Ageostrophic Wind Research Articles

The Life Cycle of a Cyclone Wave in the Southern Hemisphere. Part I: Eddy Energy Budget

Abstract The energetics of a Southern Hemisphere cyclone wave have been analyzed using ECMWF data and the results of a limited-area model simulation. An analysis of the energy budget for a storm that developed in the eastern Pacific on 4–6 September 1987 showed the advection of the geopotential height field by the ageostrophic wind to be both a significant source and the primary sink of eddy kinetic energy. Air flowing through the wave gained kinetic energy via this term as it approached the energy maximum and then lost it upon exiting. Energy removal by diffusion, friction, and Reynolds stresses was found to be small. The most important conclusion was that, while the wave grew initially by poleward advection of heat as expected from baroclinic theory, the system evolved only up to the point where this source of eddy energy and the conversion of eddy potential to eddy kinetic energy (typically denoted “ωα”) was compensated for by energy flux divergence (dispersion of energy), mainly of the ageostrophic ge...

The Structure of the Ageostrophic Wind Field in Baroclinic Waves

A high-pass filtered time series of the 500-mb height at a grid point in the Pacific storm track is regressed against the ageostrophic wind field at all grid points on the NMC grids for nine winters in order to determine the characteristic relationship between the ageostrophic wind and the geopotential distribution in baroclinic waves. The resulting ageostrophic wind pattern does not consist simply of a zonally oriented divergent circulation as sometimes pictured in theoretical models. Rather, it is dominated by the zonal component along the axis of the baroclinic waves at the 250-mb level, and the meridional component along the poleward and equatorward flanks of the waves at the 850-mb level. Diagnosis of the disturbance fields indicates that the finite meridional width of the baroclinic jet and its associated wave disturbances is the key factor in determining the distribution of the ageostrophic wind. It is shown that the ageostrophic flow based on a variable Coriolis parameter should not be used alone to estimate the divergent circulation since the divergence of the geostrophic wind is of the same order of magnitude and partly compensates the divergence of the ageostrophic wind at upper levels and augments it at lower levels.

Structure and Evolution of Baroclinic Waves as Inferred from Regression Analysis

The structure of transient disturbances with periods shorter than a week is documented an the basis of one-point regression maps and longitude–height sections derived from the National Meteorological Center (NMC) operational analyses for nine winter seasons. The reference time series for the regression coefficients is highpass filtered, normalized 500-mb height at a grid point in the Pacific stormtrack. The field variables that were regreesed on the reference time series were not filtered. The regression statistics yield quantitative estimates of the amplitudes of the high-frequency fluctuations in the geopotential height, temperature, wind, ageostrophic wind, vorticity, divergence, vertical velocity, and outgoing longwave radiation fields. The observed high-frequency fluctuations exhibit a vertical and meridional structure suggestive of baroclinic waves evolving through the mature phase of their life cycle. Most of the results are in qualitative agreement with previous studies, but the ageostrophic wind field in the lower troposphere exhibits systematic fluctuations in the meridional component that had not been anticipated. An analysis of the temperature and vorticity tendencies in the waves yielded dynamically plausible results, despite the deficiencies in the NMC divergence and vertical velocity fields. The vertical velocity field at the 500-mb level was found to be consistent with the forcing in the quasi-geostrophic omega equation, which exhibits a simple wavelike pattern in the Q-vector field.

Frontogenesis Processes in the Middle and Upper Troposphere

Abstract Basic issues regarding upper-level frontogenesis addressed in this paper are: (i) simulated frontogenesis influenced by the initial flow, (ii) upper-level frontogenesis as essentially a two-dimensional process, and (iii) frontal-scale positive feedback between vertical advection of momentum and vorticity advection by the ageostrophic wind, which is important for the intensification of upper-level frontal zones. The methodology for investigation is based on analysis of simulated upper-level frontogenesis with a three-dimensional primitive-equation model. The model is a simplified version of the UCLA GCM, with 21 layers in the vertical, horizontal resolution of 1.2° lat × 1.5° long; a 60° sector of one hemisphere as periodic domain, and physics reduced to horizontal diffusion and dry convective adjustment. Simulations initialized with jet streams symmetric about the latitude of maximum wind at each pressure level produce—in the middle troposphere—the strongest frontal zones downstream of the trough...

Reducing the Error in a Time-Split Finite-Difference Scheme Using an Incremental Technique

It is shown both in theory and in practice that the accuracy of time-split finite-difference methods may be increased by the very simple device of incrementally adding a portion of the advection term in the adjustment step of the method. In the first part of this study, the theory of the approach is described, and large gains in accuracy are demonstrated by comparison with an exact solution to a forced oscillatory system. The technique is then applied to a considerable number (100) of 48-h forecasts with a barotropic numerical weather prediction (NWP) model using 500-hPa data on the Australian region NWP domain. It is found that the incrementally split scheme is more accurate than the conventional split scheme, which generates more noise and has unrealistically larger values of the root-mean-square ageostrophic wind. A further advantage of the new scheme is that it has superior initialization properties.

Cold and Warm Frontal Circulations in an Idealized Moist Semigeostrophic Baroclinic Wave

Abstract Diagnoses are presented of the three-dimensional vertical circulation for a coupled cold-warm frontal system in an idealized moist semi-geostrophic (SG) baroclinic wave. The vertical circulation is computed in SG space where the solution corresponds to its quasi-geostrophic (QG) counterpart in physical space. It is shown for this QG solution that (i) the vertical motion is enhanced in the moist region due to small moist stability and strong local geostrophic forcing; (ii) the cyclonic/anti-cyclonic ageostrophic wind vorticity is induced locally by the geostrophic wind deformation/rotation; (iii) the along-flow/cross-flow ageostrophic wind divergence is associated with the along-flow curvature/speed variation of the system-relative geostrophic wind. When the solution is transformed back to physical space, the vertical motion is dramatically enhanced and concentrated along the cold and warm fronts in the moist region due to time-integrated ageostrophic wind convergence and ageostrophic feedback to ...

Analysis and Diagnosis of the Composition of Ageostrophic Circulations in Jet-Front Systems

Abstract Diagnostic case studies of ageostrophic circulations in upper-tropospheric jet-front systems are presented, using the numerical analyses of the ECMWF model as data sources. The impact of the ECMWF analysis procedure on the retrieved ageostrophic fields is first discussed. Upper-level fields of the ageostrophic wind and its divergence are computed in the natural coordinates system in order to analyze the composition of the transverse (cross-stream) and the alongstream ageostrophic components that are induced by alongstream wind variations and curvature effects respectively. The analysis shows that the curvature effects can contribute in a predominant way to the upper-tropospheric divergence field in entrance or exit regions of jet streaks, so as to sometimes inhibit the expected transverse circulations. Several transverse indirect circulations are displayed in the exit regions of jet streaks. A case of lateral shift towards the anticyclonic side of the jet axis of an indirect circulation is discus...

A Case Study Using Kinematic Quantities Derived from a Triangle of VHF Doppler Wind Profilers

Horizontal divergence, relative vorticity, kinematic vertical velocity, and geostrophic and ageostrophic winds are computed from Colorado profiler network data to investigate an upslope snowstorm in northeastern Colorado. Horizontal divergence and relative vorticity are computed using the Gauss and Stokes theorems, respectively. Kinematic vertical velocities are obtained from the surface to 9 km by vertically integrating the continuity equation. The geostrophic and ageostrophic winds are computed by applying a finite differencing technique to evaluate the derivatives in the horizontal equations of motion. Comparison of the synoptic-scale data with the profiler network data reveals that the two datasets are generally consistent. Also, the profiler-derived quantities exhibit coherent vertical and temporal patterns consistent with conceptual and theoretical flow fields of various meteorological phenomena. It is suggested that the profiler-derived quantities are of potential use to weather forecasters in that they enable the dynamic and kinematic interpretation of weather system structure to be made and thus have nowcasting and short-term forecasting value.

A Diagnosis of Unbalanced Flow in Upper Levels during the AYE-SESAME I Period

Abstract Terms of the balance equation were calculated at 300 mb to diagnose unbalanced flow in the upper troposphere both prior to and during a period of strong convection which took place during the AVE-SESAME I period. The sum of the balance equation terms displayed large imbalances(>50×10−10s−2)over the Red River Valley as early as 2100 UTC. This region grew in magnitude, expanding over Oklahoma during the next 3–6 hours. The vorticity and Laplacian terms in the balance equation dominated this imbalance. An examination of ageostrophic, geostrophic and actual 300 mb winds at 2100 UTC revealed that the ageostrophic winds over Oklahoma were directed towards lower geopotential heights, indicating that the flow was neither in geostrophic balance nor merely responding to curved flow as described by the gradient wind equation. Such imbalance led to substantial increases in divergence and midlevel upward vertical motion. The remaining terms of the divergence equation were computed and summed. These terms part...

Balanced and Unbalanced Upper-Level Frontogenesis

The dynamics of frontogenesis at upper levels are investigated using a hierarchy of three numerical models. They are, in order of decreasing sophistication, the anelastic (AN), the geostrophic momentum (GM), and the quasi-geostrophic (QG) approximations to the full equations of motion. Each model is two-dimensional and assumes the same basic-state, which incorporates the frontogenetical mechanisms of confluence and horizontal shear. The dependence of the numerical solutions on the initial vertical shear of the cross-front component of the geostrophic wind, λ, and its associated along-front temperature gradient is examined in detail. For the values of λ chosen, the along-front temperature gradient is either zero (λ = 0) or such that cold air is advected along the upper front (λ < 0). Intercomparison of the broad-scale structure of the upper-level jet–fronts as described by the AN and GM models shows close agreement. For zero or weak shears (λ = 0 s−1 or λ = −2 × 10−3 s−1), the solutions are essentially identical. Vertical shear in the cross-front geostrophic wind serves to increase the amplitude of the cross-front circulation and displace the subsiding branch toward the warmer air. In the cases of weak or zero shear, the dominant mechanism for generating vertical vorticity at upper levels is the stretching of preexisting vertical vorticity, whereas for stronger shear (λ = −5.741 × 10−3 s−1) the key process becomes the tilting of horizontal vorticity into the vertical by differential vertical motion. In contrast, the QG model exhibits marked differences with its AN and GM counterparts, which become even more pronounced as |λ| is increased. These differences are related largely to the neglect of vortex tilting in generating vertical vorticity in the OG model. The GM and QG models assume cross-front thermal wind balance at all time. A posteriors examination of the numerical solutions shows this to be an excellent approximation when the vertical shear in the cross-front geostrophic wind is weak. For strong vertical shear of the cross-front geostrophic wind, the unbalanced along-front ageostrophic wind is proportional to the vertical advection of the cross-front velocity. Diagnoses of these simulations reveal thermal wind balance to be less well satisfied. It is shown that in contrast to the GM and QG models, wherein the along-front ageostrophic velocity is passive and thus cannot contribute to the evolution of the jet–front system, the unbalanced along-front flow contributes significantly to the dynamics as described by the AN model.

Comparison of LIMS temperatures and geostrophic winds with Berlin radiosonde temperature and wind measurements

The compatibility of radiosonde (RAOB) and satellite temperature and wind data in the lower–middle stratosphere is evaluated via comparison of daily measurements from the Berlin (Tempelhof) RAOB station and colocated synoptically mapped temperature retrievals and geostrophic winds obtained from the Nimbus 7 Limb Infrared Monitor of the Stratosphere (LIMS) experiment for November 1978 to May 1979. Both on a slowly varying (seasonal) time scale and during large‐amplitude sudden warming episodes, RAOB and LIMS are found to agree quite closely in terms of the local tendency of temperature and horizontal wind speed and direction. The overall similarity between RAOB winds and the LIMS‐derived geostrophic values suggests that the horizontal wind flow in the lower‐middle stratosphere over Berlin is essentially in thermal wind balance. This finding implies that ageostrophic wind motions in this region are rather small in magnitude, approaching the estimated level of uncertainty in the measurement/derivation of RAOB and LIMS winds. Low‐frequency fluctuations are found to dominate the RAOB and LIMS time series. Nevertheless, on occasion, high‐frequency structure in altitude and time appears in the RAOB data. These small‐scale features, which generally possess small amplitudes, are either attenuated or virtually absent from the LIMS time series, consistent with the comparative sampling properties of the two observing systems. The results demonstrate that the LIMS‐mapped temperature and wind analyses represent a valuable resource for investigating large‐scale dynamics and transport.

Linear wavetrains in models of the stratosphere

Experiments with a primitive equation model of the stratosphere and mesosphere are described which investigate the effect on the stratosphere of the growth of localized, stationary forcing in the troposphere. Two basic states are used which are representative of stratospheric flow in early and late winter in the southern hemisphere. Simple mechanistic models are then introduced to examine the wave phenomena which occur during the first 10 days of the experiments when the dynamics are known to be linear. The localized forcing grows to a peak amplitude of 600 gpm and half-width of 15° of latitude, values typical of the mid-latitude upper troposphere. Stratospheric travelling waves are excited with phase velocities which are sensitive to the zonal mean winds and to the speed of the switch-on of the forcing, and wave activity spreads quickly from the localized forcing with a typical vertical group velocity of 10 km d−1. Strong horizontal curvature in the basic states can profoundly affect the vertical and horizontal structures of the wavetrains: mid-latitude regions of negative northward potential vorticity gradient in the stratosphere (as seen in the southern hemisphere in late winter) lead to the confinement and amplification of waves in high latitudes. Three-dimensional diagnostics of wave activity propagation reveal wave reflections missed by zonally-averaged EP flux diagnostics. Significant wave reflection and trapping occur on the flow from late winter, eventually producing a steady linear wavetrain with an intense local centre of circulation in the upper stratosphere, the maximum anomaly being ∼1000gpm at 1mb. It is argued that such wavetrains are unrealistic because nonlinear ageostrophic winds drive the dynamics away from any linear steady state.

Linear wavetrains in models of the stratosphere

AbstractExperiments with a primitive equation model of the stratosphere and mesosphere are described which investigate the effect on the stratosphere of the growth of localized, stationary forcing in the troposphere. Two basic states are used which are representative of stratospheric flow in early and late winter in the southern hemisphere. Simple mechanistic models are then introduced to examine the wave phenomena which occur during the first 10 days of the experiments when the dynamics are known to be linear.The localized forcing grows to a peak amplitude of 600 gpm and half‐width of 15° of latitude, values typical of the mid‐latitude upper troposphere. Stratospheric travelling waves are excited with phase velocities which are sensitive to the zonal mean winds and to the speed of the switch‐on of the forcing, and wave activity spreads quickly from the localized forcing with a typical vertical group velocity of 10 km d−1. Strong horizontal curvature in the basic states can profoundly affect the vertical and horizontal structures of the wavetrains: mid‐latitude regions of negative northward potential vorticity gradient in the stratosphere (as seen in the southern hemisphere in late winter) lead to the confinement and amplification of waves in high latitudes. Three‐dimensional diagnostics of wave activity propagation reveal wave reflections missed by zonally‐averaged EP flux diagnostics.Significant wave reflection and trapping occur on the flow from late winter, eventually producing a steady linear wavetrain with an intense local centre of circulation in the upper stratosphere, the maximum anomaly being ∼1000gpm at 1mb. It is argued that such wavetrains are unrealistic because nonlinear ageostrophic winds drive the dynamics away from any linear steady state.

The interface effect and the formation of a low-level jet along the east side of the rocky mountains

Based upon the analysis of several different causes for the low-level jet along the east side of the Rocky Mountains, the concept of “Interface Effect” is established. The basic mechanism for the formation of the low-level jet in North America has been found to be the compression and divergence under the driving of ageostrophic winds, of the air columns between two surfaces—the ground and the bottom of inversion-which slope with different patterns in a cross-section normal to the jet stream. As a result, the air parcel is accelerated along the stream-line and the anticyclonic shear of the current increased. Also, the diurnal variation of the jet is determined by the interface effect.

The Diagnosis of Upper Tropospheric Divergence and Ageostrophic Wind Using Profiler Wind Observations

Wind fields derived from a network of three VHF Doppler radars are used to calculate the mean kinematic properties of the wind field over Colorado and an area-averaged geostrophic and ageostrophic wind. A numerical technique that is equivalent to the line integral method is used to compute the kinematic quantities. Details of this technique, termed the linear vector point function method (LVPF) are discussed. The behavior of the vorticity, divergence, deformation, geostrophic wind and ageostrophic wind are examined for two case studies when the synoptic scale weather patterns over Colorado are dominated by moderately intense upper-level troughs and jet streams. We find that the computed quantities of divergence, absolute vorticity, deformation, geostrophic and ageostrophic wind are modified by the passage of the weather systems in a manner consistent with our present understanding of upper-level dynamics. In addition, temporal variations in the kinematic properties, geostrophic wind and ageostrophic wind are revealed that are beyond the resolution of the existing rawinsonde network.

Viscous pumping and the spin‐down of thermospheric gyres and jets

Strong gyres and jets can be generated at auroral latitudes in the thermosphere by enhanced electric fields during geomagnetic substorms. Typical height profiles of ion density suggest that the ion drag force should generate large curvature in the vertical profile of the winds in the highly viscous region of the thermosphere above about 200 km. It is proposed that the poststorm spin‐down of these gyres and jets proceeds via Ekman circulations driven by the curvatures in the height profiles of the winds. Analytic and numerical calculations of the ageostrophic winds forced by curvature in model geostrophic wind profiles show that the ageostrophic wind speeds and directions depend mainly on the kinematic viscosity in the region of curvature and the total change in shear in the geostrophic wind. Ageostrophic wind speeds for typical thermospheric jets can exceed 200 m s−1 (about 50% of the jet winds). Spin‐down times of thermospheric jets and cyclonic gyres by the Ekman pumping mechanism are estimated at ≲6 hours.

Interpretation of Ageostrophic Winds and Implications for Jet Stream Maintenance

Abstract The use of ageostrophic flow to infer the presence of vertical circulations in the entrances and exits of the climatological jet streams is questioned. Problems of interpretation arise because of the use of different difinitions of geostrophy in theoretical studies and in analyses of atmospheric data. The nature and role of the ageostrophic flow based on constant and variable Coriolis parameter difinitions of geostrophy vary. In the latter the geostrophic divergence cannot be neglected, so the vertical motion is not associated solely with the ageostrophic flow. Evidence is presented suggesting that ageostrophic flow in the climatological jet streams is primarily determined by the kinematic requirements of wave retrogression rather than by a forcing process. These requirements are largely met by the rotational flow, with the divergent circulations present being geostrophically forced, and so playing a secondary, restoring role.

A Case Study of the Effects of Random Errors in Rawinsonde Data on Computations of Ageostrophic Winds

Data input for the AVE-SESAME I experiment are utilized to describe the effects of random errors in rawinsonde data on the computation of ageostrophic winds. Computer-generated random errors for wind direction and speed and temperature are introduced into the station soundings at 25 mb intervals from which isentropic data sets are created. Except for the isallobaric and the local wind tendency, all winds are computed for Apr. 10, 1979 at 2000 GMT. Divergence fields reveal that the isallobaric and inertial-geostrophic-advective divergences are less affected by rawinsonde random errors than the divergence of the local wind tendency or inertial-advective winds.

The inland boundary layer at low latitudes

Observations from the Koorin boundary-layer experiment in Australia (latitude 16 °S) were analysed in a study of the nocturnal jet development. For geostrophic winds in the range 10–20 m s-1, ageostrophic wind magnitudes of 5–10m s-1 were common above the surface layer near sunset, with cross-isobar flow angles of about 40 °. The jet that then developed by midnight was probably the result of these large ageostrophic winds, strong surface cooling and favourable baroclinity and sloping terrain. The analysis is supported by numerical model calculations with special emphasis on the role of long-wave radiative cooling on turbulent decay. Decay is rapid in the presence of radiation, although there is little influence on stress divergence levels. Evidence of sea-breeze influences on the jet evolution, and on features of deeply penetrating sea breezes in general, will be presented and discussed in part 2 of this study (submitted to Boundary-Layer Meteorol.).

Numerical Simulations of a Transverse Indirect Circulation and Low-Level Jet in the Exit Region of an Upper-Level Jet

Uccellini and Johnson present a case study of a severe weather event in Ohio on 10–11 May 1973 to show evidence for coupling between an upper-tropospheric jet streak and a low-level jet within an indirect transverse circulation found in the exit region of the upper-level jet. The differential advection of moisture and temperature created by the shear between the upper- and low-level jets reduced convective stability, thereby enhancing the potential for severe convection. Two 12 h numerical simulations of the 10–11 May 1973 case are studied to determine 1) if a transverse indirect circulation with a low-level jet imbedded in its lower branch can be diagnosed in the exit region of the upper-level jet and studied using the model output at 3 h intervals and 2) if the initial magnitude and structure of the upper-level jet have a significant effect on the subsequent development of the low-level jet and the decrease in convectivc stability due to differential advection. In an adiabatic model simulation, an indirect transverse circulation having a low-level jet within its lower branch occurs in the exit region of the upper-level jet. The simulated vertical distribution of mass divergence and ageostrophic flow in the exit region agree with the diagnoses of Uccellini and Johnson. At upper levels, mass divergence (convergence) occurs on the cyclonic (anticyclonic) side of the exit region, while the opposite occurs at low levels. The, upper branch of the indirect circulation is dominated by the inertial–advective contribution to the ageostrophic wind which is related to the alongstream isotach gradient in the exit region. The lower branch is dominated by the wind tendency contribution to the ageostrophic wind. Ageostrophic shear associated with this circulation contributes to the development of differential moisture and temperature advection, which act to destabilize the preconvective environment. A second simulation using a smoothed, nondivergent initialization with a weaker upper-level jet streak and weaker alongside isotach gradient in the exit region of the upper-level jet produces a weaker indirect transverse circulation even though diabatic heating effects are present. The indirect circulation for this simulation is marked by smaller vertical motions, a weaker low-level return branch, and weaker low-level thermal and moisture advection associated with the low-level flow. Comparison of the two simulations suggests that the indirect circulation in the exit region of the upper-level jet is strongly responsive to dynamical processes associated with the initial structure of the jet streak.