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Terry L. Clark and W. D. Hall

Abstract

A three-dimensional numerical model is used to study the effect of small-scale supersaturation fluctuations on the evolving droplet distribution in the first 150 m above cloud base. The primary purpose of this research is to determine whether the irreversible coupling between the thermodynamics and dynamics due to finite phase relaxation time scales τs is sufficient to produce significant small-scale horizontal variations in supersaturation. Thus, the paper is concerned only with this internal source for thermodynamic variability. All other source terms, such as the downgradient flux of the variance of thermodynamic fields, have purposely been neglected.

Lagrangian particle experiments were run in parallel with the basic Eulerian model. The purpose of these experiments is to relax some of the microphysical parameterization assumptions with respect to assumed distribution shape and as a result add credibility to the results of distribution broadening.

Model results of five cases are presented, representing the cloud condensation nuclei characteristics of typical continental and maritime cumulus with mean dissipation rate of −100 cm2 s−3. The results show that for a maritime case of N≈100 cm−3 and =0.5 m s−1 the standard deviation of the supersaturation is as large as its horizontal mean. The horizontal variability of all thermodynamic fields is shown to increase significantly with τs. The droplet broadening response to this irreversible coupling effect is found to be significant for the larger values of τs in the Eulerian experiments. The Lagrangian particle experiments showed a somewhat reduced but still significant effect.

Although the experiments do show a broadening effect caused by finite values of τs, in no case were we able to show a continual increase in distribution broadening with height as reported from cumulus observations.

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Terry L. Clark and Thomas R. Karl

Abstract

A linear multiple regression equation was developed for each of 27 ozone monitoring sites in the north-eastern United States to forecast the next day's maximum 1 h average ozone concentration. Thirty-five prognostic meteorological variables, the climatological daily maximum surface temperature, the length and direction of 12 and 24 h backward trajectories, and three air quality variables relating to the seasonality or the upwind ozone concentrations were considered as possible predictors in each of the regression equations. Data pertaining to 244 randomly selected days formed the developmental or the dependent data set, while the data pertaining to the remaining 122 days in the months of June, July, August and September of 1975, 1976 and 1977 were used to assess the performance of the regression equations. Performance was assessed and compared to that of persistence, via statistical evaluations of site-specific forecasts. In addition, areas of the Northeast where the 1 h ozone standard was predicted to be exceeded, were compared to the areas where the standard was exceeded.

The results indicated that approximately half of the predictions generated from the independent data set were within 20% of the observations, while 77% were within 40% of the observations. A tendency for the underprediction of the maximum concentrations was noted. Overall, the regression equations performed best in forecasting the trends and patterns of the daily 1 h average ozone concentrations.

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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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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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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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Terry L. Clark, Larry Radke, Janice Coen, and Don Middleton

Abstract

A good physical understanding of the initiation, propagation, and spread of crown fires remains an elusive goal for fire researchers. Although some data exist that describe the fire spread rate and some qualitative aspects of wildfire behavior, none have revealed the very small timescales and spatial scales in the convective processes that may play a key role in determining both the details and the rate of fire spread. Here such a dataset is derived using data from a prescribed burn during the International Crown Fire Modelling Experiment. A gradient-based image flow analysis scheme is presented and applied to a sequence of high-frequency (0.03 s), high-resolution (0.05–0.16 m) radiant temperature images obtained by an Inframetrics ThermaCAM instrument during an intense crown fire to derive wind fields and sensible heat flux. It was found that the motions during the crown fire had energy-containing scales on the order of meters with timescales of fractions of a second. Estimates of maximum vertical heat fluxes ranged between 0.6 and 3 MW m−2 over the 4.5-min burn, with early time periods showing surprisingly large fluxes of 3 MW m−2. Statistically determined velocity extremes, using five standard deviations from the mean, suggest that updrafts between 10 and 30 m s−1, downdrafts between −10 and −20 m s−1, and horizontal motions between 5 and 15 m s−1 frequently occurred throughout the fire.

The image flow analyses indicated a number of physical mechanisms that contribute to the fire spread rate, such as the enhanced tilting of horizontal vortices leading to counterrotating convective towers with estimated vertical vorticities of 4 to 10 s−1 rotating such that air between the towers blew in the direction of fire spread at canopy height and below. The IR imagery and flow analysis also repeatedly showed regions of thermal saturation (infrared temperature > 750°C), rising through the convection. These regions represent turbulent bursts or hairpin vortices resulting again from vortex tilting but in the sense that the tilted vortices come together to form the hairpin shape. As the vortices rise and come closer together their combined motion results in the vortex tilting forward at a relatively sharp angle, giving a hairpin shape. The development of these hairpin vortices over a range of scales may represent an important mechanism through which convection contributes to the fire spread.

A major problem with the IR data analysis is understanding fully what it is that the camera is sampling, in order physically to interpret the data. The results indicate that because of the large amount of after-burning incandescent soot associated with the crown fire, the camera was viewing only a shallow depth into the flame front, and variabilities in the distribution of hot soot particles provide the structures necessary to derive image flow fields. The coherency of the derived horizontal velocities support this view because if the IR camera were seeing deep into or through the flame front, then the effect of the ubiquitous vertical rotations almost certainly would result in random and incoherent estimates for the horizontal flow fields. Animations of the analyzed imagery showed a remarkable level of consistency in both horizontal and vertical velocity flow structures from frame to frame in support of this interpretation. The fact that the 2D image represents a distorted surface also must be taken into account when interpreting the data.

Suggestions for further field experimentation, software development, and testing are discussed in the conclusions. These suggestions may further understanding on this topic and increase the utility of this type of analysis to wildfire research.

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

Abstract

The effect of surface heating on the flow past an isolated obstacle is examined with the aid of a nonlinear numerical model. These simulations extend the results of Part I, which considered the adiabatic, stratified flow around the obstacle. When the obstacle is heated, substantial low-level shear develops in the lee as the flow converges at low levels and diverges above. A linear model is developed to explain some of the details of this shear pattern. In this model, vertical shear is produced by differential heating and removed by mixing.

Some of the small-scale circulations that develop in the convective boundary layer are then discussed. A thermal instability predominates in the lowest levels of the boundary layer with its axis aligned along the low-level shear vector. Higher in the boundary layer, a transverse mode appears and breaks the thermal instability into three-dimensional maxima. The transverse nature of this mode, the existence of an inflection point, and the low Richardson number suggest that this mode is a shearing instability.

The convergence/vorticity zone in the lee of the obstacle (described in Part I) is then examined in detail. Several small-scale vortices develop along this zone at points where the thermal instabilities intersect. Observational studies have indicated that these boundary layer vortices often spawn tornadoes. It is shown that the vertical vorticity in these circulations is due to stretching of the preexisting vorticity along the convergence zone.

The small-scale circulations in the boundary layer force a gravity wave response (with λ∼10 km) in the stratified atmosphere above. The vertical velocity in these waves exceeds 1 m s−1 in certain regions of the flow. A model is developed to explain how the boundary layer eddies with horizontal scales of ∼2–4 km can force a 10 km wave response above. This model depends on the fact that the vertical group velocity is inversely proportional to the horizontal wavelength as well as on a feedback process in which the gravity waves modulate the boundary layer eddies.

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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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