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William D. Hall

Abstract

A two-dimensional anelastic cloud model which incorporates detailed treatments of the water and ice phase is presented. The liquid phase processes considered include condensation, quasi-stochastic coalescence, fallout and breakup, while the ice phase processes include diffusional and accretional growth of ice particles. Results of two cloud simulations are presented. The first case assumes an atmosphere with maritime cloud condensation nuclei (CCN) activation characteristics and considers the warm rain processes only. It was found that with the appearance of precipitation, the model-predicted supersaturations within updraft regions often reach values larger than 5% with respect to water. The second case assumes an atmosphere with continental CCN characteristics and includes the ice phase processes leading to the formation of graupel. The results of the second case illustrate the importance of cloud vertical motions in transporting ice particles from preferential nucleation regions in the upper portions of the cloud to preferential accretional growth regions of larger liquid water content. Discrepancies between model results and observations are discussed briefly and future research applications of this model are mentioned.

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Terry L. Clark and William D. Hall

Abstract

This note describes how to generate vertically stretched grids within the context of vertical nesting that are consistent with the conservative interpolation formula used by Clark and Farley. It is shown that all nested grids derive their structure directly from the parent grid, where the only flexibility allowed for nested grids is their grid ratio relative to the parent grid. Formulas are presented that can he used to analyze resulting nested grid structures, and an example showing how these formulas were used to generate relatively smooth inner meshes is described. Suggestions for further improvements in grid design are also provided.

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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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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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Charles A. Knight, L. Jay Miller, and William D. Hall

Abstract

The development of convective cells within anvil precipitation, in a region of moderate convective activity that might be called a small mesoscale convective system, is described and discussed. The presence of precipitation-sized hydrometeors in the air as the convection develops makes early stages visible to radar that might not otherwise be seen. Two kinds of convective initiation are illustrated. In one, a vigorous cell is initiated over an outflow boundary, but within light precipitation. In the other, the initiation is evidently by an instability created by the melting layer, perhaps by a mechanism first discussed by Findeisen. In this latter type, the new convective elements are not severe but they generate supercooled cloud within the anvil, extend entirely through the anvil to altitudes above 12 km MSL, and produce graupel showers with rain at the ground exceeding 50 dBZ. The instability itself may be generated in large part by moistening and cooling the sounding by the falling precipitation.

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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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Charles A. Knight, William D. Hall, and Philip M. Roskowski

Abstract

Using quantitative analysis of time-lapse motion pictures from aircraft and a sensitive meteorological radar, the cloud top history is related to the early radar echo development in 12 vigorous, summer, convective cloud turrets in northeastern Colorado. At a threshold of about 5 dB(Z), the first echoes appear typically 5–10 min after the cloud top passes the -−20°C level. The first echo either appears at cloud top or reaches the top very quickly. It sometimes appears at a well-defined height, but sometimes nearly simultaneously over an altitude range of 3 km or more. Radar echo at 5 dB(Z) typically fills the visual cloud 5–10 min after first echo. In terms of overall cloud lifetime there is plenty of time for the particles responsible for the first echo to form by the ice process. A detailed model of the rates of ice particle formation by vapor growth followed by riming gives a 5 dB(Z) radar echo within 7–10 min at concentrations as low as 1 m−3, at most temperatures between −10 and −20°C and in cloud conditions realistic for northeast Colorado. The natural echo development may often result from the transport of embryonic ice particles into regions with vigorous updraft and high liquid water content where growth by accretion is rapid, rather than from growth entirely within the vigorous updrafts, for which the time may often be insufficient.

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Kyoko Ikeda, Roy M. Rasmussen, William D. Hall, and Gregory Thompson

Abstract

Observations of supercooled drizzle aloft within two storms impacting the Oregon Cascades during the second Improvement of Microphysical Parameterization through Observational Verification Experiment (IMPROVE-2) field project are presented. The storms were characterized by a structure and evolution similar to the split-front model of synoptic storms. Both storms were also characterized by strong cross-barrier flow. An analysis of aircraft and radar data indicated the presence of supercooled drizzle during two distinct storm periods: 1) the intrafrontal period immediately following the passage of an upper cold front and 2) the postfrontal period. The conditions associated with these regions of supercooled drizzle included 1) temperatures between −3° and −19°C, 2) ice crystal concentrations between 1 and 2 L−1, and 3) bimodal cloud droplet distributions of low concentration [cloud condensation nuclei (CCN) concentration between 20 and 30 cm−3 and cloud drop concentration <35 cm−3].

Unique to this study was the relatively cold cloud top (<−15°C) and relatively high ice crystal concentrations in the drizzle region. These conditions typically hinder drizzle formation and survival; however, the strong flow over the mountain barrier amplified vertical motions (up to 2 m s−1) above local ridges, the mountain crest, and updrafts in embedded convection. These vertical motions produced high condensate supply rates that were able to overcome the depletion by the higher ice crystal concentrations. Additionally, the relatively high vertical motions resulted in a near balance of ice crystal fall speed (0.5–1.0 m s−1), leading to nearly terrain-parallel trajectories of the ice particles and a reduction of the flux of ice crystals from the higher levels into the low-level moisture-rich cloud, allowing the low-level cloud water and drizzle to be relatively undepleted.

One of the key observations in the current storms was the persistence of drizzle drops in the presence of significant amounts of ice crystals over the steepest portion of the mountain crest. Despite the high radar reflectivity produced by the ice crystals (>15 dBZ) in this region, the relatively high condensate supply rate led to hazardous icing conditions. The current study reveals that vertical motions generated by local topographic features are critical in precipitation processes such as drizzle formation and thus it is essential that microphysical models predict these motions.

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Teddie L. Keller, Stanley B. Trier, William D. Hall, Robert D. Sharman, Mei Xu, and Yubao Liu

Abstract

At 1818 mountain standard time 20 December 2008, a Boeing 737 jetliner encountered significant crosswinds while accelerating for takeoff at the Denver International Airport (DIA), ran off the side of the runway, and burst into flames. Passengers and crew were able to evacuate quickly, and, although there were injuries, there were no fatalities. Winds around the time of the accident were predominantly from the west, with substantial spatial and temporal speed variability across the airport property. Embedded in this mostly westerly flow were intermittent gusts that created strong crosswinds for the north–south runways. According to the report from the National Transportation Safety Board, it was one of these strong gusts that initiated the events that led to the runway excursion and subsequent crash of the aircraft. Numerous aircraft reported significant mountain-wave activity and turbulence over Colorado on the day of the accident. To determine whether wave activity may have contributed to the strong, intermittent gustiness at DIA, a high-resolution multinested numerical simulation was performed using the Clark–Hall model, with a horizontal grid spacing of 250 m in the inner domain. Results from this simulation suggest that the surface gustiness at DIA was associated with undulations in a train of lee waves in a midtropospheric stable layer above the airport, creating regions of higher-velocity air descending toward the surface. In contrast, a simulation with horizontal grid spacing that was similar to that of a state-of-the-art operational forecast model (3 km) did not predict strong winds at DIA.

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Xiaoqing Wu, William D. Hall, Wojciech W. Grabowski, Mitchell W. Moncrieff, William D. Collins, and Jeffrey T. Kiehl

Abstract

A two-dimensional cloud-resolving model with a large domain is integrated for 39 days during the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE) to study the effects of ice phase processes on cloud properties and cloud radiative properties. The ice microphysical parameterization scheme is modified based on microphysical measurements from the Central Equatorial Pacific Experiment. A nonlocal boundary layer diffusion scheme is included to improve the simulation of the surface heat fluxes. The modified ice scheme produces fewer ice clouds during the 39-day simulation. The cloud radiative properties show significant improvement and compare well with various observations. Both the 39-day mean value (202 W m−2) and month-long evolution of outgoing longwave radiative flux from the model are comparable with satellite observations. The 39-day mean surface shortwave cloud forcing is −110 W m−2, consistent with other estimates obtained for TOGA COARE. The 39-day mean values of surface net longwave, shortwave, latent, and sensible fluxes are −46.2, 182.9, −109.9, and −7.8 W m −2, respectively, in line with the IMET buoy data (−54.6, 178.2, −102.7, and −10.6 W m−2).

The offline radiation calculations as well as the cloud-interactive radiation simulations demonstrate that a doubled effective radius of ice particles and enhanced shortwave cloud absorption strongly affect the radiative flux and cloud radiative forcing but have little impact on the cloud properties. The modeled albedo is sensitive to the effective radius of ice particles and/or the shortwave cloud absorption in the radiation scheme. More complete satellite observations and theoretical studies are required to fully understand the physical processes involved.

The results suggest that the ice microphysical parameterization plays an important role in the long-term simulation of cloud properties and cloud radiative properties. Future field observations should put more weight on the microphysical properties, cloud properties, and high-quality radiative properties in order to further improve the cloud-resolving modeling of cloud systems and the understanding of cloud–radiation interaction.

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