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Han-Ru Cho and Terry L. Clark

Abstract

The structure of vorticity fields of cumulus clouds is studied using a three-dimensional numerical convection model developed by Clark (1977, 1979. 1981). The analysis of the model results suggests that 1) it is justified to neglect the solenoidal effect in cloud vorticity dynamics; and 2) the effects of vertical advection and twisting of vorticity, while both are very important to the local structure, cancel each other when averaged over a cloud horizontal cross-section. Consequently, 3) the cloud vorticity in the mean is controlled mainly by horizontal convergence/divergence of vorticity through cloud boundary and satisfies a very simple conservation equation. Furthermore, the model results also suggest that 4) clouds can induce a very strong horizontal eddy flux of vertical vorticity. The magnitude of this flux is of the order 10−4 m s−2 on the basis of a unit fractional cloud coverage. These results support the hypothesis introduced by Cho and Cheng (1980).

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Terry L. Clark and R. D. Farley

Abstract

The Clark nonhydrostatic anelastic code is extended to allow for interactive grid nesting in both two and three spatial dimensions. Tests are presented which investigate the accuracy of three different quadratic interpolation formulae which are used to derive boundary conditions for the fine mesh model. Application of the conservation condition of Kurihara and others is shown to result in significant improvements in the treatment of interactive nesting. A significant improvement in the solutions for interactive versus parasitic nesting is also shown in the context of forced gravity wave flow. This result, for the anelastic system, is in agreement with the earlier results of Phillips and Shukla, who considered the hydrostatic shallow water system of equations.

The interactive nesting model is applied to the simulation of the severe downslope windstorm of 11 January 1972 in Boulder using both two and three spatial dimensions. The three-dimensional simulation results in a gustiness signature in the surface wind speed. The cause of this gustiness is attributed to the development of turbulent eddies in the convectively unstable region of the topographically forced wave. These eddies are transported to the surface by downdrafts formed in the leading edge of the convectively unstable region. A type of periodicity to the wind gustiness signature is then produced by a competition between the two physical processes of wave build up via forced gravity wave dynamics and wave breakdown via convective instability. The actual source/sink terms for the turbulence are still under investigation. Some preliminary comparisons between the two- and three-dimensional windstorm simulations are also presented.

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Terry L. Clark, William D. Hall, and Robert M. Banta

Abstract

Simulations of the 9 January 1989 Colorado Front Range windstorm using both realistic three-dimensional (3D) orography and a representative two-dimensional (2D) east–west cross-sectional orography are presented. Both Coriolis forcing and surface friction (drag law formulation) were included for all experiments. The model results were compared with analyses of Doppler lidar scan data available from the surface to 4 km MSL provided by the Environmental Technology Laboratory of the National Oceanic and Atmospheric Administration (NOAA).

The fully three-dimensional simulations with realistic orography used time-dependent inflow boundary conditions. These experiments were designed, in part, to assess the ability of mesoscale models to predict the onset and general characteristics of downslope windstorms. The present experiments highlight the sensitivity of wind storm onset and positioning of surface gusts to both model resolution and surface physics, which is in agreement with previous findings.

These realistic orography experiments show that the major east–west canyons in the vicinity of Boulder produce a north–south broken structure to the strong updraft jump patterns. However, as the model resolution is increased from 3.33 to 1.11 km, the modulating effects of the canyons, with the exception of the Big Thompson, actually decreased. This tendency is attributed to an increasingly dominant role of the nonlinear internal fluid dynamics as the model resolution increases. Comparisons of model simulations with the lidar observations showed good agreement on the spatial and temporal scales of lee eddies. A north–south scale of ∼10 km occurred in both the realistic orography model results and observations.

A relatively strong Coriolis effect was shown to result from the super- and subgeostrophic flows caused by the nonlinear gravity wave dynamics. A northerly wind component of as much as 12 m s−1 at low levels over the foothills and plains is shown to be a direct result of Coriolis forcing. The turning of the wind with height as a result of this effect is supported by the observations.

The transition from two to three dimensions showed some dramatic changes to the structure of the windstorm gusts in the idealized 2D orography simulations. The 3D simulations showed a smooth distribution of energy centered about a scale of ∼3 km. These gust structures were close to isotropic in the horizontal as they propagated out onto the plains. Again this type of structure was supported by the observations.

Three sources of surface gustiness are discussed in the paper. Surface gusts produced by vortex tilting and advected out of the wave-breaking region, as described in previous studies, occur in the present simulations. This mechanism is evidenced by the accompanying strong vertical vorticity. Propagating gust structures, similar in appearance to those obtained by others, are also obtained in both the 2D and 3D experiments using the idealized 2D orography. Rather than resulting from local Kelvin–Helmholtz instabilities, the propagating gusts in the present experiments appear to arise from high-amplitude lee waves that propagate as a result of the transient character of the wave-breaking region modulating the shape of the effective waveguide.

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Terry L. Clark, James R. Scoggins, and Robert E. Cox

Abstract

The formulation of algebraic functions, involving synoptic-scale atmospheric parameters as variables, capable of predicting clear-air turbulence within 7000 ft sub-layers of the stratosphere was attempted. The data sample used was composed of 153 turbulent and non-turbulent regions identified from 46 stratospheric flights of the XB-70 aircraft over the western United States during the period March 1965 to November 1967, and the values of 69 synoptic-scale parameters determined from rawinsonde data associated with each of the regions. After the XB-10 regions and the values of the synoptic-scale parameters were grouped into one or more of five overlapping categories, or sub-layers, determined by the altitude of the aircraft at the time the turbulence or non-turbulence was reported, discriminant function analysis was employed in each sub-layer to construct functions which could discriminate the turbulent from the non-turbulent regions. Those discriminant functions yielding the best results in each sub-layer were tested from 23 stratospheric flights of the YF-12A aircraft over the same area during the period March 1970 to January 1972. For each sub-layer, five discriminant functions yielding the best results were used to derive a forecasting procedure. This procedure correctly identified approximately 85% of the turbulent and non-turbulent regions in each of the five sub-layers.

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Terry L. Clark, Mary Ann Jenkins, Janice Coen, and David Packham

Abstract

The object of this paper is to describe and demonstrate the necessity and utility of a coupled atmosphere-fire model: a three-dimensional, time-dependent wildfire simulation model, based on the primitive equations of motion and thermodynamics, that can represent the finescale dynamics of convective processes and capture ambient meteorological conditions.

In constructing this coupled model, model resolution for both the atmosphere and the fuel was found to be important in avoiding solutions that are physically unrealistic, and this aspect is discussed. The anelastic approximation is made in the equations of motion, and whether this dynamical framework is appropriate in its usual form for simulating wildfire behavior is also considered.

Two simple experiments-the first two in a series of numerical simulations using the coupled atmosphere- fire model-are presented here, showing the effect of wind speed on fire-line evolution in idealized and controlled conditions. The first experiment considers a 420-m-long fire line, and the second considers a 1500-m-long fire fine, where wind speeds normal to the initial fire lines vary from 1 to 5 m s−1. In agreement with some general observations, the short fire line remains stable and eventually develops a single conical shape, providing the wind speed is greater than about 1–2 m s−1, while under similar conditions, the longer fire line breaks up into multiple conical shapes. In both cases, the conical shapes are attributed to a feedback between the hot convective plumes and the near-surface convergence at the fire front. The experimental results reveal a dynamical explanation for fire-line breakup and geometry, demonstrating that the model is a valuable tool with which to investigate fire dynamics, and eventually it may be able to provide a credible scientific basis for policy decisions made by the meteorological and fire-management communities.

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Koichiro Wakasugi, Ben B. Balsley, and Terry L. Clark

Abstract

The VHF Doppler radar has become a powerful tool for probing structures and motions of the clear air. In this paper, we discuss the capability of VHF radar as a tool for cloud and precipitation studies. Large fluctuations of refractive index from the cloudy air can be anticipated because of an abundance of water in clouds. Due to the difficulties in obtaining the necessary fine-scale observational data within clouds, we base our analysis of cloud-echoing properties on the numerical simulation of nonprecipitating cumulus by Klaassen and Clark. The Bragg scatter echo intensity is estimated from the temperature and humidity fields obtained from the cloud model. We find that the echo is enhanced at the boundary between the cloud and environment because of enhanced water vapor fluctuations. Although echoes from nonprecipitating clouds can be detected by UHF and VHF radars, only VHF radars can discriminate echoes due to large precipitation particles from the Bragg scatter echo of cloudy air. With UHF radars, the precipitation echoes totally mask the Brag scatter echoes.

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Todd P. Lane, Michael J. Reeder, and Terry L. Clark

Abstract

Although convective clouds are known to generate internal gravity waves, the mechanisms responsible are not well understood. The present study seeks to clarify the dynamics of wave generation using a high-resolution numerical model of deep convection over the Tiwi Islands, Australia. The numerical calculations presented explicitly resolve both the mesoscale convective cloud cluster and the gravity waves generated. As the convective clouds evolve, they excite gravity waves, which are prominent features of the model solutions in both the troposphere and stratosphere. The source location is variable in time and space but is related to the development of individual convective cells. The largest amplitude gravity waves are generated when the cloud tops reach the upper troposphere.

A new analysis technique is introduced in which the nonlinear terms in the governing equations are taken as the forcing for linear gravity waves. The analysis shows that in the present calculation, neither the shear nor the diabatic heating are the dominant forcing terms. Instead, the wave source is most easily understood when viewed in a frame of reference moving with the wind at the level of neutral buoyancy, whereupon the source may be described as a vertically oriented, oscillating convective updraft. This description is consistent with the properties of the modeled stratospheric waves.

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N. Andrew Crook, Terry L. Clark, and Mitchell W. Moncrieff

Abstract

A numerical model is used to study the “Denver Cyclone,” a mesoscale vortex that develops in eastern Colorado under southerly to southeasterly flow. Diurnal effects (e.g., surface heating/cooling) have been excluded from these simulations, but are discussed in Part II of this study. Simulations are first performed with a southerly ambient wind (i.e., parallel to the Continental Divide). If the Froude number of the flow based on the height of the Palmer Divide (an east–west ridge to the south of Denver) is small, then a vortex forms over the Denver region. This circulation then propagates northward at just over half the upstream flow velocity, leaving essentially stagnant air in the lee of the Palmer Divide. It is shown that Coriolis turning has a negligible effect on the flow when the ambient wind is from the south. However, when the ambient wind is at an angle to the Continental Divide, northerly winds can develop along the foothills due to Coriolis turning of the flow as it approaches the Divide.

Idealized experiments of low Froude number flow past a bell-shaped hill are also performed to explore the connection between lee vortices and the stagnation aloft predicted by linear theory. In one experiment, a low Froude number (Fr = 0.3) flow is approached from a Fr ∼ 1 flow, where linear theory gives useful predictions, by applying a reverse pressure gradient to decelerate the flow. During this deceleration, the flow reverses aloft at the point predicted by linear theory. Wave breaking then follows and the region of flow reversal descends to the surface. This experiment suggests that the intense, concentrated vorticity that develops in the lee of obstacles at low Froude numbers is due to the tilting of horizontal vorticity as gravity waves overturn and break.

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Roelof T. Bruintjes, Terry L. Clark, and William D. Hall

Abstract

A case study showing comparisons between observations and numerical simulations of the passage of a winter storm over complex terrain is presented. The interactions between the mesoscale and cloud environments and the microphysical and dynamical processes are addressed using both observations and numerical simulations.

A three-dimensional, time-dependent nested grid model was used to conduct numerical simulations of the three-dimensional airflow and cloud evolution over the Mogollon Rim and adjacent terrain in Arizona. The modeling results indicated that the flow patterns and cloud liquid water (CLW) were closely linked to the topography. To a large extent, gravity waves excited by the flow over the mountains determine the distribution of clouds and precipitation. The waves extend through deep layers of the atmosphere with substantial updrafts and downdrafts, at times exceeding 5 m s−1. The simulated vertical velocities and horizontal wavelengths of about 20 km were in good agreement with the aircraft observations. The CLW regions associated with the waves extended through much deeper layers of the atmosphere and in quantities a factor of 2 larger than those associated with the forced ascent over the ridges. The CLW associated with waves may provide an additional source for precipitation development not previously considered in cloud seeding experiments. In addition, synoptic-scale flow patterns over the area change from one storm system to the next and even during one storm system. Consequently, both the winds and the evolution of clouds over the area are highly space and time dependent

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Roelof T. Bruintjes, Terry L. Clark, and William D. Hall

Abstract

A three-dimensional, time-dependent, nested-grid model is used to calculate the targeting of tracer or Seeding material over complex terrain in northern Arizona. Good agreement with measurements of SF6 tracer is reported in three case studies. Released in upwind valleys, the tracer movement and dispersion are strongly influenced by both valley flow and gravity waves excited by the mountains, as well as by changes in the synoptic flow, which can change substantially even during a single storm. The interaction between the airflow and the topography seem to be the dominant factor determining the dispersion and transport of tracer material.

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