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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 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

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 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

It is shown that the low-level southerly jet of the continental United States is susceptible to a dynamic instability similar to symmetric instability and Lilly's (1966) parallel instability. The possible role of this instability in organizing the squall lines of 3 April 1974 is discussed. It is further speculated that the instability was instrumental in producing the widespread tornado outbreak on this date, due to its ability to concentrate low-level vorticity into narrow shear lines.

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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

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

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

The predictions of propagation velocity made by a linear model of moist convective instability are compared with actual case studies of severe local storms. Excellent agreement between observation and theory is found for a certain class of storms, namely those that propagate in a continuous fashion. The calculations suggest that such storms take the form of convectively forced internal gravity waves. The pattern of low-level convergence provided by the gravity wave organizes the convection, which in turn drives the wave.

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David J. Raymond

Abstract

A new model of the Madden–Julian oscillation (MJO) is presented. Cloud–radiation interactions in this model make the tropical atmosphere susceptible to large-scale radiative–convective overturning. The modeled MJO takes the form of such an instability, though its behavior is substantially modified by the effects of surface heat flux variability. The dynamics of the disturbance in the model are quasi-balanced, in the sense that the low-level flow in the disturbance is more associated with the vorticity than with the divergence. The cumulus parameterization used in the model allows a lag of several days to exist between the strongest surface heat flux into a column and the development of heavy precipitation in that column. This lag plays a key role in model dynamics.

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