxes of heat and moisture across this discontinuity cause the development of an ageostrophic solenoidal circulation and the creation of an atmospheric boundary layer convergence zone. This resulting frontal zone, or Gulf Stream atmospheric front (GSAF), is a commonly observed feature in this region during the warm season.

Simulations using The Pennsylvania State University–National Center for Atmospheric Research mesoscale model are combined with data gathered from the High-Resolution Remote Sensing Experiment to study the effects of the Gulf Stream on mesoscale circulations in the warm-season marine atmospheric boundary layer. Particular attention was given to determining whether a model with resolution and physics similar to those of operational mesoscale forecast models can adequately predict this phenomenon. Although limitations in the horizontal and vertical resolution of the model prevent detailed reproduction of the meso-γ-scale structure of the GSAF, the model does produce a significant meso-β boundary layer convergence zone in response to the local SST maximum associated with the Gulf Stream. The magnitude of the modeled response is primarily a function of air–sea temperature difference, the local wind vector, and the depth of the boundary layer.

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George S. Young
and
Richard H. Johnson

Abstract

Data from the NOAA BAO (Boulder Atmospheric Observatory) tower and the PROFS (Program for Regional Observing and Forecasting Services) surface mesonetwork have been used to detect the meso- and microscale flow patterns associated with the passage of a shallow cold front over complex terrain. This front moved across the PROFS surface mesonetwork and the BAO tower site on the morning of 3 December 1981. Partial blocking of the cold airflow by the higher terrain to the north led to a westward movement of the cold front in the upper reaches of the South Platte River drainage basin. A meso-β-scale anticyclonic eddy subsequently formed in the lee of this terrain obstruction.

The micro-α-scale vertical eddy structure in the cold frontal zone is defined using data from the BAO tower. Updrafts of >6 m s−1 at 200 m AGL occurred at the wind-shift and temperature-drop line. Immediately behind this feature, micro-α-scale eddies entrained air through the frontal surface, diluting the low-level flow which was overtaking the front from behind. These features have been observed by others in atmospheric and laboratory gravity currents.

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