Daniel Keyser

A Review of the Structure and Dynamics of Upper-Level Frontal Zones

Abstract This article presents a review of upper-level fronts with the intent of synthesizing observational and modeling studies into a conceptual and dynamical description of these fronts and their evolution relative to the life cycle of midlatitude baroclinic waves. The discussion begins by tracing present-day concepts concerning the structure of upper-level frontal systems, which are based on composite analyses of radiosonde and aircraft data, from their origins in the pioneering analyses of upper-air data in the 1930s. Perspectives from scales both smaller and larger than upper-level frontal systems are provided respectively by considering the effects of turbulent processes on frontal structure and dynamics and by relating variations in frontal structure to the evolution of the baroclinic waves that provide the dynamical environment for upper-level frontogenesis. The dynamics of upper-level fronts are shown to comprise the interactions between the primary (geostrophic) and secondary (ageostrophic) cir...

Numerical Simulation of Frontogenesis in a Moist Atmosphere

Abstract This paper describes the effect of condensation and evaporation on mesoscale frontal circulations in a two-dimensional numerical model. Utilizing an explicit scheme for the prediction of water vapor, cloud water and rainwater, the model is used to investigate the interactions between convection and the larger-scale environment. The model results are qualitatively compared with results of theoretical and observational studies, including those from the recent Severe Environmental Storms and Mesoscale Experiment-Atmospheric Variability Experiment (SESAME-AVE). Three major differences are observed in a comparison of the moist and dry simulations: 1) The speed of the upper- and lower-level jets was significantly higher in the moist case, 2) The intensity of the ageostrophic circulations in the moist simulation was much stronger, 3) The vertical velocity field in the moist case was characterized by a banded structure not present in the dry case.

Fronts, Jet Streams and the Tropopause

The advent of kite and balloon-borne meteorograph soundings during the early 1900s and the subsequent deployment of regional rawinsonde networks provided the observational basis for the study of the spatial and temporal evolution of fronts, jet streams and the tropopause. During the mid-century years (1935–1965), researchers focused on the structural characteristics of fronts and their associated jet streams near the tropopause, and on the diagnosis of the frontogenetic processes and secondary circulations governing their life cycles. The pioneering observational study by J. Bjerknes and E. Palm n (1937) showed fronts to be transitional zones of finite width (~100 km) and depth (~1 km), rather than near zero-order discontinuities extending from the surface to the tropopause. Newton (1954) presented the most comprehensive diagnosis of all components of upper-level frontogenesis during this period, and Sawyer (1956) and Eliassen (1962) derived the diagnostic theory for geostrophically forced secondary circulations about fronts based on the semigeostrophic equations, which was later expanded to the temporal dimension by Hoskins (1971) and Hoskins and Bretherton (1972).

A Composite Study of the Interactions between Tropical Cyclones and Upper-Tropospheric Troughs

Abstract The objective of this study is to understand how interactions with upper-tropospheric troughs affect the intensity of tropical cyclones. The study includes all named Atlantic tropical cyclones between 1985 and 1996. To minimize other factors affecting intensity change, times when storms are over subcritical sea surface temperatures (≤26°C) or near landfall are removed from the sample. A trough interaction is defined to occur when the eddy momentum flux convergence calculated over a 300–600-km radial range is greater than 10 (m s−1) day−1. The trough interaction cases are separated into four composites: (i) favorable superposition [tropical cyclone intensifies with an upper-tropospheric potential vorticity (PV) maximum within 400 km of the tropical cyclone center], (ii) unfavorable superposition, (iii) favorable distant interaction (upper PV maximum between 400 and 1000 km from the tropical cyclone center), and (iv) unfavorable distant interaction. For comparison, two additional composites are cre...

The Effect of Large-Scale Flow on Low-Level Frontal Structure and Evolution in Midlatitude Cyclones

Abstract Observational and modeling studies documented in the literature indicate that the large-scale flow has an important effect on the structure and evolution of low-level fronts in midlatitude cyclones. The purpose of this paper is to address the role of the large-scale flow on low-level cyclone/frontal structure and evolution through a combined observational and idealized modeling approach. Analyses of two observed cyclone cases embedded in large-scale diffluence and confluence, respectively, are presented to illustrate two possible cyclone/frontal structures and evolutions. Specifically, the cyclone moving into a diffluent, high-amplitude ridge becomes meridionally elongated and possesses a strong meridionally oriented cold front and a weak warm front. The cold front rotates into the warm front, forming an occluded front in the manner of the Norwegian cyclone model, as indicated by the narrowing of the thermal ridge connecting the warm sector to the cyclone center. In contrast, the cyclone moving i...

The Ohio Valley Wave-Merger Cyclogenesis Event of 25–26 January 1978. Part II: Diagnosis Using Quasigeostrophic Potential Vorticity Inversion

Abstract The dynamical interactions between precursor disturbances during the wave-merger cyclogenesis event of 25–26 January 1978 over eastern North America are diagnosed using quasigeostrophic potential vorticity (QGPV) inversion. This case is characterized by two prominent preexisting upper-level disturbances that induce rapid surface cyclogenesis as they come into close proximity. Static QGPV inversion is used to attribute a particular geopotential height field to the QGPV associated with each precursor disturbance. The full flow is partitioned into the following components: the northern upper precursor, the southern upper precursor, and the background flow. Prognostic QGPV inversion is used to quantify the instantaneous geopotential height tendencies attributable to each of these flow components. The static-inversion results for the upper precursors exhibit the structure of baroclinic vortices with maximum amplitude near the tropopause. During the 48-h period spanning the period of study of this even...

The Influence of Planetary Boundary Layer Physics on Frontal Structure in the Hoskins-Bretherton Horizontal Shear Model

Abstract A series of numerical experiments with the Hoskins-Bretherton horizontal shear model of frontogenesis in an, amplifying, two-dimensional baroclinic wave is performed. The analytic solutions from the Boussinesq, semi-geostrophic model provide initial conditions for numerical integrations with a two-dimensional, dry version of the fully compressible, hydrostatic primitive equation (PE) model of Anthes and Warner with 40 km horizontal resolution. The PE model is integrated 1) without planetary boundary layer (PBL) physics; 2) with a one-layer bulk-drag scheme; and 3) with a high-vertical-resolution PBL model. The lower boundary is thermally insulated in order to isolate the effect of the internal mixing of heat in the PBL. The simulation with the high-resolution PBL physics resolves several realistic features including 1) a narrow updraft at the top of the PBL above the sea-level pressure trough at the warm edge of the frontal zone; 2) a stable layer capping the PBL to the rear of the frontal zone; ...

A Two-Dimensional Primitive Equation Model of Frontogenesis Forced by Confluence and Horizontal Shear

Abstract A two-dimensional primitive equation model of frontogenesis forced by a combination of confluence and horizontal shear is formulated for dry, nearly adiabatic and inviscid conditions. The frontogenetical forcing mechanisms are included by respectively specifying the cross-front and vertical variation of the cross-front geostrophic wind component. The results of three numerical integrations containing confluent forcing are analyzed and discussed in detail. The first is the case of pure confluence, in which the vertical shear of the cross-front geostrophic component is zero. The second and third cases respectively consist of negative and positive vertical shear of the cross-front geostrophic component, which correspond to cold and warm advection at upper levels for the configuration of the alongfront wind component. The above frontogenetical forcing and the resulting frontal structures are related to typical flow patterns occurring within midlatitude baroclinic waves during various stages in their ...

Tests of a Fine-Mesh Model over Europe and the United States

Abstract Thirty–two 24 h forecasts have been run over western Europe and the eastern United States using a six-layer, 60 km mesh primitive equation model. The forecasts show considerable skill in forecasting cyclogenesis over the Mediterranean and the United States in spite of the inadvertent neglect of surface friction over half of the domain. The average 24 h S 1score for sea level pressure is 39.1 compared to an average of 45.9 for Fleet Numerical Weather Centrals operational model and 73.4 for persistence. The initialization scheme is based on an objective analysis of the horizontal wind field. Following the wind analysis, we infer geopotential and temperature from the rotational part of the wind with a nonlinear form of the balance equation. We present detailed results from one initial analysis and error statistics from 30 analyses occurring from December 1977 through April 1978. Typical root-mean-square (rms) differences between first-guess and balanced analyses of geopotential and temperature are ...

The Applicability of a Mixed–Layer Model of the Planetary Boundary Layer to Real-Data Forecasting

Abstract The mesoscale numerical model of the planetary boundary layer (PBL), which Lavoie applied to lake-effect snowstorm and to airflow over the Hawaiian Islands, is modified and utilized to assess the feasibility of producing short-range real-data forecasts of low-level flow patterns. The dry model atmosphere comprises three layers. A parameterized surface layer of fixed depth (50 m) follows the variable terrain and allows vertical fluxes of heat and momentum to affect the flow in the overlying PBL or “mixed layer.” The horizontal wind velocity and potential temperature, both prognostic variables, are assumed to be independent of height in the mixed layer. The height of the top of the mixed layer is an additional prognostic variable. A parameterized stable layer, characterized by a vertically constant potential temperature lapse rate, overlies the mixed layer. Synoptic-scale patterns of pressure and potential temperature are specified at the top of this uppermost layer as upper boundary conditions. En...