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I. Orlanski and B. B. Ross

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B. B. Phillips and Ross Gunn

Abstract

The electrical charge transferred to highly insulated spheres by the diffusion of ions from ionized moving air is directly measured. The equilibrium charge is proportional to the logarithm of the ratio of the positive and negative ion conductivities, and depends upon the velocity of the sphere relative to its ionized environment. The measurements agree with theory.

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Ross Gunn and B. B. Phillips

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Measurements in a giant expansion chamber show that the size of newly formed cloud droplets depends critically upon the cleanliness of the processed air. Droplets formed in ordinary air are small, but droplets large enough to precipitate are immediately formed by condensation whenever the condensation-nuclei density is sufficiently reduced. The product of mean drop mass and the activated nuclei density approximates the available free water per unit volume. Cloud droplets formed from polluted surface air are usually too small to precipitate, but large droplets formed overhead by condensation in sufficiently clean air may fall through the polluted cloud and initiate rain through association processes. Since the activated nuclei density is normally observed to decrease with increasing altitude, the probability of generating droplets sufficiently large to initiate rain increases as the vertical development of a cloud system increases. The population densities of large cloud droplets normally observed near the tops of precipitating clouds may be explained in terms of an overlying parcel of cooled air that is initially relatively free of nuclei.

Since pollution is swept out of the atmosphere by diffusion onto cloud droplets, and by droplet movement, it is suggested that periods of general cloudiness and precipitation reduce the original nuclei density and permit the subsequently formed droplets to grow still larger, thus increasing the probability of appreciable precipitation. The rain producing cycle is, therefore, equipped with a feed-back or regenerative mechanism which normally proceeds, in a given mass of air, until the air is appreciably desiccated.

Condensation nuclei as well as water vapor normally accumulate simultaneously in fair weather. The presence of nuclei may delay the initiation of precipitation until sufficient vertical instability can be established to lift or cool the relatively clean overlying layers. The precipitation cycle may then be re-established. The investigation shows that rain may be formed directly from the vapor in clean air, without the production of clouds.

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I. Orlanski and B. B. Ross

Abstract

The transient behavior of an idealized dry frontal system is investigated using a two-dimensional numerical model. The development of a cross-stream circulation within stationary and moving cold fronts is determined for various frontal and synoptic conditions. In the stationary front, a circulation is generated by symmetric baroclinic instability, but nonlinear effects restrict this circulation to remain very weak. In the moving cold front, the vertical shear of the synoptic wind which advects the front produces an ageostrophic residue as a result of the differential advection of the vertical shear of the frontal jet and the horizontal temperature gradient across the front. This residue, which depends upon the vertical synoptic shear and the thermal wind structure of the frontal system, will generate a cross-stream circulation which maintains the cold front in a quasi-steady state. The resulting motion field is described well by the streamfunction balance equation. The lifting produced by the cross-stream circulation in the moving cold front system may be sufficient to trigger deep convection under favorable conditions in the moisture and synoptic wind fields.

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B. B. Ross and I. Orlanski

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The effect of moisture upon the dynamics of mature idealized cold front systems is investigated using a two-dimensional numerical model. Lifting produced by the initial cross-stream frontal circulation studied by Orlanski and Ross (1977) is shown to saturate the warm moist air above the nose of the front when initial humidity levels are sufficiently high. If the atmosphere is convectively unstable, this saturated air will develop into deep convection with the convection-induced circulation overwhelming the initial frontal circulation. The initial development of convection is also shown to produce a gravity wave exhibiting similar scales to those of the convective zone. This wave propagates into the warm air at a much faster speed than the moving front-cloud system. Comparisons are made of the intensity of convection for different initial humidity and temperature conditions and when a low-level capping inversion is present. Also a comparison is made of cloud development caused by a combination of frontal lifting and surface heating when temperature inversions of different intensifies are present. The stronger inversion is shown to suppress convection produced by surface heating alone with the combined effect of frontal lifting and surface heating required to release the convective instability.

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ROBERT B. ROSS

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Bruce B. Ross

Abstract

The effects of mesoscale forcing and diabatic heating on the development of convective systems have been investigated using a simplified numerical model to simulate the squall line and the convective system preceding it that occurred over Texas and Oklahoma on 10– 11 April 1979. A simulation run without including latent but showed both systems to be initiated and maintained by convergence produced by larger-scale forcing. The first cloud system formed downwind of the convergence zone that was produced by the confluence of airstreams along a dryline. A cloud front approaching from the west then merged with this dryline, destroying its horizontal gradients through diffusive effects and replacing it with a frontal convergence line that was alinged with the low-level flow. This new configuration was then favorable for the formation of the squall line that developed in the simulation.

When latent heat was included the continuous cloud in the first convective system broke down into isolated cells which moved downstream from the convergence zone. In the non-latent heat case, the primary mechanism for providing moisture to this cloud was vertical diffusion from the moist surface layer. When latent heat was added, vertical advection within cell updraft provided a more efficient means to supply moisture to the convective system.

In the simulated squall line, latent heat release produced a deeper cloud system while intensifying and maintaining the low-level convergence. However, unlike the earlier system, the squall line did not break into convective cells when latent but was included in the simulation.

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R. B. ROSS

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Isidoro Orlanski and Bruce B. Ross

Abstract

A detailed analysis is made of a three-dimensional numerical simulation of the evolution of an observed moist frontal system over a 48 h period. The simulated front undergoes an initial period of frontogenetic growth, characterized by an alignment of vertical vorticity and horizontal convergence near the surface. The front then evolves to a mature, quasi-steady state as the line of maximum convergence moves ahead of the maximum vorticity. This phase shift is shown to be the result of a negative feedback mechanism which inhibits further vorticity growth while reducing the amount of viscous damping required to achieve a steady state. The influence of viscosity and surface drag upon this mechanism is also assessed.

When moisture is included in the numerical solution, the squall line which develops along the front exhibits a dual updraft structure with low-level convergence near the nose of the front and midlevel convergence located 100 km to the rear at a height of 3 km. This configuration is very similar to that found by Ogura and Liou in their analysis of an Oklahoma squall line not associated with a cold front.

Analysis of the equations of motion within the convective zone of the mature squall line shows the diabatic heating to be closely balanced by adiabatic cooling due to vertical temperature advection. As a result, the only net warming within this region occurs as adiabatic warming in the clear air outside of the cloud zone.

A linear, two-layer, dry model containing stable lower and unstable upper layers is shown to reproduce the dual updraft structure for certain low-level wind intensities without requiring microphysics. Also, for all wind conditions, this simple model produces strong convergence at the interface between the two layers. This suggests that the occurrence of a convergence maximum at the level of free convection should be a common feature of convectively unstable cloud systems.

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Isidoro Orlanski and Bruce B. Ross

Abstract

An investigation is made of the stability of a convectively unstable atmosphere in the presence of a stably stratified layer beneath, which is moving with a constant velocity relative to the upper air. This work is an extension of the linear model presented as part of the recent study of Orlanski and Ross in which they sought to explain the structure of their simulated squall line. A stability analysis shows that two modes are possible: 1) the gravitational or convective mode due to the unstable stratification in the upper layer which modifies the stable region below and 2) the classical Kelvin-Helmholtz mode due to shear across the interface. The Kelvin–Helmholtz mode is of limited physical interest in this case. On the other hand, the gravitational mode produces an updraft structure similar to updrafts in the stable lower layer of a convective system. Analysis of the horizontal displacement of the surface convergence for this mode relative to the convergence in the convective zone shows this displacement to depend primarily on the wind, stratification, and depth of the stable lower layer. The resulting relationship provides a method for determining whether a dual or single updraft will occur in a convective system.

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