Richard W. Moore

The Modulation of the Subtropical and Extratropical Atmosphere in the Pacific Basin in Response to the Madden–Julian Oscillation

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 (c...

Reexamining the Dynamics of Short-Scale, Diabatic Rossby Waves and Their Role in Midlatitude Moist Cyclogenesis

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...

Analysis of an Idealized, Three-Dimensional Diabatic Rossby Vortex: A Coherent Structure of the Moist Baroclinic Atmosphere

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 s...

A Global Climatology of Baroclinically Influenced Tropical Cyclogenesis

AbstractTropical cyclogenesis is generally considered to occur in regions devoid of baroclinic structures; however, an appreciable number of tropical cyclones (TCs) form in baroclinic environments each year. A global climatology of these baroclinically influenced TC developments is presented in this study. An objective classification strategy is developed that focuses on the characteristics of the environmental state rather than on properties of the vortex, thus allowing for a pointwise “development pathway” classification of reanalysis data. The resulting climatology shows that variability within basins arises primarily as a result of local surface thermal contrasts and the positions of time-mean features on the subtropical tropopause. The pathway analyses are sampled to generate a global climatology of 1948–2010 TC developments classified by baroclinic influence: nonbaroclinic (70%), low-level baroclinic (9%), trough induced (5%), weak tropical transition (11%), and strong tropical transition (5%). All ...

The Integral Role of a Diabatic Rossby Vortex in a Heavy Snowfall Event

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 conjunct...

Mesoscale convective vortex formation in a weakly sheared moist neutral environment

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...

Genesis Criteria for Diabatic Rossby Vortices: A Model Study

AbstractA 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...

The Baroclinic Moisture Flux

AbstractAs 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 circulat...