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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

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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Piotr K. Smolarkiewicz, Roy M. Rasmussen, and Terry L. Clark

Abstract

This study focuses on basic island scale forcing mechanisms for the formation and evolution of a band cloud typically present upwind of the island of Hawaii. By means of numerical experiments and verification of our results against observations and laboratory experiments reported in the literature, we show that the band cloud is a complex three-dimensional phenomenon which is inseparable from the airflow around the island. In particular, we demonstrate that the event needs to be analyzed in terms of the basic fluid dynamics of strongly stratified flow past a three-dimensional obstacle. The band cloud is found to arise primarily from the dynamic interaction of the trade winds with the island. The upwind surface flow forms a separation line with an associated stagnation point. A low-level convergence zone forms along this line, resulting in an updraft line. If the updrafts are strong enough, a band cloud forms. Formation and characteristics of such a system are mostly controlled by the environmental stability and strength of the trade wind. A simple criterion for the occurrence of a strong band cloud is offered in terms of the height of the island, trade-wind speed, environmental stability, and the lifted condensation and/or free convection level.

A series of controlled experiments addresses questions on the role of the thermal forcing in the formation and evolution of the band cloud. In particular, we show that the band cloud is not primarily related to the diurnal cycle (as was anticipated in the literature), but that the diurnal effects are relatively weak modulations of the primary effects of a strongly fluid flow past the island.

The possibility of vortex shedding in the lee of the island and its implications for the band cloud are also discussed.

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Robert E. Eskridge, Francis S. Binkowski, J. C. R. Hunt, Terry L. Clark, and Kenneth L. Demerjian

Abstract

A finite-difference highway model is presented which uses surface layer similarity theory and a vehicle wake theory to determine the atmospheric structure along a roadway. Surface similarity is used to determine the wind profile and eddy diffusion profiles in the ambient atmosphere. The ambient atmosphere is treated as a basic-state atmosphere on which the disturbances due to vehicle wakes are added. A conservation of species equation is then solved using an upstream-flux corrected technique which insures positive concentrations. Simulation results from the highway model are compared with 58 half-hour periods of data (meteorological and SF6 tracer) taken by General Motors. The results show that the predictions of this model are closer to the observations than those of the Gaussian-formulated EPA highway model (HIWAY).

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Todd P. Lane, Robert D. Sharman, Terry L. Clark, and Hsiao-Ming Hsu

Abstract

An investigation of the generation of turbulence above deep convection is presented. This investigation is motivated by an encounter between a commercial passenger aircraft and severe turbulence above a developing thunderstorm near Dickinson, North Dakota, on 10 July 1997. Very high-resolution two- and three-dimensional numerical simulations are used to investigate the possible causes of the turbulence encounter. These simulations explicitly resolve the convection and the turbulence-causing instabilities. The configurations of the models are consistent with the meteorological conditions surrounding the event.

The turbulence generated in the numerical simulations can be placed into two general categories. The first category includes turbulence that remains local to the cloud top, and the second category includes turbulence that propagates away from the convection and owes its existence to the breakdown of convectively generated gravity waves. In both the two- and three-dimensional calculations, the local turbulence develops rapidly and occupies a layer about 1 km deep above the top of convective updrafts after their initial overshoot into the stratosphere. This local turbulence is generated by the highly nonlinear interactions between the overshooting convective updrafts and the tropopause. Gravity wave breakdown is only present in the two-dimensional calculation and occurs in a layer about 3 km deep and 30 km long. This gravity wave breakdown is attributed to an interaction between the gravity waves and a critical level induced by the background wind shear and cloud-induced wind perturbations above cloud top.

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Terry L. Clark, Teddie Keller, Janice Coen, Peter Neilley, Hsiao-ming Hsu, and William D. Hall

Abstract

Numerical simulations of terrain-induced turbulence associated with airflow over Lantau Island of Hong Kong are presented. Lantau is a relatively small island with three narrow peaks rising to between 700 and 950 m above mean sea level. This research was undertaken as part of a project to better understand and predict the nature of turbulence and shear at the new airport site on the island of Chek Lap Kok, which is located to the lee of Lantau. Intensive ground and aerial observations were taken from May through June 1994, during the Lantau Experiment (LANTEX). This paper focuses on flow associated with the passage of Tropical Storm Russ on 7 June 1994, during which severe turbulence was observed.

The nature of the environmental and topographic forcing on 7 June 1994 resulted in the turbulence and shear being dominated by the combination of topographic effects and surface friction. High-resolution numerical simulations, initialized using local sounding data, were performed using the Clark model. The simulation results indicate that gravity-wave dynamics played a very minor role in the flow distortion and generation of turbulence. As a result of this flow regime, relatively high vertical and horizontal resolution was required to simulate the mechanically generated turbulence associated with Tropical Storm Russ.

Results are presented using a vertical resolution of 10 m near the surface and with horizontal resolutions of both 125 and 62.5 m over local, nested domains of about 13–24 km on a side. The 125-m model resolution simulated highly distorted flow in the lee of Lantau, with streaks emanating downstream from regions of sharp orographic gradients. At this resolution the streaks were nearly steady in time. At the higher horizontal resolution of 62.5 m the streaks became unstable, resulting in eddies advecting downstream within a distorted streaky mean flow similar to the 125-m resolution simulation. The temporally averaged fields changed little with the increase in resolution; however, there was a three- to fourfold increase in the temporal variability of the flow, as indicated by the standard deviation of the wind from a 10-min temporal average. Overall, the higher resolution simulations compared quite well with the observations, whereas the lower resolution cases did not. The high-resolution experiments also showed a much broader horizontal and vertical extent for the transient eddies. The depth of orographic influence increased from about 200 m to over 600 m with the increase in resolution. A physical explanation, using simple linear arguments based on the blocking effects of the eddies, is presented. The nature of the flow separation is analyzed using Bernoulli’s energy form to display the geometry of the separation bubbles. The height of the 80 m2 s−2 energy surface shows eddies forming in regions of large orographic gradients and advecting downstream.

Tests using both buoyancy excitation and stochastic backscatter to parameterize the underresolved dynamics at the 125-m resolution are presented, as well as one experiment testing the influence of static stability suppressing turbulence development. All these tests showed no significant effect. Implications of these results to the parameterization of mechanically induced turbulence in complex terrain are discussed.

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Terry L. Clark, William D. Hall, Robert M. Kerr, Don Middleton, Larry Radke, F. Martin Ralph, Paul J. Neiman, and David Levinson

Abstract

Results from numerical simulations of the Colorado Front Range downslope windstorm of 9 December 1992 are presented. Although this case was not characterized by severe surface winds, the event caused extreme clear-air turbulence (CAT) aloft, as indicated by the severe structural damage experienced by a DC-8 cargo jet at 9.7 km above mean sea level over the mountains. Detailed measurements from the National Oceanic and Atmospheric Administration/Environmental Research Laboratories/Environmental Technology Laboratory Doppler lidar and wind profilers operating on that day and from the Defense Meteorological Satellite Program satellite allow for a uniquely rich comparison between the simulations and observations.

Four levels of grid refinement were used in the model. The outer domain used National Centers for Environmental Prediction data for initial and boundary conditions. The finest grid used 200 m in all three dimensions over a 48 km by 48 km section. The range of resolution and domain coverage were sufficient to resolve the abundant variety of dynamics associated with a time-evolving windstorm forced during a frontal passage. This full range of resolution and model complexity was essential in this case. Many aspects of this windstorm are inherently three-dimensional and are not represented in idealized models using either 2D or so-called 2D–3D dynamics.

Both the timing and location of wave breaking compared well with observations. The model also reproduced cross-stream wavelike perturbations in the jet stream that compared well with the orientation and spacing of cloud bands observed by satellite and lidar. Model results also show that the observed CAT derives from interactions between these wavelike jet stream disturbances and mountain-forced internal gravity waves. Due to the nearly east–west orientation of the jet stream, these two interacting wave modes were orthogonal to each other. Thermal gradients associated with the intense jet stream undulations generated horizontal vortex tubes (HVTs) aligned with the mean flow. These HVTs remained aloft while they propagated downstream at about the elevation of the aircraft incident, and evidence for such a vortex was seen by the lidar. The model and observations suggest that one of these intense vortices may have caused the aircraft incident.

Reports of strong surface gusts were intermittent along the Front Range during the period of this study. The model showed that interactions between the gravity waves and flow-aligned jet stream undulations result in isolated occurrences of strong surface gusts in line with observations. The simulations show that strong shears on the upper and bottom surfaces of the jet stream combine to provide an episodic “downburst of turbulence.” In the present case, the perturbations of the jet stream provide a funnel-shaped shear zone aligned with the mean flow that acts as a guide for the downward transport of turbulence resulting from breaking gravity waves. The physical picture for the upper levels is similar to the surface gusts described by Clark and Farley resulting from vortex tilting. The CAT feeding into this funnel came from all surfaces of the jet stream with more than half originating from the vertically inclined shear zones on the bottom side of the jet stream. Visually the downburst of turbulence looks similar to a rain shaft plummeting to the surface and propagating out over the plains leaving relatively quiescent conditions behind.

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Jonathan J. Gourley, Yang Hong, Zachary L. Flamig, Ami Arthur, Robert Clark, Martin Calianno, Isabelle Ruin, Terry Ortel, Michael E. Wieczorek, Pierre-Emmanuel Kirstetter, Edward Clark, and Witold F. Krajewski

Despite flash flooding being one of the most deadly and costly weather-related natural hazards worldwide, individual datasets to characterize them in the United States are hampered by limited documentation and can be difficult to access. This study is the first of its kind to assemble, reprocess, describe, and disseminate a georeferenced U.S. database providing a long-term, detailed characterization of flash flooding in terms of spatiotemporal behavior and specificity of impacts. The database is composed of three primary sources: 1) the entire archive of automated discharge observations from the U.S. Geological Survey that has been reprocessed to describe individual flooding events, 2) flash-flooding reports collected by the National Weather Service from 2006 to the present, and 3) witness reports obtained directly from the public in the Severe Hazards Analysis and Verification Experiment during the summers 2008–10. Each observational data source has limitations; a major asset of the unified flash flood database is its collation of relevant information from a variety of sources that is now readily available to the community in common formats. It is anticipated that this database will be used for many diverse purposes, such as evaluating tools to predict flash flooding, characterizing seasonal and regional trends, and improving understanding of dominant flood-producing processes. We envision the initiation of this community database effort will attract and encompass future datasets.

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Brian A. Klimowski, Robert Becker, Eric A. Betterton, Roelof Bruintjes, Terry L. Clark, William D. Hall, Brad W. Orr, Robert A. Kropfli, Paivi Piironen, Roger F. Reinking, Dennis Sundie, and Taneil Uttal

The 1995 Arizona Program was a field experiment aimed at advancing the understanding of winter storm development in a mountainous region of central Arizona. From 15 January through 15 March 1995, a wide variety of instrumentation was operated in and around the Verde Valley southwest of Flagstaff, Arizona. These instruments included two Doppler dual-polarization radars, an instrumented airplane, a lidar, microwave and infrared radiometers, an acoustic sounder, and other surface-based facilities. Twenty-nine scientists from eight institutions took part in the program. Of special interest was the interaction of topographically induced, storm-embedded gravity waves with ambient upslope flow. It is hypothesized that these waves serve to augment the upslope-forced precipitation that falls on the mountain ridges. A major thrust of the program was to compare the observations of these winter storms to those predicted with the Clark-NCAR 3D, nonhydrostatic numerical model.

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Morris L. Weisman, Robert J. Trapp, Glen S. Romine, Chris Davis, Ryan Torn, Michael Baldwin, Lance Bosart, John Brown, Michael Coniglio, David Dowell, A. Clark Evans, Thomas J. Galarneau Jr., Julie Haggerty, Terry Hock, Kevin Manning, Paul Roebber, Pavel Romashkin, Russ Schumacher, Craig S. Schwartz, Ryan Sobash, David Stensrud, and Stanley B. Trier

Abstract

The Mesoscale Predictability Experiment (MPEX) was conducted from 15 May to 15 June 2013 in the central United States. MPEX was motivated by the basic question of whether experimental, subsynoptic observations can extend convective-scale predictability and otherwise enhance skill in short-term regional numerical weather prediction.

Observational tools for MPEX included the National Science Foundation (NSF)–National Center for Atmospheric Research (NCAR) Gulfstream V aircraft (GV), which featured the Airborne Vertical Atmospheric Profiling System mini-dropsonde system and a microwave temperature-profiling (MTP) system as well as several ground-based mobile upsonde systems. Basic operations involved two missions per day: an early morning mission with the GV, well upstream of anticipated convective storms, and an afternoon and early evening mission with the mobile sounding units to sample the initiation and upscale feedbacks of the convection.

A total of 18 intensive observing periods (IOPs) were completed during the field phase, representing a wide spectrum of synoptic regimes and convective events, including several major severe weather and/or tornado outbreak days. The novel observational strategy employed during MPEX is documented herein, as is the unique role of the ensemble modeling efforts—which included an ensemble sensitivity analysis—to both guide the observational strategies and help address the potential impacts of such enhanced observations on short-term convective forecasting. Preliminary results of retrospective data assimilation experiments are discussed, as are data analyses showing upscale convective feedbacks.

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