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John H. E. Clark

Abstract

The response to pressure falls near a coastline is studied with a linear shallow-water representation of the inversion-capped marine layer. The aim is to examine Catalina eddy development in the southern California bight region. Of particular interest is response sensitivity to background flow parallel to the coastline, as quantified by a Froude number (Fr). A conceptual model of eddy evolution is formulated that focuses on a pair of Kelvin waves triggered by the pressure falls. For subcritical Fr,|Fr| < 1, the waves move out of the regions of most favored growth located to the immediate north and south of the pressure fall center. Only a weak residual circulation remains in the bight region and an eddy does not form, For supercritical Fr, the waves tend to be held in place by the background flow. They thus undergo considerable amplification before dissipation eventually halts growth. A vigorous eddy circulation results in the bight region that resembles observed structures.

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John H. E. Clark

Abstract

The structure and vertical propagation of a quasi-geostrophic disturbance in the presence of a constant zonal flow which has been distorted by a finite-amplitude forced stationary wave is considered. The vertical structure of the finite-amplitude wave is given by an eigenvalue of the basic state potential vorticity equation. The disturbance is described by a truncated set of Fourier modes in the east-west direction whose vertical structure is found by solving a second-order Green's function equation. Variations with respect to time are ignored and the usual inviscid, adiabatic assumption is made.

It is found that the distortion of the basic flow is manifested, as far as the disturbance is concerned, as a horizontal sheet of horizontal potential transport, and the resulting solution behaves as if a critical level occurred at this level in spite of the fact that the zonally-averaged east-west ambient flow is height-independent.

It is shown that heat and energy propagation by a disturbance of central wavenumber 2 can be significantly enhanced by the presence of a wavenumber 1 distortion of ambient westerlies. On the other hand, transports by a disturbance of central wavenumber 1 are normally suppressed and, in fact, can be in the opposite direction to what would be expected with an undistorted zonal flow. These effects on wavenumber 2 agree with the suggestion of Matsuno after his numerical study, which did not account for the distortion of the ambient flow. Also a node-like vertical structure introduced into wavenumber 1 agrees with the observational study by Sato.

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John H. E. Clark

Abstract

Adiabatic, inviscid, quasi-geostrophic perturbations on a β plane are forced at some initial time by the switch-on of a vertical velocity or temperature disturbance at the base of a stratified atmosphere. The vertical variation of the basic zonal wind is represented by three simple layered models. A Laplace-transformed potential yorticity equation is solved in each layer, and after solutions are matched across interfaces, inversion integrals are evaluated numerically and asymptotically. After initial high-frequency planetary waves have died out by radiating energy to infinity, the response primarily consists of a standing-wave mode and a traveling barotropic Rossby mode. In spite of the fact that the barotropic model propagates energy vertically with an infinite group velocity, a signfficant build-up of wave energy is not realized until many days after the switch-on. The time dependence of energy and heat fluxes is caused by the interaction of these modes. The role of unstable modes and other vertically propagating groups of planetary waves is considered. The propagation of energy into the upper atmosphere is shown to be strongly affected by the presence of westerly wind maxima where the speed exceeds the critical value for propagation of energy by stationary planetary waves. Delays in the arrival of energy by a day or more can be caused by such regions.

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John H. E. Clark

Abstract

Quasigeostrophic disturbances on a midlatitude β-plane channel forced by radiative heating perturbations due to synoptic-scale variations of low-level stratiform cloud are considered. The longitudinal phase of the cloud is linked to that of the low-level streamfunction. Cloudiness is wavelike with cooling above cloudy areas that decreases exponentially with height. No perturbation cooling occurs above cloud-free areas. Two background flows are considered: one is constant and the other has a jet centered near the tropopause. Forced linear steady waves are found. Though infinitesimal amplitude disturbances are considered, the problem is nonlinear because of the coupling between cloud and winds. The resulting structures are sensitive to the phase shift between cloud and streamfunction with strongest coupling for cloud to the west of surface troughs. The waves have vertical scales on the order of the troposphere depth. The stationary structures capture the summertime pattern of stratocumulus off California and its linkage to the mid-Pacific ridge. When zonal-mean cloud cooling is allowed, the mean westerlies are strengthened in the northern half of the channel near the lower boundary. Synoptic-scale amplitudes respond to the mean cloud cooling by increasing (decreasing) just above the cloud in the northern (southern) part of the channel. Mean cloud cooling also renders the background flow baroclinically unstable in the northern part of the domain.

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John H. E. Clark

Abstract

A cyclogenesis event that occurred over Colorado in early March of 1981 is the focus of this study. Two features that seemed to play a role in storm initiation were a traveling upper troposphere disturbance associated with an undulation on the subtropical front and a warm-cored shallow surface trough that was guided along the eastern slope of the Rockies from Canada to Colorado. The arrival of the latter feature initiated a sudden shift of the surface flow from upslope to downslope on the eastern side of the continental divide. A time-dependent quasi-geostrophic model was used to study the interaction of the traveling short wave and a broad topographic surface ridge in the presence of a baroclinic mainly westerly background flow. Westerly and easterly background surface winds were used to determine whether the surface trough arrival had any influence on the vigor of lee cyclogenesis initiated by the upper troposphere short-wave trough. With surface westerlies rapid cyclogenesis occurred, while with surface easterlies little cyclogenesis was found to the east of the Rockies. Thus the shallow surface trough's arrival seemed to be crucial to storm initiation. These findings were based on a linear model. It is shown, however, that the height of the Rockies necessitates the inclusion of finite amplitude effects associated with the lower boundary into the model. A simplified calculation was carried out to examine the effect of forcing associated with the interaction of the background thermal wind and a lower boundary enhancement due to finite amplitude topography. Lee cyclogenesis is further enhanced with surface westerlies and further suppressed with surface easterlies. Thus the shallow surface trough takes on an even more important role triggering storm growth.

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John H. E. Clark

Abstract

Observed convective systems, such as mesoscale convective complexes (MCCs), often undergo repeated cycles of nocturnal growth and daytime decay especially during the summer. The gravity wave response to these pulsations is poorly understood. The motivation for this study is to understand this response and especially its sensitivity to environmental factors such as horizontal wind and static stability. A semianalytical approach is used that focuses on the roles of singularities in a complex horizontal wavenumber space. The model is linear, Boussinesq, hydrostatic, and rotating with uniform ambient conditions. Prescribed, 2D, pulsating, upright, convective heating drives the waves. The Fourier transform technique is used to unravel the response into a discrete and continuous horizontal spectrum.

Many features of the response depend on a Froude number, F = πU/(DN) where U = background wind, D = depth of source region, and N = buoyancy frequency. The most efficient forcing of the gravity wave field occurs near criticality (F = 1). For typical values of U and D associated with midlatitude convective systems, the critical value of N is about one-fourth of average tropospheric values. The resulting enhanced loss of energy from the convective system due to gravity waves could limit the intensity of convective systems near criticality. At subcriticality (F < 1), the pulsating upstream response is dominated by unbalanced ageostrophic propagating modes upstream of the source region and by a balanced geostrophic mode downstream of the source. The latter advances with a speed equal to the background flow. No far-upstream response occurs for supercriticality (F > 1). The downstream response for F > 1 is dominated by the geostrophic mode in the pressure, temperature, and source-parallel wind. In addition, a vigorous ageostrophic mode advances downstream from the subcritical source region giving rise to alternating regions of rising motion and subsidence. It is hypothesized that the latter could trigger new lines of convection in the downstream subcrticial Froude number regime.

Mature MCCs often develop low-level cooling in response to evaporative cooling. This cooling primarily triggers the advective inertial gravity wave mode, which propagates downstream at the background wind speed. It is shown that subcritical flows (F < 1; weak ambient flow and/or strong static stability) are tuned to strongly respond to this mode. It is suggested that the development of new convection might be suppressed near the system under subcritical conditions. Low-level cooling has little effect on the supercritical response.

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JOHN H. E. CLARK

Abstract

A six-level quasi-geostrophic model including radiative and photochemical processes in the manner suggested by Lindzen and Goody is run from a state of joint radiative-photochemical equilibrium for midwinter conditions. The spectral method is used to integrate the equations where all dependent variables are represented by a set of spherical harmonics with east-west wave numbers 0, 1, 2, 3, and 6 included. The winter storage of ozone in the polar lower stratosphere is simulated, and the importance of horizontal planetary scale transports and the vertical eddy diffusion of ozone is demonstrated. The rapid dissipation of upper stratospheric temperature disturbances by joint radiative-photochemical relaxation is discussed, and the importance of tropospheric forcing and nonlinear exchanges of kinetic energy between the planetary scale waves is demonstrated. The energetics of downward-propagating spontaneous warmings is discussed. A full-scale warming is triggered by strengthening the north-south lower tropospheric temperature gradient. Its main energy source is found to be a greatly increased forcing of the stratosphere from below.

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John H. E. Clark
and
Lauren Taylor Morone

Abstract

A series of at least daily rocket soundings of the mesosphere at Wallops Island, Virginia (37°50′N, 75°29′W), in August and September 1976 reveal near simultaneity between rapid temperature rises and tropospheric convection in the form of squall lines.

A multilevel numerical model is developed to test the hypothesis that the convection and warnings are related via internal gravity waves. Some features of the model are: 1) the wave energy source is expressed in terms of cloud-base mass flux, plume diameter and buoyant updraft velocity; and 2) the turbulent-viscous gravity wave dissipation is limited to above 55 km and is parameterized on the basis of findings by Hines (1965).

Significant findings are: 1) mesospheric heating rates of the same order as those observed result for reasonable values of the convective parameters and in situ dissipation time scales; 2) only gravity waves confined to a well-defined wavelength and frequency interval are able to propagate upward to mesospheric altitudes; 3) heating rates are strongly dependent on plume diameter and updraft velocity; and 4) for a given cloud-base mass flux, heating rates are optimized for a plume updraft velocity of 10 m s−1.

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John H. E. Clark
and
Scott R. Dembek

Abstract

The Catalina eddy that existed from 5 July to 12 July 1987 during FIRE (First ISSCP Regional Experiment) over offshore California is analyzed. There were two stages to the eddy's lifecycle. During the first, from 5 July to 1200 UTC 9 July, the eddy formed just south of Santa Barbara and drifted southeastward parallel to the coastline. This motion is attributed to an equivalent β effect associated with gradients of marine layer depth perpendicular to the coast. The eddy's thermal structure was characterized by an elevated marine inversion with surface temperatures 2°–4°C higher than beyond the periphery. Over offshore regions a sharp edge to the eddy was noted with a sudden change in mixed layer depth, wind speed and direction, and temperature. The eddy's influence on coastal winds was most notable during the nighttime and early morning. The strong local sea-breeze circulation overwhelmed the coastal eddy circulation during daytime. A pronounced diurnal wind fluctuation was observed at San Nicolas Island during this period, associated with a perturbation wind parallel to the California coastline. We conclude that it is due to either an extended coastal sea-breeze influence (latitudes in this region are close to the critical latitude according to linear theory) or northward-propagating coastally trapped Kelvin waves. The eddy's second stage was initiated on 9 July by the formation of a cutoff low in the middle troposphere immediately above the eddy. During this period the eddy expanded horizontally, moved southwestward away from the coastline, and eventually weakened. For a brief time, a coherent meso-α structure existed from the surface to about 500 hPa.

Eddy formation was precipitated by the passage of a low-level trough that strengthened the northerly flow across the mountains north of Santa Barbara. Froude numbers at the time of eddy formation suggest considerable lee troughing as the airflow was forced over and possibly around the topography.

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Mark R. Schoeberl
and
John H. E. Clark

Abstract

A global model of planetary wave propagation in a spherical atmosphere is used to examine the spectrum of free or resonant planetary waves of the solstitial stratosphere. These free modes are located by forcing the model with a weak periodic vertical velocity along the lower boundary and looking for a resonant response in wave amplitude. The modes correspond to the natural traveling oscillations in the earth's atmosphere, of which the 5-day wave is the best known example.

The 15-day wave observed by Madden (1978) and others is found to be such a resonant mode. We find that the strong stratospheric winds cause the 15-day wave to become baroclinic by trapping the wave between the earth's surface and the strong winds at the stratopause. The strong winds effectively reduce the atmospheric damping which greatly reduces the amplitude of barotropic waves with periods >10 days. The computed meridional structure of the 15-day wave is in reasonable agreement with Madden's (1978) observations at extratropical latitudes. Our results indicate that a mode resembling the H⅓ Hough function represents the principal resonant component.

Other resonances at periods longer than 15 days for zonal harmonies 1, 2 and 3 are shown, and these modes are also baroclinic. At very long periods (50–100 days) broad resonant peaks are observed for all three zonal harmonics. These peaks indicate that the structure of stationary planetary waves is very sensitive to changes in the mean zonal wind (frequency changes in this model) as has been noted by other authors.

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