Cross-isobaric Flow Research Articles

Energetics of Indian winter monsoon

The Indian subcontinent is characterized by complex topography and heterogeneous land use-land cover. The Himalayas and the Tibetan Plateau are spread across the northern part of the continent. Due to its highly variable topography, understanding of the prevailing synoptic weather systems is complex over the region. The present study analyzes the energetics of Indian winter monsoon (IWM) over the Indian subcontinent using outputs of mesoscale model (MM5) forced with National Center for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR), US, initial and boundary conditions. MM5 modeling framework, designed to simulate or predict mesoscale atmospheric circulations, is having a limited-area, non-hydrostatic and terrain following 12 sigma levels. The IWM energetics is studied using MM5 model outputs. Prior to this model’s validity and deviation from the corresponding observations (NCEP/NCAR) is assessed. The model’s overestimation/underestimation of wind, temperature and specific humidity at upper troposphere proves that the model has difficulty in picking up corresponding fields at all the model grid points because of terrain complexity over the Himalayas and Tibetan Plateau. Hence, the model fields deviate from the corresponding observations. However, model results match well with the winter global energy budget calculated using reanalysis dataset by Peixoto and Oort (1992). It suggests MM5 model’s fitness in simulating large scale synoptic weather systems. And, thus the model outputs are used for calculation of energetics associated with IWM. It is observed that beyond \(15^{{\circ }}\hbox {N}\) lower as well as upper level convergence of diabatic heating, which represents continental cooling and sinking of heat from atmosphere to land mass (i.e., surface is cooler than surrounding atmosphere) dominates. The diabatic heating divergence (cooling of continents) is found over ocean/sea and whole of the China region, Tibetan and central Himalayas (because of excess condensation than evaporation). The adiabatic generation of kinetic energy depends on the cross isobaric flow (north to south in winter, i.e., the present study shows strong circulation during IWM). It is found that wind divergence of model concludes lower level convergence over study region (i.e., strong winter circulation in the model fields).

Frictional Convergence in a Decaying Weak Vortex

Abstract Cross-isobaric flow and Ekman pumping are investigated in the frictional spindown of an initially barotropic vortex in a stratified atmosphere. Consistent with early work by Holton and others, it is found that the stratification limits the vertical penetration of the secondary circulation driven by friction, resulting in more rapid spindown than in the unstratified case. As a result, the cross-isobaric flow and Ekman pumping are weaker and shallower than classical calculations ignoring the stratification would lead one to believe. The effect of stability becomes stronger as the vortex becomes smaller for fixed boundary layer depth. For weak geostrophic vortices with horizontal scales of several hundred kilometers or less, such as tropical easterly waves, the reduction is particularly pronounced, which raises questions about the efficacy of Ekman pumping in forcing convection in such vortices. These results suggest a revised conceptual model for the role of Ekman pumping in the atmosphere. The theory as it stands is limited to weak, linear vortices in which geostrophic balance holds approximately, corresponding to small Rossby number, though extensions of the analytical theory to stronger vortices may be possible.

A Study of the Impacts of Vertical Diffusion on the Structure and Intensity of the Tropical Cyclones Using the High-Resolution HWRF System

AbstractThe Hurricane Weather Research and Forecasting (HWRF) system was used in an idealized framework to gain a fundamental understanding of the variability in tropical cyclone (TC) structure and intensity prediction that may arise due to vertical diffusion. The modeling system uses the Medium-Range Forecast parameterization scheme. Flight-level data collected by a NOAA WP-3D research aircraft during the eyewall penetration of category 5 Hurricane Hugo (1989) at an altitude of about 450–500 m and Hurricane Allen (1980) were used as the basis to best match the modeled eddy diffusivities with wind speed. While reduction of the eddy diffusivity to a quarter of its original value produced the best match with the observations, such a reduction revealed a significant decrease in the height of the inflow layer as well which, in turn, drastically affected the size and intensity changes in the modeled TC. The cross-isobaric flow (inflow) was observed to be stronger with the decrease in the inflow depth. Stronger inflow not only increased the spin of the storm, enhancing the generalized Coriolis term in the equations of motion for tangential velocity, but also resulted in enhanced equivalent potential temperature in the boundary layer, a stronger and warmer core, and, subsequently, a stronger storm. More importantly, rapid acceleration of the inflow not only produced a stronger outflow at the top of the inflow layer, more consistent with observations, but also a smaller inner core that was less than half the size of the original.

A New Look at the Physics of Rossby Waves: A Mechanical–Coriolis Oscillation

Abstract The presence of the latitudinal variation of the Coriolis parameter serves as a mechanical barrier that causes a mass convergence for the poleward geostrophic flow and divergence for the equatorward flow, just as a sloped bottom terrain does to a crossover flow. Part of the mass convergence causes pressure to rise along the uphill pathway, while the remaining part is detoured to cross isobars out of the pathway. This mechanically excited cross-isobar flow, being unbalanced geostrophically, is subject to a “half-cycle” Coriolis force that only turns it to the direction parallel to isobars without continuing to turn it farther back to its opposite direction because the geostrophic balance is reestablished once the flow becomes parallel to isobars. Such oscillation, involving a barrier-induced mass convergence, a mechanical deflection, and a half-cycle Coriolis deflection, is referred to as a mechanical–Coriolis oscillation with a “barrier-induced half-cycle Coriolis force” as its restoring force. Through a complete cycle of the mechanical–Coriolis oscillation, a new geostrophically balanced flow pattern emerges to the left of the existing flow when facing the uphill (downhill) direction of the barrier in the Northern (Southern) Hemisphere. The β barrier is always sloped toward the pole in both hemispheres, responsible for the westward propagation of Rossby waves. The β-induced mechanical–Coriolis oscillation frequency can be succinctly expressed as , where , and λ is the angle of a sloped surface along which the unbalanced flow crosses isobars, α is the angle of isobars with the barrier’s slope, and k is the wavenumber along the direction of the barrier’s contours.

The wind-field structure in a stably stratified atmospheric boundary layer over a rough surface

Both wind turning with height and ageostrophic flow in a stably stratified atmospheric boundary layer are analyzed using a three-parameter turbulence model. For a quasi-steady state of the boundary layer, the cross-isobaric flow is determined only by turbulent stress at the surface in the direction of geostrophic wind. The “operative” prediction models, in which the first-order turbulence closure schemes are used, tend to overestimate the boundary-layer depth and underestimate the angle between the surface and geostrophic winds when compared to “research” models (schemes of high-level turbulence closure). The true value of the angle between the surface and geostrophic winds is significant for the presentation of a large-scale flow. A nocturnal low-level jet is a mesoscale phenomenon reflected in data obtained from measurements in a stably stratified atmospheric boundary layer. It is found that such jets are of great importance in transporting humidity, momentum, and air pollution. In this study, the difference between jet flows over a homogeneous underlying surface and over a spatially localized large-scale aerodynamic roughness is shown.

Analysis of Model Results for the Turning of the Wind and Related Momentum Fluxes in the Stable Boundary Layer

The turning of wind with height and the related cross-isobaric (ageostrophic) flow in the thermally stable stratified boundary layer is analysed from a variety of model results acquired in the first Global Energy and Water Cycle Experiment (GEWEX) Atmospheric Boundary Layer Study (GABLS1). From the governing equations in this particular simple case it becomes clear that the cross-isobaric flow is solely determined by the surface turbulent stress in the direction of the geostrophic wind for the quasi-steady state conditions under consideration. Most models indeed seem to approach this relationship but for very different absolute values. Because turbulence closures used in operational models typically tend to give too deep a boundary layer, the integrated total cross-isobaric mass flux is up to three times that given by research numerical models and large-eddy simulation. In addition, the angle between the surface and the geostrophic wind is typically too low, which has important implications for the representation of the larger-scale flow. It appears that some models provide inconsistent results for the surface angle and the momentum flux profile, and when the results from these models are removed from the analysis, the remaining ten models do show a unique relationship between the boundary-layer depth and the surface angle, consistent with the theory given. The present results also imply that it is beneficial to locate the first model level rather close to the surface for a proper representation of the turning of wind with height in the stable boundary layer.

The effect of the continental boundary layer on the dynamics of fronts in a 2D model of baroclinic instability. II: Surface heating and cooling

AbstractThe effect of surface heating and cooling on the structure and evolution of the surface front that develops in the finite‐amplitude Eady wave is investigated. Spatially uniform short‐wave radiative heating of the surface is weakly frontolytic, although it appears this result may be sensitive to the way in which the boundary‐layer turbulence is parametrized. The cross‐front potential temperature gradient is weakened principally by the differential heating produced by horizontal variations in the depth of the mixed layer, and to a smaller extent by horizontal variations in the surface sensible heat flux. The direct effect of enhanced daytime turbulent stresses is to retard the winds in the boundary layer, reducing the convergence in the cross‐front plane, while the indirect effect is to produce a strong cross‐isobaric flow towards the trough, thereby increasing the convergence. The net effect is to reduce the rate of frontogenesis near the surface front. During the day the front is oriented vertically in the boundary layer because of the strong mixing. Nocturnal cooling through long‐wave radiative emission from the surface is strongly frontogenetic. At night, as the turbulence decays in the trough ahead of the surface front, air parcels accelerate down the pressure gradient towards the centre of the trough, producing low‐level cross‐front surges on both sides of the trough axis. This enhanced nocturnal convergence rapidly strengthens the cross‐front potential temperature gradient and tilts the front westward with height. Such nocturnal frontogenesis is especially intense when it follows strong daytime turbulent mixing. Copyright © 2005 Royal Meteorological Society

Boundary-Layer Damping of Baroclinic Instability

Abstract A shooting method is applied to a 100-level perturbation model with an explicit Ekman layer. Growth rates for a no-slip boundary condition are similar to those found previously. It is noted that the large cross-isobaric flow of the classic Ekman solution overestimates the damping by roughly a factor of 2. For the condition of zero vertical gradient of perturbation streamfunction at the lower boundary (allowing slip), growth rates are only slightly reduced from the frictionless case. The physical bases for an implicit boundary layer, as well as the no-slip condition, are questioned.

Comments on “A Diagnostic Method for Computing the Surface Wind from the Geostrophic Wind Including the Effects of Baroclinity”

Abstract A numerical method for solving the generalized Ekman problem in which both geostrophic wind and eddy diffusivity are allowed to vary with height is used to compute surface cross-isobaric flow angles and surface winds. These are compared with Danard's calculations using similarity functions. There is remarkably good agreement in the results obtained with these two fundamentally different approaches to the momentum transfer problem.

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.).

Kinetic energy budget of an Arabian Sea Cyclone in June 1979

A diagnostic kinetic energy study of a cyclonic storm in the Arabian Sea of the southwest monsoon in June 1979 is presented in this paper. The surface and upper air dropsonde data especially taken during Monsoon Experiment in 1979 (MONEX-79) have been used for the computations. Some of the important results emerged from the study are following :
 
 (i) The kinetic energy was maximum between 900 and 700 mb in all the period under study.
 (ii) Internal as well us the external energy source was equally responsible for the generation of
 kinetic energy of the system,
 (iii) The horizontal boundary fluxes of energy is smaller but appreciable in magnitude when compared with the generation of kinetic energy from the potential energy due to cross isobaric flow in the lower level.
 (iv) The values of different terms in kinetic energy equation are larger in case of the smaller volume when compared with the values of larger volume.

Components of large-scale kinetic energy generation during an eastern Mediterranean cyclogenesis

The kinetic energy generation due to cross-isobaric flow is defined by the product — v·∇Φ). This process is considered to be adiabatic and thermodynamically reversible. In thex, y, p coordinate system this function may be resolved into itsx andy components, which involve the zonal and meridional wind components, i.e.\( - u\frac{{\partial \Phi }}{{\partial x}}\) and\( - v\frac{{\partial \Phi }}{{\partial y}}\) respectively. The contribution of each of these components to the generation function during a case of cyclogenesis over the eastern Mediterranean is studied. The depression deepened rapidly between 9th and 10th December 1980 and was accompanied by severe weather conditions all over the eastern Mediterranean.

東アジアにおける冬期総観場の運動エネルギー論

Energetics characteristics over Siberia, Northeastern Asia, China and Japan Sea are studied with two winter's upper air data.The Siberia region, which is frequently occupied by a surface high pressure and an upper anticyclonic flow, shows destruction by cross-isobaric flow and negative dissipation of kinetic energy in the winter mean balance of kinetic energy. The Northeastern Asia and Japan Sea areas, which are frequently occupied by a surface low and an upper cyclonic flow, show generations and dissipations of kinetic energy. From the comparison of the regional balances of kinetic energy, the upper atmosphere over the China and Japan Sea regions together with the adjacent oceanic areas may be considered as the most energetically active area over the northern hemisphere in winter.The studies of kinetic energy budgets for various flow patterns show that with the exception of Siberia, the kinetic energy generation, outflow and dissipation are associated with the cyclonic type flows. Even though the flow patterns over the China and Japan Sea regions are predominantly cyclonic, their kinetic energy budgets show extremely intense transformation in comparison with that of an average cyclonic flow over other regions in this study or over North America.

On the dynamics and climatology of surface flow over the Equatorial oceans

The surface circulation over the Equatorial Atlantic and Eastern Pacific is investigated using 60-year average ship data with a one-degree square resolution. At the height of the Northern summer, a zonally oriented speed maximum extends across the Eastern part of the oceans at about 4–5 N, near where the cross-equatorial flow recurves from Southeast to Southwest. This separates a band of strong divergence immediately to the North of the Equator from the convergence belt further poleward. Strongest convergence occurs about 350 km South of the confluence between Northern and Southern Hemisphere trades, and within a region of high directional steadiness of wind. A negative wind stress curl induces upwelling and low sea surface temperature immediately South of the Equator, and downwelling to the North. In most of the Northeast trades and the Southeast trades of the Southern Hemisphere during July–August, frictional energy dissipation not offset by cross-isobaric flow produces a gradual slowing of the current and downstream convergence. As the Southern Hemisphere trades flow North from the Equator over the Eastern part of the oceans, the Coriolis acceleration increases acting to the right, enhancing the cross-isobaric angle and resulting in net kinetic energy generation, the aforementioned speed maximum and attendant zone of strong divergence. Continuation of this Coriolis effect recurves the flow from Southeast to Southwest, which further downstream diminishes the cross-isobaric angle, thus impairing kinetic energy generation and resulting in a band of maximum convergence. A positive relation is found between the latitude of the discontinuity separating Northern and Southern Hemisphere trades and the latitude and intensity of the zonally oriented convergence/divergence bands, both seasonally and on a year-to-year basis. DOI: 10.1111/j.2153-3490.1978.tb00859.x

Response of the Hurricane Boundary Layer to Changes of Sea Surface Temperature in a Numerical Model

An axisymmetric, multilayer hurricane model is used to investigate the hurricane's response to sudden changes of sea surface temperature (SST). The model contains a parameterization of the planetary boundary layer (PBL) which includes matched formulations for the surface layer and the mixed layer. The heat, moisture and momentum fluxes are mutually dependent through Monin-Obukhov similarity theory. The height of the model hurricane PEL is 400–500 m, below which the potential temperature and specific humidity are nearly invariant with height. The flow in the hurricane PBL is characterized by subgradient tangential velocities and nearly uniform cross-isobaric flow angles. The sensible heating from the ocean is insignificant, but the evaporation is large. The magnitudes of the equivalent drag coefficients are approximately one-third those of the exchange coefficients for heat and moisture. As the SST is suddenly decreased (increased), the steady-state model hurricane experiences two stages of modification. The first stage consists of adjustments of the hurricane PBL featuring a weakened (enhanced) dynamic and thermodynamic coupling of the storm with the ocean. No important changes of intensity occur during this stage, which lasts several hours. The decrease (increase) of kinetic energy dissipation offsets part of the decrease (increase) of kinetic energy generation. The second stage is characterized by a steady modification of storm intensity. The fluctuations of intensity in these experiments are less pronounced than those shown by a similar model with a conventional bulk parameterization of the hurricane PBL.

On the dynamics and climatology of surface flow over the Equatorial Oceans

The surface circulation over the Equatorial Atlantic and Eastern Pacific is investigated using 60-year average ship data with a one-degree square resolution. At the height of the Northern summer, a zonally oriented speed maximum extends across the Eastern part of the oceans at about 4–5 N, near where the cross-equatorial flow recurves from Southeast to Southwest. This separates a band of strong divergence immediately to the North of the Equator from the convergence belt further poleward. Strongest convergence occurs about 350 km South of the confluence between Northern and Southern Hemisphere trades, and within a region of high directional steadiness of wind. A negative wind stress curl induces upwelling and low sea surface temperature immediately South of the Equator, and downwelling to the North. In most of the Northeast trades and the Southeast trades of the Southern Hemisphere during July–August, frictional energy dissipation not offset by cross-isobaric flow produces a gradual slowing of the current and downstream convergence. As the Southern Hemisphere trades flow North from the Equator over the Eastern part of the oceans, the Coriolis acceleration increases acting to the right, enhancing the cross-isobaric angle and resulting in net kinetic energy generation, the aforementioned speed maximum and attendant zone of strong divergence. Continuation of this Coriolis effect recurves the flow from Southeast to Southwest, which further downstream diminishes the cross-isobaric angle, thus impairing kinetic energy generation and resulting in a band of maximum convergence. A positive relation is found between the latitude of the discontinuity separating Northern and Southern Hemisphere trades and the latitude and intensity of the zonally oriented convergence/divergence bands, both seasonally and on a year-to-year basis. DOI: 10.1111/j.2153-3490.1978.tb00859.x

Subsynoptic-Scale Energy Transformations in Various Severe Storm Situations

The energy transformations ore studied with a subsynoptic-scale upper air network for three types of severe storm environments under convective, nonconvective and frontal situations. The environment of the convective storms is most active energetically with intense generation and dissipation of kinetic energy taking place in the upper layer of the atmosphere. In the case of the non-convective environment, a strong adiabatic destruction of kinetic energy through the cross-isobaric flow takes place in the upper layer of the atmosphere. In the frontal cases the upper layer of the atmosphere becomes energetically inactive for the observed scale in terms of the gross energy budget. Examination of the kinetic energy budget during the thunderstorm passage indicates the existence of subsynoptic-scale areas of adiabatic generation and destruction of kinetic energy with the maximum destruction in the middle and upper layers in the area of the maximum storm intensity. The eddy energy transformations in the disturbances are intensive in the active convective environment, and a significant amount of eddy kinetic energy is derived through the local baroclinic conversion process.

A Semi-Empirical Dynamical Model for the Trade-Wind Convective Layer

Abstract It is shown that it is feasible to investigate theoretically the steady state local structure of the trade-wind convective layer with semi-empirical models by systematically synthesizing the various published results on the mass and energy budgets for the Atlantic region. A thermodynamic model was first developed to clarify how the thickness and the virtual potential temperature of the layer depend upon the various parameters. Realistic results were obtained. A dynamical model was later constructed to examine the role of the vertical fluxes of momentum in addition to the thermodynamic processes. It was found that there are five key adjustable parameters, namely the Coriolis parameter, the imposed interior flow, the surface horizontal divergence, the radiational cooling, and the heating due to precipitation. The variations of the structure of the convective layer with these five parameters and their physical implications are discussed. In particular, the cross-isobar flow was found to have a relat...

A Numerical Study of the Influence of Advective Accelerations in an Idealized, Low-Latitude, Planetary Boundary Layer

Abstract Steady, longitudinally invariant, barotropic, boundary layer flow is numerically studied at low latitudes where advective accelerations may he large and the Coriolis parameter is small. Flow is generated by specifying the pressure gradient field independent of the flow. It is found that as the flow approaches the equator, advective terms associated with the large latitudinal variation of the Coriolis parameter become important. As the flow crosses the equator, adjective accelerations may become important to the extent that the boundary layer downstream from the equator is radically different from the Ekman boundary layer. Compared to Ekman flow, the wind vector may rotate with height in the opposite direction, and the boundary layer depth may be considerably thinner and less dependent on latitude. The cross-isobar flow of this advective boundary layer is deeper and may be stronger, so that spatial transitions between this boundary layer and a quasi-Ekman boundary layer can produce significant ver...

An approximation to boundary layer wind profiles

A procedure for computing approximate solutions of the boundary layer equations for a neutrally stratified atmosphere is described. The approximation equations, which are derived using perturbation expansion theory, are valid for cases when the cross-isobaric flow is small. The first approximation equations are integrated analytically and then applied to cases for which the gradient flow is either straight or curved. Comparison of the approximations with numerical solutions of the exact equations indicates that the approximations are generally useful under normal conditions of flow near the surface of the earth. DOI: 10.1111/j.2153-3490.1967.tb01508.x