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John E. Hart

Abstract

Limit-cycle type oscillations of the amplitude of a baroclinic wave field, which has grown by instability on a background zonal flow, have been observed experimentally in a rotating cylinder containing two immiscible fluid layers, the upper one of which is driven by a differentially rotating contact lid. The experimental results are summarized and compared with previous speculations and theories on the origins of qualitatively similar long-period modulations occurring in the rotating annulus. Results from a truncated-spectral model are presented. The solutions reproduce the fundamental modulation characteristics for several different experimental configurations involving different distributions of the basic zonal potential vorticity gradients and hence different linear stability properties. The observed modulations are related to periodic transfers of energy between the wave field and the zonal flow. This process is primarily baroclinic, dominated by available potential energy transport fluctuations, but barotropic transfers also occur. The predicted wave field modulates in shape as well as amplitude and the motion cannot be characterized as either “tilted trough” or “amplitude” vacillation.

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John E. Hart

Abstract

This paper discusses the nature of quasi-geostrophic β-plane flow over an idealized set of ridges with height h=F(y/Ly) cosx/L x. When the mountains are highly anisotropic, with scale factor ratio Lx/Ly≪1, the asymptotically exact forced solution is governed by a simple set of three nonlinear ordinary differential equations similar to those obtained by Charney and DeVore (1978). For fixed forcing, the region of parameter space where multiple, stable steady solutions exist is mapped out. A cusp catastrophe occurs in which a rapid zonal flow over the ridges drops to a very low value as a parameter like the driving Rossby number decreases slightly below a certain critical point; and the zonal flow then remains at this low value for a large range of Rossby number on either side of the bifurcation value. The existence of limit cycle solutions is discussed. Such periodic solutions are shown to exist for the f-plane case, and probably exist for the β-plane as well. However, numerical solutions indicate. that the limit cycles are unstable, with the steady solutions being favored. The stationary solutions are also shown to be stable with respect to barotropic isotropic perturbations.

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John E. Hart

Abstract

Simple expressions are presented for the corrections to the classic Ekman pumping law W e = ẑ·curl(τ0/f) due to nonlinear advection effects in the surface boundary layer. These involve products of the surface Reynolds stress, τ0 and the underlying ocean currants v0(x, y, t) and their derivatives, and products of τ0(x, y, t) and its own derivatives. The former interaction is independent of the turbulence closure, while the latter is obtained using solutions for a constant eddy viscosity. The corrections are usually small, as is assumed when the linear Ekman pumping relation is applied in ocean modeling. However, they can become significant in circumstances involving very high wind stresses (e.g., a hurricane), or in situations where a strong narrow oceanic current flows under a region of moderate but perhaps relatively uniform surface stress.

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John E. Hart

Abstract

Laboratory experiments with finite-amplitude baroclinic waves arising from instability of a two-layer f-plane shear flow are reported. They show that as the system becomes more and more supercritical, as measured by decreasing E½/Ro, where E is the Ekman number and Ro the Rossby number associated with the driving, there are a succession of wavenumber transitions to lower and lower wavenumbers. At finite amplitude, the dominant wavenumber is considerably smaller than that predicted by linear stability theory. A simple weakly nonlinear model is constructed to interpret the laboratory results. It shows that because the longer growing waves do not extract energy as rapidly from the mean flow as the shorter ones, at finite amplitude, the preferred equilibrium states are dominated by the former. The theoretical calculation also indicates that at least near the neutral curve sideband harmonics do not substantially affect the equilibration process. In addition, a mechanism that may explain the observation of extremely long equilibration times is offered.

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William Bluman and John E. Hart

Abstract

Airbone Doppler lidar wind measurements were obtained in the lee of Mount Shasta in northern California on 28 August 1984. These data consist of line of sight wind vectors at flight level (3000 m) and along planes tilted at 1, 2 and 3 degree below the 3000 m level. The observed field is confined to a rectangular box, encompassing the mountain, that extends about 40 km downwind and about 20 km crosswind. The spatial resolution of the measured wind field is approximately 330 m.

The upstream southwesterly flow tended to circumvent the mountain although some air did rise over the peak (at 4317 m) to initiate three-dimensional internal gravity waves in the lee. These waves are delineated in the two-dimensional divergence field D, determined from the downwind velocity components on each of the tilted planes with line of sight wind vector measurements. The observed field of D exhibits a peak in its power spectrum, determined along the downstream direction, at a wavelength of about 8 km with a secondary peaks at about 17 km. Data from upper air soundings at Medford, Oregon and from onboard sensors establish that the 8 km wavelength represents the free wave response, which is determined by the airstream characteristics. Comparison with the power spectrum of the mountain slope indicates that the longer wavelength is a forced response.

Qualitative aspect of the lee-wave pattern are reproduce in a linear model with uniform airstream characteristics. However, the amplitude of the free wave response is underestimated by a factor of two, and the forced wave amplitude is about three time that of the free wave. In addition, the wave disturbance produced by the linear model decays more rapidly in the downstream direction than the observed wave. These discrepancies are interpreted in relation to physical features that are contained in the linear model.

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John A. Hart and Ariel E. Cohen

Abstract

This study is an application of the Statistical Severe Convective Risk Assessment Model (SSCRAM), which objectively assesses conditional severe thunderstorm probabilities based on archived hourly mesoscale data across the United States collected from 2006 to 2014. In the present study, SSCRAM is used to assess the utility of severe thunderstorm parameters commonly employed by forecasters in anticipating thunderstorms that produce significant tornadoes (i.e., causing F2/EF2 or greater damage) from June through October. The utility during June–October is compared to that during other months. Previous studies have identified some aspects of the summertime challenge in severe storm forecasting, and this study provides an in-depth quantification of the within-year variability of severe storms predictability. Conditional probabilities of significant tornadoes downstream of lightning occurrence using common parameter values, such as the effective-layer significant tornado parameter, convective available potential energy, and vertical shear, are found to substantially decrease during the months of June–October compared to other months. Furthermore, conditional probabilities of significant tornadoes during June–October associated with these parameters are nearly invariable regardless of value, highlighting the challenge of using objective environmental data to attempt to forecast significant tornadoes from June through October.

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John A. Hart and Ariel E. Cohen

Abstract

This study introduces a system that objectively assesses severe thunderstorm nowcast probabilities based on hourly mesoscale data across the contiguous United States during the period from 2006 to 2014. Previous studies have evaluated the diagnostic utility of parameters in characterizing severe thunderstorm environments. In contrast, the present study merges cloud-to-ground lightning flash data with both severe thunderstorm report and Storm Prediction Center Mesoscale Analysis system data to create lightning-conditioned prognostic probabilities for numerous parameters, thus incorporating null-severe cases. The resulting dataset and corresponding probabilities are called the Statistical Severe Convective Risk Assessment Model (SSCRAM), which incorporates a sample size of over 3.8 million 40-km grid boxes. A subset of five parameters of SSCRAM is investigated in the present study. This system shows that severe storm probabilities do not vary strongly across the range of values for buoyancy parameters compared to vertical shear parameters. The significant tornado parameter [where “significant” refers to tornadoes producing (Fujita scale) F2/(enhanced Fujita scale) EF2 damage] exhibits considerable skill at identifying downstream tornado events, with higher conditional probabilities of occurrence at larger values, similar to effective storm-relative helicity, both findings being consistent with previous studies. Meanwhile, lifting condensation level heights are associated with conditional probabilities that vary little within an optimal range of values for tornado occurrence, yielding less skill in quantifying tornado potential using this parameter compared to effective storm-relative helicity. The systematic assessment of probabilities using convective environmental information could have applications in present-day operational forecasting duties and the upcoming warn-on-forecast initiatives.

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John E. Hart, G. V. Rao, Henry Van De Boogaard, John A. Young, and John Findlater

Abstract

This paper describes observation of the East African low-level jet stream obtained during June and July 1977 with a long-range research aircraft. We present results based on the real-time display of data on board the aircraft during the research flights. The jet stream core was located at about 40°E between 1 and 2 km altitude. The jet had a very sharp horizontal shear layer to the west of the core, with less pronounced shear to the east extending out over the ocean. Although flowing persistently from the south, it experienced a strong diurnal change over land. In addition, other changes in structure on a longer time scale were observed. This article presents the kinematic and thermal structure of the jet, and the low-level flow further upstream. Based on these data, the estimated cross-equational water vapor flux across the equator was found to be much higher than previously thought.

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John E. Yorks, Dennis L. Hlavka, William D. Hart, and Matthew J. McGill

Abstract

Accurate knowledge of cloud optical properties, such as extinction-to-backscatter ratio and depolarization ratio, can have a significant impact on the quality of cloud extinction retrievals from lidar systems because parameterizations of these variables are often used in nonideal conditions to determine cloud phase and optical depth. Statistics and trends of these optical parameters are analyzed for 4 yr (2003–07) of cloud physics lidar data during five projects that occurred in varying geographic locations and meteorological seasons. Extinction-to-backscatter ratios (also called lidar ratios) are derived at 532 nm by calculating the transmission loss through the cloud layer and then applying it to the attenuated backscatter profile in the layer, while volume depolarization ratios are computed using the ratio of the parallel and perpendicular polarized 1064-nm channels. The majority of the cloud layers yields a lidar ratio between 10 and 40 sr, with the lidar ratio frequency distribution centered at 25 sr for ice clouds and 16 sr for altocumulus clouds. On average, for ice clouds the lidar ratio slightly decreases with decreasing temperature, while the volume depolarization ratio increases significantly as temperatures decrease. Trends for liquid water clouds (altocumulus clouds) are also observed. Ultimately, these observed trends in optical properties, as functions of temperature and geographic location, should help to improve current parameterizations of extinction-to-backscatter ratio, which in turn should yield increased accuracy in cloud optical depth and radiative forcing estimates.

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Ryan C. Bunker, Ariel E. Cohen, John A. Hart, Alan E. Gerard, Kim E. Klockow-McClain, and David P. Nowicki

Abstract

Tornadoes that occur at night pose particularly dangerous societal risks, and these risks are amplified across the southeastern United States. The purpose of this study is to highlight some of the characteristics distinguishing the convective environment accompanying these events. This is accomplished by building upon previous research that assesses the predictive power of meteorological parameters. In particular, this study uses the Statistical Severe Convective Risk Assessment Model (SSCRAM) to determine how well convective parameters explain tornado potential across the Southeast during the months of November–May and during the 0300–1200 UTC (nocturnal) time frame. This study compares conditional tornado probabilities across the Southeast during November–May nocturnal hours to those probabilities for all other November–May environments across the contiguous United States. This study shows that effective bulk shear, effective storm-relative helicity, and effective-layer significant tornado parameter yield the strongest predictability for the November–May nocturnal Southeast regime among investigated parameters. This study demonstrates that November–May southeastern U.S. nocturnal predictability is generally similar to that within other regimes across the contiguous United States. However, selected ranges of multiple parameters are associated with slightly better predictability for the nocturnal Southeast regime. Additionally, this study assesses conditional November–May nocturnal tornado probabilities across a coastal domain embedded within the Southeast. Nocturnal coastal tornado predictability is shown to generally be lower than the other regimes. All of the differences highlight several forecast challenges, which this study analyzes in detail.

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