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Hwa-Young M. Yeh

Abstract

No abstract available.

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Young C. Kwon
and
William M. Frank

Abstract

The energy flows of a simulated moist hurricane-like vortex are analyzed to examine the processes that change the intensity and structure of tropical cyclones. The moist vortex used in this study is initially axisymmetric on an f plane and is placed on a uniform surface—an ocean with constant sea surface temperature of 29°C. Two simulations are performed using the following different environmental flows: one in a calm environment and the other in weak environmental vertical shear. The differences between the intensities and structures of the two simulated vortices are discussed in terms of energy flows.

While the structure and intensity of the vortex without shear are relatively steady, those of the vortex with shear experience dramatic changes. The sheared vortex shows delayed weakening, persistent wavenumber 1 asymmetry with maximum rainfall on the downshear left side, and top-down breakdown. In both vortices barotropic energy conversion is stronger than baroclinic energy conversion. However, baroclinic processes in the upper levels of the sheared vortex play an important role in weakening the vortex. The energy flow diagram and the cross section of energy conversion terms show the existence of multiple baroclinic eddy life cycles at the upper levels of the sheared vortex. The activity of the baroclinic eddies continues until ventilation of the upper-level warm-core structure is sufficient to weaken the sheared vortex.

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Young C. Kwon
and
William M. Frank

Abstract

A series of numerical simulations of dry, axisymmetric hurricane-like vortices is performed to examine the growth of barotropic and baroclinic eddies and their potential impacts on hurricane core structure and intensity. The numerical experiments are performed using the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5) with a 6-km horizontal grid. To examine internal effects on the stability of vortices, all external forcings are eliminated. Axisymmetric vortices that resemble observed hurricane structures are constructed on an f plane, and the experiments are performed without moist and boundary layer processes.

Three vortices are designed for this study. A balanced control vortex is built based on the results of a full-physics simulation of Hurricane Floyd (1999). Then, two other axisymmetric vortices, EXP-1 and EXP-2, are constructed by modifying the wind and mass fields of the control vortex. The EXP-1 vortex is designed to satisfy the necessary condition of baroclinic instability, while the EXP-2 vortex satisfies the necessary condition of barotropic instability. These modified vortices are thought to lie within the natural range of structural variability of hurricanes.

The EXP-1 and EXP-2 vortices are found to be unstable with respect to small imposed perturbations, while the control vortex is stable. Small perturbations added to the EXP-1 and EXP-2 vortices grow exponentially at the expense of available potential energy and kinetic energy of the primary vortex, respectively. The most unstable normal modes of both vortices are obtained via a numerical method. The most unstable mode of the EXP-1 (baroclinically unstable) vortex vertically tilts against shear, and the maximum growth occurs near a height of 14 km and a radius of 20 km. On the other hand, the most unstable normal mode of the EXP-2 (barotropically unstable) vortex has horizontal tilting against the mean angular velocity shear, and the maximum perturbations are located at a lower altitude (around 4 km) and at larger radius (around 100 km). Despite these differences, the normal modes of both vortices have a wavenumber-1 structure.

The energy budget analysis shows that the growing baroclinic and barotropic perturbations have opposite effects on the vortex intensity in terms of kinetic energy. Baroclinic eddies strengthen, whereas barotropic eddies weaken, the primary vortex. It is hypothesized that fluctuations in hurricane core structure and intensity can occur due to eddy processes triggered by alternating periods of barotropic and baroclinic eddy growth in the core. Once formed, these eddies may interact with the intense diabatic energy sources in real hurricanes. A similar study of eddy behaviors in a more realistic hurricane, which includes moist and boundary layer processes and uses a finer grid mesh, will be the topic of Part II.

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Young-Oh Kwon
and
Terrence M. Joyce

Abstract

Spatial and temporal covariability between the atmospheric transient eddy heat fluxes (i.e., 〈υ′T′〉 and 〈υ′q′〉) in the Northern Hemisphere winter (January–March) and the paths of the Gulf Stream (GS), Kuroshio Extension (KE), and Oyashio Extension (OE) are examined based on an atmospheric reanalyses and ocean observations for 1979–2009.

For the climatological winter mean, the northward heat fluxes by the synoptic (2–8 days) transient eddies exhibit canonical storm tracks with their maxima collocated with the GS and KE/OE. The intraseasonal (8 days–3 months) counterpart, while having overall similar amplitude, shows a spatial pattern with more localized maxima near the major orography and blocking regions. Lateral heat flux divergence by transient eddies as the sum of the two frequency bands exhibits very close coupling with the exact locations of the ocean fronts.

Linear regression is used to examine the lead–lag relationship between interannual changes in the northward heat fluxes by the transient eddies and the meridional changes in the paths of the GS, KE, and OE, respectively. One to three years prior to the northward shifts of each ocean front, the atmospheric storm tracks shift northward and intensify, which is consistent with wind-driven changes of the ocean. Following the northward shifts of the ocean fronts, the synoptic storm tracks weaken in all three cases. The zonally integrated northward heat transport by the synoptic transient eddies increases by ~5% of its maximum mean value prior to the northward shift of each ocean front and decreases to a similar amplitude afterward.

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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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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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G. S. Young
and
J. M. Fritsch

A generalized set of conventions for facilitating analysis of mesoscale boundaries is proposed, and an example of the utility of the new conventions is presented.

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H. M. Stommel
and
W. R. Young

Abstract

No abstract available.

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M. L. Banner
and
I. R. Young

Abstract

This study examines the performance of a state-of-the-art spectral wind wave model that uses a full solution to the nonlinear interaction source term. The situation investigated here is fetch-limited wind wave evolution, for which a significant observational database exists. The authors consider both the evolutionary characteristics such as the predicted development of wave energy and peak wave frequency with fetch, as well as the predicted local features of the directional wavenumber spectrum: the spectral shape of the dominant wave direction slice, together with the directional spreading function. In view of the customary practice of constraining the shape of the spectral tail region, this investigation required relaxing the constrained tail assumption. This has led to new insight into the dynamic role of the spectral tail region.

The calculations have focused on the influence of two of the source terms in the spectral evolution (radiative transfer) equation for the energy density spectrum—those due to wind input and to dissipation predominantly through wave breaking. While the form of the wind input source term exerts some influence, the major impact arises from the dissipation source term, for which the authors explore a range of variants of the quasi-linear form proposed by Hasselmann. Due to the nonlinear coupling of spectral components through the wave–wave interaction term, it is only possible to obtain a detailed physical understanding of spectral evolution through such numerical experiments.

The results point to basic shortcomings in the present source terms. These lead to predicted local spectral properties and fetch evolution characteristics that differ significantly from the available observations. It is concluded that further refinement of the dissipation source term is required to improve modeling capabilities for wind sea evolution.

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Diana J. M. Greenslade
and
Ian R. Young

Abstract

One of the main limitations to current wave data assimilation systems is the lack of an accurate representation of the structure of the background errors. One method that may be used to determine background errors is the observational method of Hollingsworth and Lönnberg. The observational method considers correlations of the differences between observations and the background. For the case of significant wave height (SWH), potential observations come from satellite altimeters. In this work, the effect of the irregular sampling pattern of the satellite on estimates of background errors is examined. This is achieved by using anomalies from a 3-month mean as a proxy for model errors. A set of anomaly correlations is constructed from modeled wave fields. The isotropic length scales of the anomaly correlations are found to vary considerably over the globe. In addition, the anomaly correlations are found to be significantly anisotropic. The modeled wave fields are then sampled at simulated altimeter observation locations, and the anomaly correlations are recalculated from the simulated altimeter data. The results are compared to the original anomaly correlations. It is found that, in general, the simulated altimeter data can capture most of the geographic and seasonal variability in the isotropic anomaly correlation length scale. The best estimates of the isotropic length scales come from a method in which correlations are calculated between pairs of observations from prior and subsequent ground tracks, in addition to along-track pairs of observations. This method was found to underestimate the isotropic anomaly correlation length scale by approximately 10%. The simulated altimeter data were not so successful in producing realistic anisotropic correlation functions. This is because of the lack of information in the zonal direction in the simulated altimeter data. However, examination of correlations along ascending and descending ground tracks separately can provide some indication of the areas on the globe for which the anomaly correlations are more anisotropic than others.

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