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David J. Raymond

Abstract

A reformulation of wave-CISK shows that gravity waves generated by the divergence of cumulus mass fluxes am responsible for the forcing of further convection. When downdrafts are included, a new, non-propagating instability arises. This mode has a growth rate much larger than the usual, propagating wave-CISK mode, but requires the downdraft mass flux to exceed a certain critical value. The nonpropagating mode apparently corresponds to air-mass thunderstorms, whereas the propagating mode suggests long-lived convection. The two modes respond very differently to wind shear.

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David J. Raymond

Abstract

The wave-CISK model of Raymond incorporating lag effects in the updraft and downdraft is implemented as an initial value problem in physical space. Examples of both midlatitude and tropical squall lines are successfully simulated. Diagnosis of the model shows that condensational heating in the updraft is the primary driving mechanism of a mature squall line, with evaporative cooling and convective momentum transfer playing subsidiary roles. However, an instability involving the displacement of boundary layer air by downdrafts apparently plays an important role in squall line initiation.

A weakness of the model is its inability to predict the direction of squall line propagation relative to the low level wind shear. This is traced to the insensitivity of the convective parameterization to midlevel entrainment. However, unlike strict two-dimensional squall lint models, the parameterization allows cross-stream mass transfer to occur in the context of overall slab symmetry. Such transfer is dynamically important for at least some squall lines.

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David J. Raymond

Abstract

A two-dimensional, hydrostatic, nonrotating numerical model with a cumulus paramelerizafion is developed to study the early stages of mesozcale convective systems. Amplifying, forced gravity waves occur when peneirative downdrafts are present. Updraft heating by itself is unable to cause convective sysiems to intensify. Propagation speeds are in rough agreement with those observed in midlatitude mesoscale convective systems. The conditionalityof the convection and the horizontal advection of precipitation by the relative wind produce las between lifting and convection that are not found in conventional wave-CISK models. These lags slow the growth and reduce the propasation speeds of forced gravity waves.

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David J. Raymond

Abstract

An extension to the Gossard-Munk impedance relation, valid for large-amplitude, dissipative disturbances, is outlined. This new relation yields information on the dissipative properties of a convergence line if the associated wind and pressure perturbations as well as the propagation speed of the line are known. When applied to the gust fronts of squall lines and tradewind showers, it is found that the fractional dissipation rate increases with the non-dimensional amplitude of the disturbance.

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David J. Raymond

Abstract

The response of a stratified atmosphere to a steady, moving source of heat is explored as a tool for understanding organized, moist convection. The results are shown to depend strongly on the ratio of the vertical wavelength of the forced gravity waves to the depth of the disturbance.

The wave-CISK mechanism is understood as a coincidence between heating and low-level upward motion that only occurs for certain values of this ratio. Wave-CISK is shown to require precipitation-induced cooling to work satisfactorily. The cooling suppresses the subsidence wave produced by latent heat release, and allows parcels from near the surface to rise to the level of free convection.

Squall lines often have a midlevel jet flowing through them from front to rear. Such a jet is seen in our simulations, and is the result of the oscillatory response of a stratified fluid to a moving beat source. The jet is strongest when evaporative cooling is included.

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David J. Raymond

Abstract

A new theoretical approach to mountain flow is presented. The boundary value problem of stratified flow over a high ridge of arbitrary shape is solved by an iterative scheme using integral equations. The dynamical effects of boundary layer heating and cooling are also obtained. The shape as well as the size of a ridge is shown to be of importance for the resulting flow. The gross effect of boundary layer heating over the ridge is to dampen the perturbation in the airflow, while cooling has the reverse effect.

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David J. Raymond

Abstract

A two-scale model of convective disturbances is developed in which the larger scale describes the disturbance as a whole and the smaller scale consists of convective turbulence. A novel formulation for the turbulence is developed in the context of a one-dimensional model for the disturbance scale flow. The convective turbulence scheme assumes that vertical mixing occurs when and only when an unstable lapse rate is present. The intensity of the mixing is explicitly computed via a linearized model for the turbulence, coupled with scaling arguments.

The model is ultimately applied to moist convection over a heated island. Intense vertical mixing occurs near the top of the resulting cloud. This mixing has a strong effect on the mean cloud circulation, resulting in inflow top and bottom and outflow around the middle. This is in marked contrast to the ascending bubble structure that occurs when such mixing is suppressed. The results confirm the arguments of Fraser (1968).

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David J. Raymond

Abstract

A system for analyzing and displaying gridded numerical data called Candis is described. The system is written in the C programming language, and is built on a standard way of representing such data. The analysis package is modular, hierarchical, and extensible. Facilities available on the UNIX operating system enhance its ease of use.

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David J. Raymond

Abstract

The mechanisms that regulate moist convection over the warm tropical oceans are not well understood. One school of thought holds that convection is caused by the convergence of moisture, which in turn is produced by an independent dynamical mechanism. Another school maintains that convection occurs as needed to just balance the production of convective instability and that the timescales to establish this balance is much less than the timescales of tropical disturbances. This is called the quasiequilibrium hypothesis.

This paper explores how convection is actually governed over the west Pacific warm pool. Convection appears to be initiated there when the boundary-layer equivalent potential temperature exceeds a threshold value that is determined by conditions just above cloud base. Given known surface flux values and the propensity for convection to inject low equivalent potential temperature air into the boundary layer, it is shown that under most circumstances convection is regulated by a balance between the respective tendencies of surface fluxes and convective downdrafts to increase and decrease boundary-layer equivalent potential temperature. This regulatory mechanism is operative on timescales of one-half of a day or greater and is denoted boundary-layer quasiequilibrium. Given additional information about the ratio of downdraft to updraft volume fluxes and the equivalent potential temperature deficit in downdrafts, it appears to be possible to infer the mean vertical velocity at cloud base over timescales for which the clear-air vertical velocity is radiatively governed.

On the basis of this analysis it is hypothesized that moisture convergence and low-level vertical motion over the west Pacific warm pool are largely a consequence rather than a cause of convection, at least on timescales of one-half of a day or greater. Externally imposed vertical motion should result in significant additional latent heat release only where the atmosphere is saturated. This typically occurs in the Tropics in the middle and upper levels of regions that are already convectively active.

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David J. Raymond

Abstract

Hadley cell simulations over a tropical ocean are reported that suggest that the emission and absorption of thermal radiation by clouds plays an important role in the dynamics of the Hadley circulation. In particular, inclusion of interactions between clouds and radiation causes the tropical atmosphere to become unstable to large-scale convective circulations driven by differential radiative heating between cloudy and clear regions, even when the sea surface temperature and the solar forcing do not vary with latitude or time. Zonally symmetric circulations take the form of an equatorially asymmetric convective cell, with ascent in one hemisphere and descent in the other. This may explain why there is usually only one intertropical convergence zone that is located away from the equator even when the sea surface temperature maximum is on or near the equator.

The conclusions of this paper are tentative because the treatments of convection, radiation, and cloudiness are highly simplified. However, if the simulated radiative–convective instability exists in nature, it also may explain the tendency of the tropical atmosphere to divide into large areas that are convectively suppressed, punctuated by smaller areas of intense convection.

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