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H. D. Orville

Abstract

The effects of ambient winds on the initiation and development of cumulus clouds over mountains are investigated. A case with the atmosphere initially at rest is compared with a case in which a wind increasing linearly with height is imposed from the ridge line upward to the top of the grid. Preliminary conclusions are that cloud initiation is stimulated in the ambient wind case. In addition, the cloud forms over the down-wind slope rather than directly over the peak and develops in situ. However, the cloud in the linear wind-shear case eventually dissipates, raising questions about the effects of the side and top boundaries on the cloud. Three more cases are integrated to test the influence of the boundary conditions. The results prompt the conclusions that the cloud will be initiated earlier because of the wind effects but will not develop as rapidly as the case with no initial wind, unless the cloud top comes within a few grid intervals of the top.

A more realistic wind is then modeled to test its effect on the initiation and development of clouds. In two cases a mountain wave is simulated, which allows motion down to the plain on either side of the mountain ridge. The initiation and development of clouds in these two cases are completely different from those in the other cases. The clouds form 1 km downwind of the ridge, develop rapidly with respect to liquid water content, and move with the wind out of the grid.

These mountain-wave cases reveal a unique generating process for the clouds. Essentially, the mountain dams the airflow upstream. Heating of the upwind slope creates a wave in the airflow upwind. Small perturbations in potential temperature and water vapor form at the upwind boundary, propagate into the grid and become superimposed on this wave. They amplify with time and are advected up the mountain slope and into that part of the model's atmosphere which has been moistened by a large circulation cell downwind. Here the warm, moist bubbles initiate clouds. This is one method of generating cloud streets or of creating pulsations in stationary clouds formed in the downwind circulation cell.

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R. D. Farley
and
H. D. Orville

Abstract

This paper is the first in a three part series describing numerical simulations of hailstorms and hailstone growth using a two-dimensional, time-dependent cloud model. In this model. cloud water, cloud ice and rain are treated via standard parameterization technique The precipitating ice field is discretized into 20 logarith-mically spaced size categories which evolve in, and interact with the time-dependent dynamic framework. Ice particles are generated by the freezing of raindrops and via a parameterization of the Bergeron process. Growth of these ice particles is based on wet and dry growth concepts applied to the continuous accretion process.

The model has been used to simulate a severe supercellular hailstorm from the National Hail Research Experiment These simulations include cases assuming various microphysical configurations of the model along with simplified cloud seeding experiments The simulations indicate many areas of agreement between the model results and observation chief among them being the characteristic sloping updraft and moving gust front, the rounded dome cloud top, the radar overhang, and the intense precipitation cascade. The major observed features which were not properly simulated were the persistent bounded weak echo region and the high concentrations of giant hail and associated high radar reflectivity values. The model results have also been compared to and are consistent with aircraft measurements of the thermodynamic structure of the subcloud region, and the basic internal structure of hailstorms.

The model simulations and the storm were prodigious producers of surface rain and hail. The model was unable to simulate the vast amounts of large hail observed for this case, mainly due to depiction of the cloud water caused by embryo generation mechanisms being too efficient, although the two-dimensionality of the model may also limit hail production. Recirculation of hall embryos from the forward overhang back down into the leading edge of the sloping updraft was important to hail production according to both the observations and the model results. The overall effect of the cloud seeding, although dependent on the magnitude and duration of the seeding, was quite similar in all cases. The primary seeding, effect was the creation of more small ice particles, most of which were carried aloft into the anvil. Dynamic effects induced by the seeding were generally insignificant. In all seeded cases the amount of hail at the surface was reduced, although the undesirable response of decreased rainfall also resulted.

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H. D. Orville
and
K. Hubbard

Abstract

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A. S. Dennis
and
H. D. Orville

Abstract

No abstract available.

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J. Y. Liu
and
H. D. Orville

Abstract

The effects of precipitation on a model of cumulus cloud initiation and development over mountains are studied by numerically integrating the equations of motion, equations of conservation of water substance, and the thermodynamic energy equation. The model is two-space dimensional with a vertical wind shear in a stable, incompressible atmosphere. Heating and evaporation at the valley and mountain interact with the initial ambient flow to initiate clouds which produce shadows on the surface and cut down both heating and evaporation. The model is restricted vertically to 3.5 km and horizontally to 7.0 km.

Several precipitation parameters are studied in this model. One, the critical water content determines when cloud water converts to rainwater. A second, the autoconversion rate, determines how rapidly the cloud water converts to rainwater. The third parameter determines how quickly the precipitation evaporates beneath the cloud. The rainwater first forms by autoconversion and is then increased by the accretion process following techniques described by Kessler and Srivastava. Berry's formulation for autoconversion is also tested.

The development of the cumulus clouds is similar for both precipitating and nonprecipitating clouds at their early stages. Virga phenomena are illustrated in these small cumulus clouds. At later stages the evaporation beneath and to the sides of the cloud makes the air cooler and creates a downdraft. Generally such effects shorten the clouds' life cycle. The shadow effects cause the clouds to move out of the model grid at a progressively faster rate and cause the clouds subsequent to the first one to be smaller.

In a symmetric model integrated both with and without precipitation and with cloud shadow effects, the shadow causes multiple growths over the ridge, the third of three clouds being the only one to accelerate until impeded by the rigid upper boundary of the grid. The first two clouds dissipate shortly after formation. The downdrafts beneath the clouds are stronger in the precipitating case.

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J. E. McDonald
and
H. D. Orville

Abstract

No abstract available.

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H. D. ORVILLE
and
L. J. SLOAN

Abstract

Crowley's second- and fourth-order nonconservative techniques for treating the advection term in numerical solutions of the hydrothermodynamic equations are tested on a model of cumulus cloud growth over mountains. The results are compared with previous integrations using upstream differencing, a first-order method.

All three methods give comparable results on a symmetric case. However, numerical damping which is a characteristic of the upstream-differencing method is considerably reduced in the Crowley techniques, as evidenced by curves of kinetic energy changes and sources and sinks for this energy.

In an ambient wind case the Crowley second-order method and the upstream-differencing method give comparable results if the eddy diffusion coefficients used in the Crowley method are twice as large as those used in the upstream-differencing method.

The results illustrate that the value of the eddy coefficients is crucial for the formation of the numerical clouds in the ambient wind model. Coefficients that are too small or too large lead to weak circulation cells created by the heated slopes and insufficient penetration into the upper flow region to form a cloud.

The smaller numerical diffusion of the Crowley second-order method is illustrated in a test of the numerical diffusion of rainwater in the cloud model. Upstream differencing causes rainwater contents downwind an order of magnitude larger than those that occur using the Crowley technique for advection.

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R. D. Farley
,
D. L. Hjermstad
, and
H. D. Orville

Abstract

This paper illustrates the potential for mesoscale models to depict the distribution of precipitation in orographic situations. The study covers a 4-day time period in April 1995. The domain of the numerical model covers much of western South Dakota and some of eastern Wyoming and is centered on the Black Hills of South Dakota. The 4-day storm period is characterized by changing atmospheric conditions, from primarily rain generation to snowfall production. Observations and climatic data of precipitation are analyzed to compare with model predictions. The model demonstrated the ability to respond appropriately to changing input conditions and produced reasonably accurate simulations of observed precipitation patterns. The model performed well for sufficiently cold, strongly forced conditions but seemed overly sensitive to the accuracy of model assumptions regarding ice initiation for warmer, weakly forced situations.

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Chester Wisner
,
H. D. Orville
, and
Carol Myers

Abstract

A numerical model of a hail-bearing cumulus cloud is presented. The model is one-dimensional and time-dependent, and it employs extensive parameterization of the microphysical processes. The raindrop and hailstone size distributions are assumed to be exponential at all times. Cloud droplets are converted to raindrops according to Berry's parameterization of the autoconversion process and are accreted by the raindrops according to Kessler's formulation. Raindrops are frozen at a rate consistent with Bigg's freezing equation, and the hailstones so formed then accrete raindrops and cloud droplets. Ice crystals are not allowed by the model, and for consistency, then, it is assumed that the cloud droplets do not freeze except when accreted by hailstones at temperatures less than0C. The melting and evaporation processes are modeled, and their impact on the results is explored.

The results of five test cases are presented. The sequence of cases is designed to illustrate the effects of the presence of hail, the melting process, and the evaporation of rain on the model by eliminating them, one at a time, from the complete model. In addition, we examine briefly the effect of a lower limit on the cloud radius as it pertains to the entrainment process. The conclusions of this study are that 1) hail is a critical component of the precipitation process, 2) a steady-state assumption is appropriate until the formation of hail in the cloud, and 3) the downdraft begins at the melting level and propagates downward.

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Harold D. Orville
,
Richard D. Farley
, and
John H. Hirsch

Abstract

Studies have been conducted to determine the cloud seeding potential of stratiform type clouds using a two-dimensional, time-dependent cloud model. An atmospheric sounding from Villanubla, Spain, in February 1980, was used to initialize the model. The model is designed to allow mesoscale convergence in the lower levels and divergence in the upper levels, which results in a stratiform-type cloud in this Spanish situation.

The seeding of clouds using either dry ice or silver iodide has been tested and rather surprising results are indicated. The silver iodide seeding simulations produce strong dynamic responses in the model clouds, even with small amounts of supercooled liquid available and a few natural ice crystals per liter in the cloud. These effects occur in a nearly moist adiabatic layer as well as in a convectively unstable layer.

The effects appear to be due to the heat released as the liquid freezes and the cloudy environment switches from liquid saturation to ice saturation. Cloud vertical motions of a few to several m s−1 are produced in the seeded cloud region. Vertical motions of 10 to 20 cm s−1 exist in comparable regions of the unseeded cloud. Precipitation is strongly affected. Consequently, this heat release is much more significant in terms of the overall energetics of the cloud than has been evident in our seeding simulation conducted in pure convective situations with much stronger updrafts.

The tests of the dry ice seeding indicate small effects, but this is largely due to the rapid fall of the dry ice pellets through the cloud and to the short time period available for the seeding to take effect.

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