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Richard W. Dixon
and
Todd W. Moore

Abstract

Tornado vulnerability depends on the incidence of and societal exposure to tornadoes for a particular location. This study assesses the vulnerability of Texas counties to tornadoes using tornado incidence and societal exposure composite scores. Three different assessment methods are used to quantify tornado vulnerability and a geographical information system is used for visualization. Using multiple assessment methods facilitates different ways of viewing tornado vulnerability. Even though the three tornado vulnerability maps produced in this study are spatially diverse, some counties were repeatedly identified as highly vulnerable. The most highly vulnerable counties were located within the northern and northeastern portions of the state, specifically in the northeastern corner within the Shreveport, Louisiana, county warning area.

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Richard W. Moore
and
Michael T. Montgomery

Abstract

The fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5) is used to conduct a number of idealized numerical simulations to confirm recent findings of an alternate growth mechanism in a moist baroclinic environment to that of traditional baroclinic instability. In this alternate growth scenario, disturbance growth depends on the presence of sufficient environmental moisture and baroclinicity. The resulting coherent structure, termed a diabatic Rossby vortex (DRV), grows as a result of an approximate phase locking and mutual amplification of two diabatically generated potential vorticity (PV) anomalies: a low-level positive (cyclonic) PV anomaly and a midtropospheric negative (anticyclonic) PV anomaly.

The three-dimensional structure of a DRV is found to be qualitatively very similar to that seen in previous two-dimensional model simulations. The most apparent structural discrepancy from the two-dimensional model is the increased strength of the midtropospheric negative PV anomaly in the three-dimensional simulations.

A sensitivity study is undertaken to better understand the dependence of the DRV dynamics on some of the more pertinent environmental and perturbation vortex parameters, and to quantify the effect of each parameter. The resulting intensity of a DRV is most sensitive to the magnitude of environmental baroclinicity and moisture content, while the vertical profile of moisture is the most dominant factor in determining the characteristic depth of the DRV. It is also found that the “size” and “amplitude” of a precursor perturbation vortex are important factors in determining the track and intensity of the ensuing DRV.

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Richard W. Moore
and
Michael T. Montgomery

Abstract

As a step toward a more comprehensive study of the physical processes that underlie explosive cyclogenesis, a two-dimensional, semigeostrophic model with a now standard parameterization of latent heat release is used to diagnose the structure, energetics, and propagation characteristics of short-scale, diabatic normal modes in a moist, baroclinic atmosphere with the Eady basic state.

Upon revisiting the inviscid problem, it is found that when a thermodynamically consistent vertical profile of latent heat release is used, the short-wave cutoff vanishes, and growth rates become independent of zonal wavelength for zonal wavelengths shorter than approximately 1900 km. The destabilized short-scale modes, identified previously as diabatic Rossby waves, owe their existence to the continuous generation of potential vorticity by moist processes associated with warm air advection, rising motion, and latent heat release.

To determine if these short-scale, diabatic Rossby wave modes continue to grow at small but finite amplitudes in the presence of frictional damping, a standard Ekman boundary layer with quadratic surface drag is employed. It is found that exponential growth is robust over a wide range of zonal wavelengths during the incipient phase of moist cyclogenesis and that, once generated, the short-scale, diabatic modes persist even in the presence of “realistic” surface friction.

Finally, observations have shown that a number of explosive cyclones exhibit two stages of development. The first stage, prior to the period of most rapid deepening, involves the spinup of surface vorticity that can occur independent of upper-level forcing. The results of this study, in conjunction with recent observational work, provide compelling evidence that a diabatic Rossby wave may possibly serve as a precursor, low-level cyclonic disturbance.

It is advocated here that an understanding of these small-scale, “moist” coherent structures is vital to describe the life cycle of extratropical cyclones in moist, baroclinic environments.

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Richard W. Moore
and
Thomas H. Vonder Haar

Abstract

Broadband radiative flux data from the Earth Radiation Budget Experiment (ERBE) are used to document the interannual variability of net cloud forcing over the lifetime of the ERBE scanner instruments. The study focuses on the high variability observed in the Pacific basin during the Northern Hemisphere winter from 1984 to 1990. This period captures the El Niño season of 1986/87 and the La Niña season of 1988/89, with an average state biased toward La Niña–type conditions. Clouds are found to cool the Pacific basin by approximately 23 W m−2 over the study period. Interannual variations of net cloud forcing vary by less than 5%, with a decrease of cloud cooling during the El Niño and an increase during the La Niña.

A key aspect of this study is the concurrent analysis of cloud type and amount information from the International Satellite Cloud Climatology Project (ISCCP). It demonstrates that the variability of middle-thick and high-thin clouds is integral to the observed changes in net cloud forcing. A subsequent analysis of ERBE net radiation measurements reveals that the geographic redistribution of cloudiness observed in the ISCCP data results in an increase of the meridional energy gradient during the El Niño and a decrease during the La Niña.

ECMWF reanalyses data are used to document and discuss the interannual variability of the meridional transport of atmospheric energy and the atmospheric circulation. During the 1986/87 El Niño, it is found that the transport of atmospheric energy from the Tropics and subtropics to higher latitudes increases by approximately 6% from the study average. Conversely, the transport decreases by about 2% during the 1988/89 La Niña. An investigation of the variability of the structure and the strength of the meridional energy transport by the mean meridional circulation, stationary eddies, and transient eddies is then used to diagnose changes to the atmospheric circulation.

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Richard W. Moore
,
Olivia Martius
, and
Thomas Spengler

Abstract

The 40-yr ECMWF Re-Analysis (ERA-40) data are combined with a number of novel climatologies to conduct a comprehensive examination of the response of the subtropical and extratropical atmosphere over the Pacific basin to an evolving Madden–Julian oscillation (MJO) event. The adopted approach constitutes a symbiosis of a climatological analysis during the Northern Hemisphere winter from 1979 to 2002 and a case study analysis of a distinct MJO event that occurred in January–February 1993. The former is designed to obtain the general characteristics observed during a composite MJO life cycle, while the latter is used to provide insight into the instantaneous mechanisms responsible for the observed composite evolution.

A primary component of the study involves the diagnosis of anomalous wave breaking activity in response to MJO forcing in the form of tropical convection and/or upper-level divergence. Wave breaking events are separated by their characteristic life cycles: LC1 (anticyclonic) and LC2 (cyclonic) events. Statistically significant anomalies in wave breaking activity are found to be prevalent during the composite MJO event. Furthermore, the dynamical distinction between LC1 and LC2 wave breaking is useful in that the two different characteristic life cycles exhibit significantly different anomalous behavior during the MJO.

Statistically significant variability is also identified in both the subtropical and extratropical flow and atmospheric blocking and surface cyclone frequency. These data, taken in conjunction with the observed evolution of the 1993 MJO event, provide a relatively coherent picture of the response of the atmosphere to MJO forcing. A schematic representation of the evolution is presented.

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Richard W. Moore
and
Thomas H. Vonder Haar

Abstract

Data from the Advanced Microwave Sounding Unit (AMSU) are used to examine a polar low that occurred in the Labrador Sea on 17–18 March 2000. During its 40-h lifetime, the polar low was observed three times by AMSU, which captured the formation and subsequent intensification of the storm. The AMSU-A channel-5 (53.6 GHz) brightness temperature field clearly identifies the warm core structure of the polar low, with storm center measurements 2–3 K higher than the background environment. Analysis of these data over time can provide a straightforward and real-time method for tracking storm motion and estimating surface wind speed. The impact of cloud, surface, and moisture variability on the measurements at 53.6 GHz is examined. Although they cannot account for the magnitude of warming, the analysis of additional AMSU frequencies illustrates not only how nonatmospheric temperature effects can subtly influence the structure of the channel-5 brightness temperature field but also how they can provide insight into the polar low and its environment.

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Richard W. Moore
,
Michael T. Montgomery
, and
Huw C. Davies

Abstract

On 24–25 February 2005, a significant East Coast cyclone deposited from 4 to nearly 12 in. (∼10–30 cm) of snow on parts of the northeastern United States. The heaviest snowfall and most rapid deepening of the cyclone coincided with the favorable positioning of an upper-level, short-wave trough immediately upstream of a preexisting surface cyclone. The surface cyclone in question formed approximately 15 h before the heaviest snowfall along a coastal front in a region of frontogenesis and heavy precipitation. The incipient surface cyclone subsequently intensified as it moved to the northeast, consistently generating the strongest convection to the east-northeast of the low-level circulation center. The use of potential vorticity (PV) inversion techniques and a suite of mesoscale model simulations illustrates that the early intensification of the incipient surface cyclone was primarily driven by diabatic effects and was not critically dependent on the upper-level wave. These facts, taken in conjunction with the observed structure, energetics, and Lagrangian evolution of the incipient surface disturbance, identify it as a diabatic Rossby vortex (DRV). The antecedent surface vorticity spinup associated with the DRV phase of development is found to be integral to the subsequent rapid growth. The qualitative similarity with a number of observed cases of explosive cyclogenesis leaves open the possibility that a DRV-like feature comprises the preexisting positive low-level PV anomaly in a number of cyclogenetic events that exhibit a two-stage evolution.

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Richard W. Moore
,
Michael T. Montgomery
, and
Huw Davies

Abstract

A suite of idealized mesoscale model simulations are conducted to examine the dynamic pathway to the genesis of short-scale, moist baroclinic disturbances. It is shown that in an initially subsaturated environment, two distinct stages of development are necessary before a preexisting surface-concentrated, warm-core vortex can begin to amplify. The first stage, termed environmental preconditioning, involves the moistening of the lower atmosphere via the transport of relatively high equivalent potential temperature air into the immediate environment of the translating vortex.

The second stage results from continuous cloud diabatic processes and it involves the emergence of a low-level positive potential vorticity (PV) anomaly with some evidence of a further, more diffuse, negative PV anomaly at higher elevations. The PV structure is characteristic of a diabatic Rossby vortex (DRV). The disturbance does not begin to amplify until the magnitude and coherence of the low-level PV structure allows for the sufficient production of eddy available potential energy through diabatic processes to overcome frictional dissipation, and this necessitates forced convection for a finite period of time. A comparison of the simulated disturbance structure with observed DRVs lends credence to the idealized model results.

Furthermore, the simulations facilitate the identification of an amplitude threshold for DRV genesis that is defined as a function of both environmental parameters (baroclinicity and moisture content) and the strength of an initial disturbance. In particular, if, given a background environment, the amplitude of an initial disturbance is not sufficiently large, frictional processes will inhibit the genesis of a growing disturbance within a realistic time frame.

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Ron McTaggart-Cowan
,
John R. Gyakum
, and
Richard W. Moore

Abstract

As subsaturated air ascends sloping isentropic surfaces, adiabatic expansion results in cooling and relative moistening. This process is an effective way to precondition the atmosphere for efficient moist processes while bringing parcels to saturation, and thereafter acts to maintain saturation during condensation. The goal of this study is to develop a diagnostic quantity that highlights circulations and regions in which the process of parcel moistening by isentropic ascent is active. Among the many features that rely on this process for the generation of an important fraction of their energy are oceanic cyclones, transitioning tropical cyclones, warm conveyor belts, diabatic Rossby vortices, and predecessor rain events. The baroclinic moisture flux (BMF) is defined as moisture transport by the component of vertical motion associated with isentropic upgliding. In warm conveyor belt and diabatic Rossby vortex case studies, the BMF appears to be successful in identifying the portion of the circulation in which this process is actively bringing parcels to saturation to promote the formation of clouds and precipitation. On a broader scale, the climatological maxima of the BMF highlight regions in which parcel moistening by isentropic ascent is anticipated to have a nonnegligible impact on the atmospheric state either through the action of the mean flow or via the repeated occurrence of isolated large-BMF events. The process-centric foundation of the BMF makes it useful as a filtering or exploratory variable, with the potential for extension into predictive applications.

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Robert J. Conzemius
,
Richard W. Moore
,
Michael T. Montgomery
, and
Christopher A. Davis

Abstract

Idealized simulations of a diabatic Rossby vortex (DRV) in an initially moist neutral baroclinic environment are performed using the fifth-generation National Center for Atmospheric Research–Pennsylvania State University (NCAR–PSU) Mesoscale Model (MM5). The primary objective is to test the hypothesis that the formation and maintenance of midlatitude warm-season mesoscale convective vortices (MCVs) are largely influenced by balanced flow dynamics associated with a vortex that interacts with weak vertical shear. As a part of this objective, the simulated DRV is placed within the context of the Bow Echo and Mesoscale Convective Vortex Experiment (BAMEX) field campaign by comparing its tangential velocity, radius of maximum winds, CAPE, and shear with the MCVs observed in BAMEX.

The simulations reveal two distinct scales of development. At the larger scale, the most rapidly growing moist baroclinic mode is excited, and exponential growth of this mode occurs during the simulation. Embedded within the large-scale baroclinic wave is a convective system exhibiting the characteristic DRV development, with a positive potential vorticity (PV) anomaly in the lower troposphere and a negative PV anomaly in the upper troposphere, and the positive/negative PV doublet tilted downshear with height. The DRV warm-air advection mechanism is active, and the resulting deep convection helps to reinforce the DRV against the deleterious effects of environmental shear, causing an eastward motion of the convective system as a whole.

The initial comparisons between the simulated DRVs and the BAMEX MCVs show that the simulated DRVs grew within background conditions of CAPE and shear similar to those observed for BAMEX MCVs and suggest that the same dynamical mechanisms are active. Because the BAMEX field campaign sampled MCVs in different backgrounds of CAPE and shear, the comparison also demonstrates the need to perform additional simulations to explore these different CAPE and shear regimes and to understand their impacts on the intensity and longevity of MCVs. Such a study has the additional benefit of placing MCV dynamics in an appropriate context for exploring their relevance to tropical cyclone formation.

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