Search Results

You are looking at 1 - 7 of 7 items for

  • Author or Editor: D. Keyser x
  • Refine by Access: All Content x
Clear All Modify Search
Gerald D. Bell and Daniel Keyser

Abstract

Equations are presented for the evolution of isobaric shear and curvature vorticity and for isentropic shear and curvature potential vorticity in natural (streamline-following) coordinates, in the case of adiabatic, frictionless flow. In isobaric coordinates, two terms of equal magnitude and opposite sign arise in the respective tendency equations for shear and curvature vorticity; these terms represent conversions between shear and curvature vorticity in the sense that their sum does not alter the total tendency of absolute vorticity. In isentropic coordinates, only the conversion terms remain in the tendency equations for shear and curvature potential vorticity, consistent with potential-vorticity conservation. The vorticity and potential-vorticity conversions arise from (i) along-stream variations in wind speed in the presence of Lagrangian changes in wind direction and (ii) flow-normal gradients of Lagrangian changes in wind speed. The assumption of horizontal nondivergence simplifies the interpretation of the vorticity-interchange process by relating the conversion terms directly to flow curvature. Schematics are developed in order to illustrate the conversion terms in idealized representations of jet-entrance and jet-exit regions and curved flow patterns; these schematics provide the basis for understanding vorticity interchanges in realistic flow regimes.

The evolution of the midtropospheric shear- and curvature-potential-vorticity fields is described for a jet- trough interaction event in northwesterly flow, leading to the formation of a well-defined midtropospheric cutoff cyclone over the eastern United States between [8 and 20 January 1986. This time period coincides with the first intensive observing period of the Genesis of Atlantic Lows Experiment. Major midtropospheric cyclogenesis begins as a jet embedded in northwesterly flow, identified as a maximum of cyclonic shear potential vorticity, propagates toward the base of a diffluent trough, identified as a maximum of cyclonic curvature potential vorticity. The potential-vorticity tendency equations reveal that for this particular stage, the interchange terms contribute both to the amplification of the trough and to the formation of a maximum of cyclonic shear potential vorticity on the downstream side of the trough. The potential-vorticity interchange process is shown to play a key role in transforming the asymmetric configuration of shear and curvature potential vorticity characteristic of the diffluent trough stage, where the cyclonic shear maximum lags the cyclonic curvature maximum, to the relatively symmetric configuration characteristic of the cutoff stage. At the culmination of the cutoff stage, the shear- and curvature-potential-vorticity maxima overlap substantially. This overlap is a consequence of the presence of a single, cyclonically curved jet within the base of the cutoff cyclone.

A second important structural change occurring during midtropospheric cyclogenesis is the transformation of the potential-vorticity anomaly corresponding to the cutoff cyclone into a circularly symmetric configuration, which is accomplished by the contraction of the northwestern extension of the potential-vorticity anomaly toward the cyclone center. This contraction process, which is shown to involve significant interchanges between shear and curvature potential vorticity, results in the detachment of the potential-vorticity anomaly from the “stratospheric reservoir” of potential vorticity located north of the cyclone.

Full access
T-W. Yu, M. Iredell, and D. Keyser

Abstract

A neural network algorithm used in this study to derive Special Sensor Microwave/Imager (SSM/I) wind speeds from the Defense Meteorological Satellite Program satellite-observed brightness temperatures is briefly reviewed. The SSM/I winds derived from the neural network algorithm are not only of better quality, but also cover a larger area when compared to those generated from the currently operational Goodberlet algorithm. The areas of increased coverage occur mainly over the regions of active weather developments where the operational Goodberlet algorithm fails to produce good quality wind data due to high moisture contents of the atmosphere. These two main characteristics associated with the SSM/I winds derived from the neural network algorithm are discussed.

SSM/I wind speed data derived from both the neural network algorithm and the operational Goodberlet algorithm are tested in parallel global data assimilation and forecast experiments for a period of about three weeks. The results show that the use of neural-network-derived SSM/I wind speed data leads to a greater improvement in the first-guess wind fields than use of wind data generated by the operational algorithm. Similarly, comparison of the forecast results shows that use of the neural-network-derived SSM/I wind speed data in the data assimilation and forecast experiment gives better forecasts when compared to those from the operational run that uses the SSM/I winds from the Goodberlet algorithm. These results of comparison between the two parallel analyses and forecasts from the global data assimilation experiments are discussed.

Full access
Daniel Keyser, Brian D. Schmidt, and Dean G. Duffy

Abstract

The problem of representing vertical circulations in frontal zones is reexamined with the objective of devising a methodology sufficiently general to apply in situations where these circulations are no longer confined to the cross-front (transverse) vertical plane and therefore must be viewed as fully dimensional. The proposed methodology, which builds upon the earlier work of Hoskins and Draghici and of Eliassen, consists of adopting a vector streamfunction that describes the vertical velocity and the horizontal irrotational flow. This generalized streamfunction, referred to as the psi vector, may be determined uniquely from the vertical velocity field over a limited region provided that suitable lateral boundary conditions on the velocity potential for the irrotational part of the horizontal velocity can be specified. A key property of the psi vector is that its projections onto arbitrary orthogonal vertical planes yield two independent vertical circulations, providing an objective means for separating a three-dimensional vertical circulation into cross- and alongfront components.

The psi-vector methodology is applied to surface and upper-level frontal zones simulated in an f plane primitive equation channel model of a finite-amplitude baroclinic wave in which all diabatic and frictional influences are neglected except for horizontal diffusion. Both along and cross-front vertical circulations are diagnosed and interpreted for upper-level frontal zones associated with jet streams and jet streaks situated within curved flowed and for surface fronts possessing attributes of observed warm and cold fronts. In all of these frontal systems, the transverse vertical circulation is dominant in the sense that the cross-front component of the vertical velocity is larger in magnitude than the corresponding alongfront component. The lateral scale of the cross-front circulation is of a frontal dimension, whereas the scale of the alongfront circulation is characteristic of the baroclinic wave. The orientations of the cross-front circulations relative to the respective frontal zones are broadly consistent with those discussed in earlier two-dimensional models. These findings support interpretations 1) that three- dimensional vertical circulations in midiatitude cyclones may be viewed conceptually as a superposition of vertical circulations associated with the baroclinic wave and with the embedded fronts, and 2) that confluence and horizontal shear forcing mechanisms, although strictly applicable to two-dimensional frontogenesis models, may carry over to describe transverse vertical circulations in three-dimensional systems.

Full access
Daniel Keyser, Brian D. Schmidt, and Dean G. Duffy

Abstract

The kinematic technique of representing three-dimensional vertical circulations in baroclinic disturbances in terms of a vector streamfunction (referred to as the psi vector) recently proposed by the authors is placed in the context of quasigeostrophic (QG) theory. A diagnostic equation is derived for the psi vector from which the vertical velocity and the irrotational part of the ageostrophic velocity can be inferred. It is shown that, for domains that are periodic or unbounded horizontally, the psi vector is forced dynamically by the irrotational part of the Q vector. It is further shown for such domains that the vertical shear of the nondivergent part of the ageostrophic velocity is proportional to the nondivergent part of the Q vector. This, in principle, completes the diagnosis of the vertical velocity and the total ageostrophic velocity from the mass field, along with diabatic and frictional effects in the thermodynamic and momentum equations.

It is demonstrated that the projection of the psi-vector equation onto the cross-front vertical plane leads to a generalization of the QG form of the Sawyer-Eliassen equation applicable to three-dimensional flows. The forcing of the scalar streamfunction for the transverse (cross-front) circulation comprises not only the confluence and horizontal shear terms from the two-dimensional case, but additional terms involving the component of the vertical velocity associated with the vertical circulation in the alongfront direction and the nondivergent component of the ageostrophic velocity in the cross-front plane. These additional terms vanish in the two-dimensional case, wherein the vertical circulation is confined to the cross-front plane and the nondivergent part of the ageostrophic velocity is restricted to the alongfront direction.

The diagnostic methodologies for the total ageostrophic flow and for the generalized Sawyer-Eliassen equation are illustrated through applications to upper-level and surface frontal zones simulated in an f-plane primitive equation (PE) channel model of a finite-amplitude baroclinic wave. It is found that discrepancies in the sense of the cross-contour flow in jet-entrance and jet-exit regions between the QG-diagnosed ageostrophic flow and that simulated by the PE model can be traced to differences in the nondivergent part of the ageostrophic flow. Qualitative consistency is found, however, in the comparison between the QG irrotational ageostrophic flow and its PE counterpart, suggesting the utility of QG diagnoses of the vertical motion field in the vicinity of curved jet-front systems, where the geostrophic-momentum approximation may break down locally. In the generalized Sawyer-Eliassen equation, it is found that the confluence and horizontal shear forcing terms are dominant, but tend to be opposed by the term involving the component of the nondivergent part of the ageostrophic flow in the cross-front plane. The dominance of these “two-dimensional” forcing terms motivates a comparison between the diagnosed frontal circulations and those obtained in previous two-dimensional front-ogenesis models. This comparison addresses the extent to which idealized frontogenetical mechanisms involving confluence and horizontal shear carry over to less restrictive, three-dimensional contexts.

Full access
Daniel Keyser, Brian D. Schmidt, and Dean G. Duffy

Abstract

In a recent paper on the kinematics of frontogenesis, Keyser et al. conjectured that partitioning the Q vector into along- and cross-isentrope components yields vertical-motion patterns that are respectively cellular and banded—the former on the scale of the baroclinic disturbance and the latter on the scale of the embedded frontal zones. This conjecture is examined diagnostically through solution of the quasigeostrophic omega equation, using the output from a nearly adiabatic and frictionless f-plane primitive equation channel model of the evolution of a baroclinic disturbance to finite amplitude. The results of the present study support the proposed conjecture, suggesting the following interpretation of the characteristic comma structure of the vertical-motion field in midlatitude baroclinic disturbances: The comma shape arises from the modification or distortion of a wave-scale dipole pattern by frontal-scale asymmetries. The dipole is associated with the along-isentrope component of the Q vector, reflecting the wavelike pattern in the potential temperature field within the baroclinic disturbance; the asymmetries are associated with the cross-isentrope component of the Q vector, reflecting the presence of frontal zones within the baroclinic disturbance.

Full access
A. L. Conaty, J. C. Jusem, L. Takacs, D. Keyser, and R. Atlas

The realism of extratropical cyclones, fronts, jet streams, and the tropopause in the Goddard Earth Observing System (GEOS) general circulation model (GCM), implemented in assimilation and simulation modes, is evaluated from climatological and case-study perspectives using the GEOS-1 reanalysis climatology and applicable conceptual models as benchmarks for comparison. The latitude-longitude grid spacing of the datasets derived from the GEOS GCM ranges from 2° × 2.5° to 0.5° × 0.5°. Frontal systems in the higher-resolution datasets are characterized by horizontal potential temperature gradients that are narrower in scale and larger in magnitude than their lower-resolution counterparts, and various structural features in the Shapiro–Keyser cyclone model are replicated with reasonable fidelity at 1° × 1° resolution. The remainder of the evaluation focuses on a 3-month Northern Hemisphere winter simulation of the GEOS GCM at 1° × 1° resolution. The simulation realistically reproduces various large-scale circulation features related to the North Pacific and Atlantic jet streams when compared with the GEOS-1 reanalysis climatology, and conforms closely to a conceptualization of the zonally averaged troposphere and stratosphere proposed originally by Napier Shaw and revised by Hoskins. An extratropical cyclone that developed over the North Atlantic Ocean in the simulation features surface and tropopause evolutions corresponding to the Norwegian cyclone model and to the LC2 life cycle proposed by Thorncroft et al., respectively. These evolutions are related to the position of the developing cyclone with respect to upper-level jets identified in the time-mean and instantaneous flow fields. This article concludes with the enumeration of several research opportunities that may be addressed through the use of state-of-the-art GCMs possessing sufficient resolution to represent mesoscale phenomena and processes explicitly.

Full access
Manuel S. F. V. De Pondeca, Geoffrey S. Manikin, Geoff DiMego, Stanley G. Benjamin, David F. Parrish, R. James Purser, Wan-Shu Wu, John D. Horel, David T. Myrick, Ying Lin, Robert M. Aune, Dennis Keyser, Brad Colman, Greg Mann, and Jamie Vavra

Abstract

In 2006, the National Centers for Environmental Prediction (NCEP) implemented the Real-Time Mesoscale Analysis (RTMA) in collaboration with the Earth System Research Laboratory and the National Environmental, Satellite, and Data Information Service (NESDIS). In this work, a description of the RTMA applied to the 5-km resolution conterminous U.S. grid of the National Digital Forecast Database is given. Its two-dimensional variational data assimilation (2DVAR) component used to analyze near-surface observations is described in detail, and a brief discussion of the remapping of the NCEP stage II quantitative precipitation amount and NESDIS Geostationary Operational Environmental Satellite (GOES) sounder effective cloud amount to the 5-km grid is offered. Terrain-following background error covariances are used with the 2DVAR approach, which produces gridded fields of 2-m temperature, 2-m specific humidity, 2-m dewpoint, 10-m U and V wind components, and surface pressure. The estimate of the analysis uncertainty via the Lanczos method is briefly described. The strength of the 2DVAR is illustrated by (i) its ability to analyze a June 2007 cold temperature pool over the Washington, D.C., area; (ii) its fairly good analysis of a December 2008 mid-Atlantic region high-wind event that started from a very weak first guess; and (iii) its successful recovery of the finescale moisture features in a January 2010 case study over southern California. According to a cross-validation analysis for a 15-day period during November 2009, root-mean-square error improvements over the first guess range from 16% for wind speed to 45% for specific humidity.

Full access