Phillip J. Smith

A Diagnosis of the Explosive Development of Two Extratropical Cyclones

Abstract This paper examines the 24-h explosive development periods of two extratropical cyclones, the first occurring over the Gulf Stream off the coast of New England from 18 to 19 January 1979 and the second occurring over the southeastern United States from 20 to 21 January 1979. The data used in this study are the First GARP (Global Atmospheric Research Program) Global Experiment (FGGE) level IIIb (SOP I) global analyses on a 4° latitude × 5° longitude grid. The parameter used to diagnose development is the geostrophic relative vorticity tendency calculated using an extended form of the Zwack-Okossi development equation. This development equation is similar to the Petterssen-Sutcliffe development equation, but is shown to be more complete by explicitly coupling surface development with forcing at all levels above the surface. Cyclonic-vorticity advection, warm-air advection, and latent heat release act to develop the two cyclones, while adiabatic cooling in the ascending air opposes development. F...

Direct and Indirect Effects of Latent Heat Release on a Synoptic-Scale Wave System

Abstract The primary goal of this paper is to diagnose, the “direct” and “indirect” effects of latent heat release on a synoptic-scale wave system containing an extratropical cyclone that developed over the eastern United States. To achieve this goal, comparisons are made between MOIST (full model physics) and DRY (latent heating removed) predictions of the wave system during the period 27–29 February 1984 using the National Meteorological Centers Limited-Area Fine Mesh (LFM) model. Both the MOIST and DRY models predict significant cyclone systems, suggesting that the background adiabatic forcing is quite important. However, the DRY model predict a weaker cyclone. The direct and indirect latent heating influences are diagnosed using eddy energy quantities and the extended height tendency equation. Direct effects are restricted to the diabatic generation of available potential energy and height tendencies forced by diabatic heating. Results show that latent heating exerts an important direct influence on ...

A Comparison of Adiabatic and Diabatic Forcing in an Intense Extratropical Cyclone System

Abstract The primary goal of this study is to compare the principal adiabatic and diabatic mechanisms responsible for the behavior of an extratropical cyclone system. To accomplish this goal, the height tendency is solved from two forms of the height tendency equation. One is the classical quasi-geostrophic (QG) form; the other is a modified form identified as the ‘extended’ height tendency equation. The latter retains the essential components of the QG form but replaces the geostrophic wind where it appears in the equation by the observed wind, adds the effects of diabatic heating (latent heat release), and allows three dimensional varying static stability. In this study, the behavior of an intense cyclone is poorly described by the QG form but is realistically represented by the extended form. Using this latter diagnostic tool, the evolution of a 9–11 January 1975 cyclone event is analyzed with the following major results: 1) a comparison of the terms included in the extended height tendency equation in...

A Composite Diagnosis of Synoptic-Scale Extratropical Cyclone Development over the United States

Abstract This paper presents a composite diagnosis of synoptic-scale forcing mechanisms associated with extratropical cyclone evolution. Drawn from 12 cyclone cases that occurred over the continental United States during the cool season months, the diagnosis provides a “climatology” of development mechanisms for difference categories of cyclone evolution ranging from cyclone weakening through three stages of cyclone intensification. Computational results were obtained using an “extended” form of the Zwack–Okossi equation applied to routine upper-air and surface data analyzed on a 230 km × 230 km grid. Results show that cyclonic vorticity advection, which maximizes in the upper troposphere, was the primary contributor to cyclone development regardless of the stage of development. A second consistent contributor to development was latent heat release. Horizontal temperature advection, often acknowledged as a development mechanism, was found to contribute to development only during more intense stages. Durin...

Problems of Large-Scale Kinetic Energy Balance—A Diagnostic Analysis in GARP

Currently available diagnostic studies on kinetic energy balance in the general circulation are reviewed as one of the basic scientific problems in GARP. The kinetic energy equation and several different approaches in the evaluation of energy variables are discussed in relation to real and modeled atmospheric data. Energy balance problems in the middle latitudes are examined in terms of linkages between processes from energy conversion to dissipation, balance within various systems of circulation, and interactions with sub-synoptic scale disturbances. The kinetic energy budget in large-scale disturbances and the general flow of the tropical circulation are contrasted with those in the middle latitudes. By clarifying the current essential problems in the energetics of the middle latitudes and tropics, ongoing diagnostic studies at the University of Missouri—Columbia and Purdue University are identified in the context of the GARP.

The Interactions between a Midlatitude Blocking Anticyclone and Synoptic-Scale Cyclones That Occurred during the Summer Season

Abstract Using the Goddard Laboratory for Atmospheres Goddard Earth Observing System 5-yr analyses and the Zwack–Okossi equation as the diagnostic tool, the horizontal distribution of the dynamic and thermodynamic forcing processes contributing to the maintenance of a Northern Hemisphere midlatitude blocking anticyclone that occurred during the summer season were examined. During the development of this blocking anticyclone, vorticity advection, supported by temperature advection, forced 500-hPa height rises at the block center. Vorticity advection and vorticity tilting were also consistent contributors to height rises during the entire life cycle. Boundary layer friction, vertical advection of vorticity, and ageostrophic vorticity tendencies (during decay) consistently opposed block development. Additionally, an analysis of this blocking event also showed that upstream precursor surface cyclones were not only important in block development but in block maintenance as well. In partitioning the basic data ...

The Impact of Latent Heat Release on Synoptic-Scale Vertical Motions and theDevelopment of an Extratropical Cyclone System

Abstract This study investigates the influence of stable and convective latent heat release on synoptic-scale vertical motions and the corresponding evolution of an extratropical cyclone during a 48 h period of strong development. The cyclones early evolution was dominated by dry dynamical processes. By midway through the period, however, forcing by latent heat release accounted for over 50% of the upward vertical motions, with the convective component dominating. The cyclones development was most intense during the second 24 h, despite a decrease in latent heat release. During the latter period, the reduced direct latent heat influence may have been augmented by an indirect influence, in which pre-existing dry dynamical forcing was enhanced by diabatic intensification of vorticity and thermal gradients.

The Importance of the Horizontal Distribution of Heating during Extratropical Cyclone Development

Abstract Diagnostic and modeling results reveal that atmospheric heating typically acts to intensify extratropical cyclones. In addition, both the Petterssen–Sutcliffe and Zwack–Okossi development equations reveal that this relationship depends on the proportionality that exists between surface geostrophic vorticity tendency and the negative of the horizontal Laplacian of atmospheric heating. Because of this Laplacian relationship, the impact of a heating field with a given magnitude and vertical distribution depends on its horizontal distribution. This paper will show how horizontal heating distributions that differ by relatively small amounts over their entire extent can yield vorticity tendency responses that could contribute to either development or decay of an underlying cyclone.

A Synoptic Analysis of the 25–26 January 1978 Blizzard Cyclone in the Central United States

Based on detailed surface analyses and National Meteorological Center (NMC) facsimile products, the evolution of the January 1978 midwestern blizzard-producing cyclone is described. The behavior of the storm is contrasted with the actions of typical winter cyclones. Forecasts by the Limited-Area Fine Mesh (LFM) numerical model are compared with observed conditions. An injection of polar air, resulting in the revitalization of the baroclinic zone, and concurrent strong upper-air flow are found to be important factors in the dramatic intensification of the low. The movement and deepening of the low, although not unprecedented, were found to deviate substantially from normal winter cyclone behavior. Although LFM forecasts underestimated the deepening of the low, the amount of predicted intensification was substantial.

A Diagnosis of a Model-Simulated Explosively Developing Extratropical Cyclone

Abstract This paper examines a simulation of the explosive development phase of the ERICA IOP 4 extratropical cyclone case obtained from the 70-km Limited Area Mesoscale Prediction System. A detailed diagnosis of the simulated cyclone is performed using the Zwack–Okossi equation to examine the forcing mechanisms influencing the near-surface synoptic-scale pressure tendency. The diagnosis includes two special features, the inclusion of forcing terms representing synoptic-subsynoptic exchange processes and a unique approach to evaluating the contribution of individual levels to the vertically integrated forcing. Explosive cyclogenesis was initiated by the combined effects of warm-air advection, cyclonic vorticity advection, and latent heat release, the first two maximizing above 400 mb and the latter below 750 mb. At the cyclone center the temperature advection profiles also featured a secondary warm-air advection maximum below 800 mb whose impact on surface pressure tendency was often equal to or greater t...