Abstract

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Abstract

Trough merger, defined as the amalgamation of two separate vorticity centers in distinct branches of westerlies into one coherent vorticity center, is studied for a case In November 1980. The analysis approach takes advantage of the failure of the operational Limited-Area Fine-Mesh Model (LFM) to simulate the mow process. A diagnostic analysis of the observed and forecast vorticity and thermodynamic structure is used to help isolate physical processes occurring during trough merger.

During the period 17–19 November 1980 major cyclogenesis occurred along the coast of North America as a deepening 500 mb trough in the northern branch of the westerlies merged with a weaker trough in the southern branch of the westerlies. Cyclonic vorticity in the middle troposphere ahead of the northern trough was generated by: 1) horizontal convergence in the lower and middle troposphere and the upward advection of this vorticity and, 2) horizontal advection in the middle and upper troposphere. Frontogenesis along the 500 mb asymptote of confluence contributed to deep tropospheric ascent and enhanced the baroclinity. The overall baroclinic zone shifted northward ahead of the deepening northern trough as warming due to horizontal advection overwhelmed adiabatic cooling due to ascent.

The LFM forecasts were deficient in all of these processes. In particular, the failure of the model to generate deep tropospheric ascent combined with an overprediction of the cooling due to adiabatic ascent precluded the model from shifting the principal baroclinic zone northward. A major quantitative precipitation forecast error resulted from the failure to predict trough merger.

Abstract

The methodology developed by Keyser et al. for representing and diagnosing three-dimensional vertical circulations in baroclinic disturbances using a two-dimensional vector streamfunction, referred to as the psi vector, is restricted to f-plane channel-model geometry. The vertical circulation described by the psi vector consists of the irrotational (or divergent) part of the ageostrophic wind and the vertical velocity. A key property of the psi vector is that its projections onto arbitrarily oriented orthogonal vertical planes yield independent vertical circulations, allowing separation of a three-dimensional vertical circulation into two two-dimensional components, and thus objective assessment of the extent to which a three-dimensional vertical circulation is oriented in a preferred direction. Here the methodology for determining the psi vector is modified to be suitable for real-data applications. The modifications consists of reformulating the diagnostic equations to apply to conformal map projections and to limited-area domains; despite the desirability of incorporating topography, this is deferred to future research. The geostrophic wind is defined in terms of constant Coriolis parameter, rendering it nondivergent and thus confining the horizontal divergence to the ageostrophic wind. The ageostrophic wind is partitioned into harmonic, rotational, and divergent components. This three-field with its counter-part determined from the psi-vector calculation.

The modified psi-vector methodology is illustrated for two well-documented East Coast midlatitude cyclones. The first case (the President's Day storm: 1200 UTC 19 February 1979) considers an interpretation that ascent in the vicinity of a curved upper-level jet-front system may be viewed as a superposition of contributions from cross-stream divergent ageostrophic flow associated wit a jet streak and from alongstream divergent ageostrophic flow associated with a baroclinic wave. The second case (the megalopolitan strom: 1200 UTC 11 February 1983) addresses the hypothesis of Uccellini and Kocin that vertical circulations transverse to meridionally displaced upper-tropospheric jet streaks are coupled in a lateral sense. In both of these cases, the diagnoses reveal that the cross-stream component of the divergent ageostrophic circulations isolates meaningful mesoscale signatures coinciding with regions of precipitation and ascent in the vicinity of upper-level jet-front systems whereas the alongstream component is indicative of synoptic-scale vertical motion. Furthermore, it is found that the cross-contour ageostrophic flow, necessary for a Lagrangian rates of change of kinetic energy in jet entrance and exit regions, is due primarily to the nondivergent (i.e., harmonic plus rotational) ageostrophic wind. This result suggests that the practice of linking cross-contour ageostrophic winds and vertical motions in jet entrance and exit regions in the qualitative assessment of energy tranformations in these regions may be problematic in the case of upper-level jet-front system situated in three-dimensional flows.

The Institute for Naval Oceanography, in cooperation with Naval Research Laboratories and universities, executed the Data Assimilation and Model Evaluation Experiment (DAMÉE) for the Gulf Stream region during fiscal years 1991–1993. Enormous effort has gone into the preparation of several high-quality and consistent datasets for model initialization and verification. This paper describes the preparation process, the temporal and spatial scopes, the contents, the structure, etc., of these datasets.

The goal of DAMÉE and the need of data for the four phases of experiment are briefly stated. The preparation of DAMÉE datasets consisted of a series of processes: 1) collection of observational data; 2) analysis and interpretation; 3) interpolation using the Optimum Thermal Interpolation System package; 4) quality control and reanalysis; and 5) data archiving and software documentation.

The data products from these processes included a time series of 3D fields of temperature and salinity, 2D fields of surface dynamic height and mixed-layer depth, analysis of the Gulf Stream and rings system, and bathythermograph profiles. To date, these are the most detailed and high-quality data for mesoscale ocean modeling, data assimilation, and forecasting research. Feedback from ocean modeling groups who tested this data was incorporated into its refinement.

Suggestions for DAMÉE data usages include 1) ocean modeling and data assimilation studies, 2) diagnosis and theorectical studies, and 3) comparisons with locally detailed observations.

Abstract

Within confluent northwesterly flow of an intensifying baroclinic wave over North America in late October 1963, an intense frontal zone developed in 12 h near the inflection point in the middle and upper troposphere. By 24 h after its initial appearance, the zone extended roughly from 400 to 700 mb and from the inflection point to just beyond the downstream trough. Horizontal gradients of potential temperature reached 15–20 K (100 km)⁻¹. Air within the frontal zone was extremely dry.

As the accompanying trough approached the east coast of he United States, surface frontogenesis occurred offshore, remaining distinct from the upper-level front. A region of subsidence, elongated in the direction of the upper-level flow, displayed maximum descent on the warm edge of the frontal zone and played a frontogenetical role through tilting of the isentropic surfaces.

Analysis of isentropic potential vorticity showed significant increase of this quantity near the cold base and a probable decrease near the top as the front developed. Turbulent beat flux, associated with reduced Richardson numbers within the frontal zone, was likely responsible for this nonconservation of potential vorticity and for the propagation of the zone to lower colder values of potential temperature.

Vertical wind shear through the frontal layer was supergeostrophic in the upper ridge and subgeostrophic in the trough. An inertial oscillation at the top of the layer began as air in the ridge flowed toward lower geopotential, forming a jet streak and then flowing toward higher geopotential near the inflection point, a region of intense individual frontogenesis.

Abstract

This paper describes a case of unexpected weak cyclogenesis over the northwestern Gulf of Mexico from 16 to 19 September 1984 based upon manually prepared and European Centre for Medium-Range Weather Forecasts (ECMWF) gridded analyses. Noteworthy aspects of the case include: 1) upward of 50 cm of rain along the extreme southern coast of Texas and 2) the brief occurrence of minimal strength tropical-storm conditions in a weak baroclinic marine environment. A crucial antecedent condition to rainstorm formation was the creation of a low-level baroclinic zone over the northwestern Gulf of Mexico due to the southward advance of drier and slightly cooler air behind a cold front that penetrated into northeastern Mexico. Four factors were responsible for rainfall concentration along the coast: 1) a northward-moving 700-mb trough and embedded vorticity maximum in the easterlies over the western Gulf of Mexico, 2) an eastward-propagating upper-tropospheric disturbance in the midlatitude westerlies over the southern United States to the north of a subtropical ridge line over Texas and Louisiana, 3) the formation of a weak midtropospheric baroclinic zone over the extreme north-western Gulf of Mexico, along which cyclonic-vorticity advection by the thermal wind contributed to a favorable environment for deep convection and cyclogenesis, and 4) the existence and maintenance of a weak north-south-oriented baroclinic zone along the Mexican coast in the lower troposphere.

The coastal baroclinic zone was associated with a quasi-stationary axis of ascent that maximized at 700 mb and lay 200–300 km to the cast of a persistent band of frontogenesis along the Sierra Madre Oriental Mountains of Mexico. Frontogenesis (∼2−4×10⁻¹⁰°C m⁻¹ s⁻¹) was dominated by the twisting term as relatively cool air over coastal Mexico was forced to ascend in the 700-mb easterly flow, favoring the northward movement of an area of cyclonic vorticity along the coast.

The results from this study are compared and contrasted with a similar September (1979) heavy-rain event that also occurred in southern Texas. In both cases, cyclogenesis occurred in a weakly baroclinic environment with embedded convection concentrated along the boundary of the surface baroclinic zone. The paper concludes with a discussion of tropical-storm formation in a baroclinic environment. It is speculated that the apparent, but short-lived, minimal strength tropical-storm development in this case could not be sustained because of the absence of a significant upstream cyclonic-vorlicity maximum aloft, despite otherwise favorable indicators for tropical cyclogenesis.

Abstract

Multiple equilibria as well as periodic and aperiodic solution regimes are obtained in a barotropic model of the midlatitude ocean’s double-gyre circulation. The model circulation is driven by a steady zonal wind profile that is symmetric with respect to the square basin’s zonal axis of north–south symmetry, and dissipated by lateral friction.

As the intensity of the wind forcing increases, an antisymmetric double-gyre flow evolves through a pitchfork bifurcation into a pair of steady mirror-symmetric solutions in which either the subtropical or the subpolar gyre dominates. In either one of the two asymmetric solutions, a pair of intense recirculation vortices forms close to and on either side of the point where the two western boundary currents merge to form the eastward jet. To the east of this dipole, a spatially damped stationary wave arises, and an increase in the steady forcing amplifies the meander immediately to the east of the recirculating vortices. During this process, the transport of the weaker gyre remains nearly constant while the transport of the stronger gyre increases.

For even stronger forcing, the two steady solution branches undergo Hopf bifurcation, and each asymmetric solution gives rise to an oscillatory mode, whose subannual period is of 3.5–6 months. These two modes are also mirror-symmetric in space. The time-average difference in transport between the stronger and the weaker gyre is reduced as the forcing increases further, while the weaker gyre tends to oscillate with larger amplitude than the stronger gyre. Once the average strength of the weaker gyre on each branch equals the stronger gyre’s, the solution becomes aperiodic. The transition of aperiodic flow occurs through a global bifurcation that involves a homoclinic orbit. The subannual oscillations persist and stay fairly regular in the aperiodic solution regime, but they alternate now with a new and highly energetic, interannual oscillation. The physical causes of these two oscillations—as well as of a third, 19-day oscillation—are discussed. During episodes of the high-amplitude, interannual oscillation, the solution exhibits phases of either the subtropical or subpolar gyre being dominant. Even lower-frequency, interdecadal variability arises due to an irregular alternation between subannual and interannual modes of oscillation.

Abstract

A long-lived severe convective system in the southern United States from 2–4 May 1978 is documented. The distinguishing feature of the convection was its origin in a region of deep synoptic scale ascent and its subsequent steady motion away from that ascent toward increasingly warmer and more moist, unstable boundary layer air. On 2 May 1978 a north-south oriented squall line originated above and within a shallow cold air mass in west central Texas north of a quasi-stationary east-west oriented frontal boundary. Potential instability was generated by a warm, moist airmass from the Gulf of Mexico that advected westward beneath a warm, dry plume of air moving northward ahead of the trough aloft. Convection first appeared along an inverted cough in western Texas that separated southward-flowing cool air, dammed up against the New Mexico mountains, from slightly warmer northeasterly and easterly flow to the east. Frontogenetical processes associated with the inverted trough played a crucial role in triggering and focusing the convection.

The squall line, once formed, intensified eastward across Texas as it ingested increasingly unstable air. By 1200 UTC 3 May 1978 the squall line had propagated away from the synoptic scale ascent center that spawned it and had reached the Alabama-Mississippi border beneath the downstream ridge line aloft. Over the next 12 h the decaying squall line and other isolated patches of thunderstorms amalgamated into a large mesoscale convective system that moved steadily toward warmer air. The main precipitation area was continuous throughout, progressing regularly, shrinking somewhat to the north and broadening and strengthening to the south. A wake trough trailing the squall line exhibited gravity-wave-like characteristics. Vigorous new convective elements erupted in a region of maximum surface frontogenesis along the intersection of the decaying squall line boundary with an old east-west frontal boundary that had been locally strengthened by previous convection. During the mature phase of the organized mesoscale convective system the synoptic scale forcing was distinguished by its lack of vertical coherence. The convection remained fastened to the convergence of Q-vector forcing at 850 mb while the synoptic scale ascent in the middle and upper troposphere remained well to the west.

By 1200 UTC 4 May 1978 the mesoscale convective system had become identified with an active squall line within a moist, unstable warm airmass across northern Florida and the extreme eastern Gulf of Mexico. New convective cells grew continuously southward from the original focal point along the east-west frontal boundary into the Gulf of Mexico ahead of an advancing cold front.