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B. R. Sutherland

Abstract

It is proposed that shear instability of the upper flank of the equatorial undercurrent may generate, under a broad range of conditions, downward propagating internal gravity waves (IGW) of large amplitude. The generation mechanism is shown to require only that the background stratification is weak where the shear is large (i.e., in the mixing region) and that the stratification is sufficiently large in the far field (i.e., near the thermocline). In a series of studies, the generation of IGW from unstable shear flows is examined. Linear theory is used to predict under what circumstances the generation of IGW may be large, and fully nonlinear simulations restricted to two dimensions are employed to provide estimates of the degree of vertical mixing and of the vertical transport of horizontal momentum by IGW. In particular, the simulations demonstrate that, when large amplitude IGW are generated by shear instability, the mean flow itself is significantly decelerated in the mixing region. The momentum flux associated with the radiating IGW is large, and it is proposed that these may act in part as a momentum source to the deep equatorial countercurrents.

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B. R. Sutherland

Abstract

Linear theory for modes in a nonuniformly stratified, semi-infinite shear flow demonstrates that Rayleigh waves (stable waves propagating in fluid with spatially varying shear) couple with evanescent internal waves. If the bulk Richardson number (the squared ratio of the buoyancy frequency and shear) lies between 1/4 and 1, the waves have infinite e-folding depth for waves with critical relative horizontal wavenumbers. Fully nonlinear numerical simulations examine the effect of Rayleigh wave–internal wave coupling when the shear layer is localized and is thus Kelvin–Helmholtz unstable. Diagnostics examining profiles of the wave-induced mean flow show that if the bulk Richardson number is of order unity, significant momentum is extracted from a shear layer as a consequence of transport by waves. The work is extended to the study of unstable jet flows and applications of this work for internal wave generation by dynamic instability of the upper flank of the jet stream are discussed.

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B. R. Sutherland and W. R. Peltier

Abstract

A mechanism is investigated whereby large amplitude internal gravity waves (IGWs) may be excited by the tropospheric jet stream when this is driven to parallel shear instability following a rapid external forcing of the mean flow a circumstance that might be realized, for example, through ageostrophic effects in the process of baroclinic wave development. A series of mean states are examined, first on the basis of linear theory, to determine the characteristics of the most unstable normal mode, which is expected to dominate the initial stages of flow evolution. Two-dimensional nonlinear numerical simulations of stratified, incompressible, Boussinesq flow in a periodic channel are also performed. On the basis of linear theory, the authors show that it is possible to assess whether IGWs will be strongly excited by examining whether the initial instability satisfies an easily calculable criterion that has been previously termed the “penetration condition.” In cases in which the penetration condition is satisfied, large amplitude IGWs with the same horizontal phase speed and wavelength as the most unstable mode of linear theory are shown in the nonlinear simulations to radiate freely into the stratosphere, and the process of stabilization of the mean state is thereafter significantly influenced by the extraction and upward vertical transport of horizontal momentum by IGWs away from the mixing region. In cases in which the penetration condition is not satisfied only very weak internal wave emission is observed. These small amplitude waves evidently develop through nonlinear mechanisms. On the basis of model calculations performed using initial conditions intended to simulate realizable midlatitude zonal flows, the authors demonstrate that the vertical flux of horizontal momentum delivered into the stratosphere by such sheer instability could reach levels comparable to the fluxes associated with topograhically forced internal waves that develop during severe down-slope windstorm events. This raises the question as to whether there may be circumstances in which the action of gravity wave drag on the general circulation could be affected by the process of emission rather than by the process of absorption. The basis of all current IGW drag parameterization schemes employed in large-scale models is that momentum is being deposited into the mean flow by IGW breaking. The authors question the universal validity of this assumption.

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B. R. Sutherland, C. P. Caulfield, and W. R. Peltier

Abstract

Two mechanisms are proposed whereby internal gravity waves (IGW) may radiate from a linearly unstable region of Boussinesq parallel flow that is characterized in the far field by constant horizontal velocity and Brunt-Väisälä frequency. Through what is herein referred to as “primary generation,” IGW may be directly excited by linear instability of the initial-state parallel shear flow. Characteristically, these waves propagate with horizontal phase speed and wavenumber equal to that of the most unstable mode of linear stability theory. Through the second mechanism, referred to as “secondary generation,” IGW may be excited via nonlinear modification of the initial instability into a form that couples strongly to a large amplitude outgoing internal wave field. The authors propose that the primary generation of IGW may occur provided a penetration condition, which is derived on the basis of linear theory, is satisfied. The penetration condition provides a limit on the growth rate of a disturbance of any particular frequency that is capable of propagating into the far field. This hypothesis is supported by a sequence of representative nonlinear numerical simulations in two spatial dimensions for both free mixing layer and jet flows with horizontal velocity profiles U(z) = tanh (z) and U(z) = sech2(z), respectively. For the purpose of these analyses, the fluid density is taken to be such that the square of the Brunt–Väisälä frequency is given by N 2(z) = J tanh2(z/R). Such stratification allows both for the development of large-scale eddies in the region of low static stability and, in the far field where N 2J is positive and approximately constant, for the radiation of a broad frequency spectrum of IGW.

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R. A. Sutherland, R. D. McPeters, G. B. Findley, and A. E. S. Green

Abstract

We report on experimental and analytical techniques for extending the useful range of sunphotometry and spectral radiometry to ultraviolet wavelengths < 360 nm. These entail modifications of the Bouguer-Langley method which allows for the spectral structure in the ozone absorption band, the detailed nature of the extraterrestrial solar flux, and round-earth corrections. Our techniques take advantage of photographic measurements of the solar aureole to provide estimates of the scattered light contributions when the instrumental solid angle exceeds that subtended by the sun.

These results are also used in conjunction with absolute irradiance measurements at the ground level made with a single monochromator equipped with solar blind photomultiplier and a long-wavelength NiSO4 rejection filter. The comparison of the observations with calculations based upon the detailed extraterrestrial spectrum of Arvesen is used to determine the ozone thickness and the aerosol optical depth. We discuss the limitations of our results and work which remains to be accomplished.

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Renee A. McPherson, Christopher A. Fiebrich, Kenneth C. Crawford, James R. Kilby, David L. Grimsley, Janet E. Martinez, Jeffrey B. Basara, Bradley G. Illston, Dale A. Morris, Kevin A. Kloesel, Andrea D. Melvin, Himanshu Shrivastava, J. Michael Wolfinbarger, Jared P. Bostic, David B. Demko, Ronald L. Elliott, Stephen J. Stadler, J. D. Carlson, and Albert J. Sutherland

Abstract

Established as a multipurpose network, the Oklahoma Mesonet operates more than 110 surface observing stations that send data every 5 min to an operations center for data quality assurance, product generation, and dissemination. Quality-assured data are available within 5 min of the observation time. Since 1994, the Oklahoma Mesonet has collected 3.5 billion weather and soil observations and produced millions of decision-making products for its customers.

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