George Chimonas

Steps, Waves and Turbulence in the Stably Stratified Planetary Boundary Layer

The stably-stratified planetary boundary layer contains small-vertical-scale, step-like structures, waves on a multitude of scales, large horizontal eddies and small-scale turbulence, all of which constantly interact with, and modify, one another. Current knowledge of how the various components act in the vicinity of the step-like structures is surveyed. It is concluded that packets of internal waves are the main conduit for interaction within and across the boundary layer, and low-intensity critical-level absorption at the fringes of their spectrum probably maintains the step-like structures. Further investigation of the processes requires intensive observations of the four-dimensional structure of the region, but such an investigation will need a new generation of high-resolution sensing systems.

Wave Exchange between the Ground Surface and a Boundary-Layer Critical Level

Abstract Gravity waves induced by two- and three-dimensional terrain features are examined theoretically in the planetary boundary layer (PBL) using a linear wave model that includes reabsorption at a critical level. The PBL structure is characterized by a constant Brunt-Vaisala frequency and a hyperbolic tangent wind speed profile, which can be adjusted to produce critical levels. It is found that for typical values of wind speed and thermal stratification in the stable PBL and for even mild terrain disturbances, the Reynolds stress and surface drag caused by surface-generated waves can be at least as large as those conventionally associated with surface friction. The wave drag will act on the PBL flow where wave dissipation occurs, for example, at a critical level or in regions of wave breaking. The drag over a given crosswind section of a two-dimensional ridge is about twice as great as that over a three-dimensional of approximately the same horizontal area. An entirely new result is the prediction tha...

Wave drag in the planetary boundary layer over complex terrain

The concepts of mountain-induced wave drag are applied to the smaller scale problem of the boundary layer over complex terrain. It is found that the Reynolds stress and surface drag caused by surface-generated waves can be at least as large as those conventionally associated with turbulence. Conditions in which wave effects are important are identified.

On internal gravity waves associated with the stable boundary layer

The stable planetary boundary layer at the baseof the residual layer supports internalwaves that are unambiguously ‘boundary layer’ incharacter. Some of these wavesare instabilities and some are neutrally stable modes, but they all have critical levelsin the residual layer. These waves exist for a broad range of conditions and should bea major component of any ducted disturbance that propagates within ninety degreesof the wind direction. The wave properties can be computed without the numericaldifficulties usually associated with critical-level systems.

The excitation of ducted modes by passing internal waves

A local peak in the atmospheric wind or stability profile supports ducted (vertically trapped) modes. If the duct is rather high in the atmosphere it cannot be directly excited by the major atmospheric sources such as convection, but an internal gravity wave radiating out of such sources transfers energy to the duct via nonlinear processes related to the resonance triad interaction. The amplitude of such a transfer is computed. The modal excitations remain after the internal wave has left the region, and while each passing wave adds only a small amount of activity to the duct such contributions are cumulative and drive the modal activity up to a level where it is limited by local dissipation. This suggests the possibility of monitoring the mean flux of internal waves through the region by measuring activity within elevated ducts, a procedure that offers several advantages over monitoring the wave flux directly.

The Transfer of Angular Momentum from Vortices to Gravity Swirl Waves

Gravity swirl waves are three-dimensional internal gravity waves in cylindrical coordinates. A radially propagating gravity swirl wave transports angular momentum from a central source region to distant regions of the atmosphere. Accordingly, atmospheric conditions that support propagating internal waves allow a vortex to decay by radiating away its rotation, while regions of neutral stability around the vortex insulate it from such losses. This may be a factor in determining whether convective storm cells attain strong rotation.

Waves and the Middle Atmosphere Wind Irregularities

Abstract Middle atmosphere wind irregularities with vertical scales less than about 5 km are modeled as gravity shear waves that terminate abruptly on attaining Hodges’s condition for overturning, N2 ≈ 0. A quasi-random spectrum of such waves is constructed, and it reproduces the main features of the winds, including the ragged profiles observed in individual soundings and the short vertical wavelength tail of the wind’s power spectrum. Shear analysis of observations reveals that around 100-km altitude the control of instability passes from small-scale internal waves to the tides, a change that probably has some bearing on the position of the turbopause.

A Thunderstorm Bow Wave

Abstract The thunderstorm solitary gust or bow wave, observed by Doviak and Ge, is examined from the viewpoint of boundary layer wave theory. It is concluded that all its well defined characteristics are consistently modeled as a bow wave of ducted atmospheric modes accompanying the traveling storm. Secondary features, such as the later onset of turbulence, the solitary echo in the radar return, and the apparent rarity of such events can also be understood through a bow wave model. It is also suggested that the radar echo return cannot be attributed to a homogeneous distribution of scattering centers, and more investigation into the actual scattering process is needed.

The combined Rayleigh, Kelvin–Helmholtz problem

Stability characteristics of a model tilted shear flow are studied. The model allows a formulation that can be readily solved for all orientations of the shear from the Rayleigh case through to the Kelvin–Helmholtz case. It is found that any tilt away from the strict Kelvin–Helmholtz situation destabilizes the flow, no matter how large the Richardson number. The transition from the Rayleigh limit to the Kelvin–Helmholtz limit proceeds smoothly, with the stability curves displaying a characteristic pattern of evolution.

Pressure gradient amplification of shear instabilities in the boundary layer

Abstract Jeffreys’ instability mechanism is applied to a boundary layer that supports shear instability. The combined instability is an order of magnitude more effective than shear instability alone. The increase in effectiveness is highly sensitive to the stratification and the wind speed near the ground.