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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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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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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 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

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

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 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 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 two-scale model (Raymond, 1979) is used to simulate a well-documented Colorado thunderstorm. Good agreement is found with the early and middle stages of the storm life cycle. Intense vertical mixing in the early stages is shown to have a crucial effect on both cloud dynamics and microphysics. In particular, precipitation particles formed at high levels in the storm are rapidly mixed down to low levels against the mean updraft even when sedimentation is suppressed. The termination of this mixing not only enables the simulated storm to develop explosively, but also sets the stage for its eventual demise.

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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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