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Lisa S. Darby and Gregory S. Poulos

Abstract

A lee-wave–rotor system interacting with an approaching cold front in the lee of Pike’s Peak near Colorado Springs, Colorado, on 1 April 1997 is studied observationally and numerically. Dynamical effects associated with the approaching cold front caused the amplification of the evolving lee wave and rotor, creating increasingly more hazardous flight conditions for nearby airports. The rapidly evolving winds measured by a Doppler lidar and 915-MHz wind profilers, and simulated by the Regional Atmospheric Modeling System (RAMS), produced light-to-moderate turbulence for a research aircraft making missed approaches at the Colorado Springs Airport during the wave amplification phase. As the cold front approached the foothills, the lee-wave–rotor system ended abruptly, reducing hazardous flight conditions.

The Doppler lidar’s detailed measurements of the lee-wave–rotor system allowed for an evaluation of RAMS ability to capture these complex wind features. Qualitative and quantitative comparisons between the lidar range–height measurements and model x–z cross sections are presented. In a broad sense, the numerical simulations were successful in the prediction of the prefrontal amplification and the postfrontal decay of the waves as measured by the lidar. RAMS also predicted observed wind reversals above the lee waves, which were indicators of breaking wave instability. At times RAMS performed poorly by over- or underpredicting the wind speeds in the lee wave, as well as the horizontal extent of the lee wave or rotor.

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Gregory S. Poulos and James E. Bossert

Abstract

The Atmospheric Studies in Complex Terrain Program conducted a field experiment at the interface of the Rocky Mountains and the Great Plains in the winter of 1991. Extensive meteorological observations were taken in northeastern Colorado near Rocky Flats to characterize overnight conditions in the region. Simultaneously, a tracer dispersion experiment using over 130 samplers to track plume development was conducted by Rocky Flats facility personnel. These two datasets provided an opportunity to investigate the accuracy and applicability of a fully prognostic, primitive equation, mesoscale model to the simulation of complex terrain dispersion.

Meteorological conditions in the Rocky Flats region are forecast for selected case nights using the Regional Atmospheric Modeling System initialized with sounding data taken during the experiment. The forecast winds and temperature are used in a Lagrangian particle dispersion model to predict tracer plume transport. The results of both models are compared to observations taken during the experimental period and qualitatively and quantitatively assessed. It is found that this modeling system is able to reproduce many features of the observed meteorology and dispersion for four overnight cases. Quantitatively, maximum ground concentrations are generally found to be within a factor of 2 of observations and located radially within approximately 50° of azimuth of the observed location. Additional model sensitivity simulations define the role of local terrain features on Rocky Flats area dispersion and indicate the need for improved model initialization techniques when multiple data sources are available. These experiments reveal a promising future for the application of prognostic mesoscale models to emergency response problems in regions of complex terrain.

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Gregory S. Poulos and Sean P. Burns

Abstract

High-rate near-surface overnight atmospheric data taken during the Cooperative Atmosphere–Surface Exchange Study-1999 (CASES-99) is used to quantify the representativeness of surface layer formulations under statically stable conditions. Combined with weak wind shear, such conditions generate large dynamic stability (Ri > 1.0), intermittency, and nonstationarity, which violate the underlying assumptions of surface layer theory. Still, such parameterizations are applied in atmospheric numerical models from large-eddy to global circulation.

To investigate two formulas, their parameterized sensible heat flux and friction velocity (u∗) values are compared, when driven by CASES-99 measurements, to CASES-99 measurements of the same from various heights. Significant inaccuracies in the magnitude and sign of flux are found with 1) a frequent, large underprediction of heat flux for Rib > ∼1.0, 2) an overprediction of negative sensible heat flux and u∗ for ∼0.2 < Rib < ∼0.8, 3) a systematic underprediction of u∗ for Rib > 1.0 for one of the schemes tested, and 4) a misrepresentation of natural heat and u∗ intermittency by both schemes for Ri > ∼1.0. Failures of the “constant flux assumption” for a given height are proposed as a partial source for the errors. Using experimental data, a surface layer of O[1–10] m is found during dynamically stable conditions. Rather than suggest a revised algebraic fit to the observations, an alternate approach to surface layer parameterization is proposed.

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C. David Whiteman, Sebastian W. Hoch, and Gregory S. Poulos

Abstract

At slope and valley floor sites in the Owens Valley of California, the late afternoon near-surface air temperature decline is often followed by a temporary temperature rise before the expected nighttime cooling resumes. The spatial and temporal patterns of this evening warming phenomenon, as seen in the March/April 2006 Terrain-Induced Rotor Experiment, are investigated using a widely distributed network of 51 surface-based temperature dataloggers. Hypotheses on the causes of the temperature rises are tested using heavily instrumented 34-m meteorological towers that were located within the datalogger array. The evening temperature rise follows the development of a shallow temperature deficit layer over the slopes and floor of the valley in which winds blow downslope. Background winds within the valley, freed from frictional deceleration from the earth’s surface by this layer, accelerate. The increased vertical wind shear across the temperature deficit layer eventually creates shear instability and mixes out the layer, creating the observed warming near the ground. As momentum is exchanged during the mixing event, the wind direction near the surface gradually turns from downslope to the background wind direction. After the short period of warming associated with the mixing, ongoing net radiative loss causes a resumption of the cooling.

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Gregory S. Poulos, Douglas A. Wesley, John S. Snook, and Michael P. Meyers

Abstract

Over the 3-day period of 24–26 October 1997, a powerful winter storm was the cause of two exceptional weather phenomena: 1) blizzard conditions from Wyoming to southern New Mexico along the Front Range of the Rocky Mountains and 2) hurricane-force winds at the surface near Steamboat Springs, Colorado, with the destruction of about 5300 ha of old-growth forest. This rare event was caused by a deep, cutoff low pressure system that provided unusually strong, deep easterly flow over the Front Range for an extended period. The event was characterized by highly variable snowfall and some very large snowfall totals; over a horizontal distance of 15 km, in some cases, snowfall varied by as much as 1.0 m, with maximum total snowfall depths near 1.5 m. Because this variability was caused, in part, by terrain effects, this work investigates the capability of a mesoscale model constructed in terrain-following coordinates (the Regional Atmospheric Modeling System: RAMS) to forecast small-scale (meso γ), orographically forced spatial variability of the snowfall. There are few investigations of model-forecast liquid precipitation versus observations at meso-γ-scale horizontal grid spacing. Using a limited observational dataset, mean absolute percent errors of precipitation (liquid equivalent) of 41% and 9% were obtained at horizontal grid spacings of 5.00 and 1.67 km, respectively. A detailed, high-temporal-resolution (30-min intervals) comparison of modeled versus actual snowfall rates at a fully instrumented snow measurement testing site shows significant model skill. A companion paper, Part II, will use the same RAMS simulations to describe the observations and modeling of the simultaneous mountain-windstorm-induced forest blowdown event.

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Michael P. Meyers, John S. Snook, Douglas A. Wesley, and Gregory S. Poulos

Abstract

A devastating winter storm affected the Rocky Mountain states over the 3-day period of 24–26 October 1997. Blizzard conditions persisted over the foothills and adjoining plains from Wyoming to southern New Mexico, with maximum total snowfall amounts near 1.5 m. ( of this two-part paper describes the observations and modeling of this blizzard event.) During the morning of 25 October 1997, wind gusts in excess of 50 m s−1 were estimated west of the Continental Divide near Steamboat Springs in northern Colorado. These winds flattened approximately 5300 ha (13 000 acres) of old-growth forest in the Routt National Forest and Mount Zirkel Wilderness. Observations, analysis, and numerical modeling were used to examine the kinematics of this extreme event. A high-resolution, local-area model (the Regional Atmospheric Modeling System) was used to investigate the ability of a local model to capture the timing and strength of the windstorm and the aforementioned blizzard. Results indicated that a synergistic combination of strong cross-barrier easterly flow; very cold lower-tropospheric air over Colorado, which modified the stability profile; and the presence of a critical layer led to devastating downslope winds. The high-resolution simulations demonstrated the potential for accurately capturing mesoscale spatial and temporal features of a downslope windstorm more than 1 day in advance. These simulations were quasi forecast in nature, because a combination of two 48-h Eta Model forecasts were used to specify the lateral boundary conditions. Increased predictive detail of the windstorm was also found by decreasing the horizontal grid spacing from 5 to 1.67 km in the local-area model simulations.

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Gregory S. Poulos, James E. Bossert, Thomas B. McKee, and Roger A. Pielke Sr.

Abstract

Via numerical analysis of detailed simulations of an early September 1993 case night, the authors develop a conceptual model of the interaction of katabatic flow in the nocturnal boundary layer with mountain waves (MKI). A companion paper (Part I) describes the synoptic and mesoscale observations of the case night from the Atmospheric Studies in Complex Terrain (ASCOT) experiment and idealized numerical simulations that manifest components of the conceptual model of MKI presented herein. The reader is also referred to Part I for detailed scientific background and motivation.

The interaction of these phenomena is complicated and nonlinear since the amplitude, wavelength, and vertical structure of the mountain-wave system developed by flow over the barrier owes some portion of its morphology to the evolving atmospheric stability in which the drainage flows develop. Simultaneously, katabatic flows are impacted by the topographically induced gravity wave evolution, which may include significantly changing wavelength, amplitude, flow magnitude, and wave breaking behavior. In addition to effects caused by turbulence (including scouring), perturbations to the leeside gravity wave structure at altitudes physically distant from the surface-based katabatic flow layer can be reflected in the katabatic flow by transmission through the atmospheric column. The simulations show that the evolution of atmospheric structure aloft can create local variability in the surface pressure gradient force governing katabatic flow. Variability is found to occur on two scales, on the meso-β due to evolution of the mountain-wave system on the order of one hour, and on the microscale due to rapid wave evolution (short wavelength) and wave breaking–induced fluctuations. It is proposed that the MKI mechanism explains a portion of the variability in observational records of katabatic flow.

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Gregory S. Poulos, James E. Bossert, Thomas B. McKee, and Roger A. Pielke

Abstract

The mutual interaction of katabatic flow in the nocturnal boundary layer (NBL) and topographically forced gravity waves is investigated. Due to the nonlinear nature of these phenomena, analysis focuses on information obtained from the 1993 Atmospheric Studies in Complex Terrain field program held at the mountain–canyon–plains interface near Eldorado Canyon, Colorado, and idealized simulations. Perturbations to katabatic flow by mountain waves, relative to their more steady form in quiescent conditions, are found to be caused by dynamic pressure effects. Based on a local Froude number climatology, case study analysis, and the simulations, the dynamic pressure effect is theorized to occur as gravity wave pressure perturbations are transmitted through the atmospheric column to the surface and, through altered horizontal pressure gradient forcing, to the surface-based katabatic flows. It is proposed that these perturbations are a routine feature in the atmospheric record and represent a significant portion of the variability in complex terrain katabatic flows.

The amplitude, wavelength, and vertical structure of mountain waves caused by flow over a barrier are themselves partly determined by the evolving structure of the NBL in which the drainage flows develop. For Froude number Fr > ∼0.5 the mountain wave flow is found to separate from the surface at higher altitudes with NBL evolution (increasing time exposed to radiational cooling), as is expected from Fr considerations. However, flow with Fr < ∼0.5 behaves unexpectedly. In this regime, the separation point descends downslope with NBL evolution. Overall, a highly complicated, mutually evolving, system of mountain wave–katabatic flow interaction is found, such that the two flow phenomena are, at times, indistinguishable. The mechanisms described here are expanded upon in a companion paper through realistic numerical simulations and analysis of a nocturnal case study (3–4 September 1993).

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Juerg Schmidli, Gregory S. Poulos, Megan H. Daniels, and Fotini K. Chow

Abstract

The dynamics that govern the evolution of nighttime flows in a deep valley, California’s Owens Valley, are analyzed. Measurements from the Terrain-Induced Rotor Experiment (T-REX) reveal a pronounced valley-wind system with often nonclassical flow evolution. Two cases with a weak high pressure ridge over the study area but very different valley flow evolution are presented. The first event is characterized by the appearance of a layer of southerly flow after midnight local time, sandwiched between a thermally driven low-level downvalley (northerly) flow and a synoptic northwesterly flow aloft. The second event is characterized by an unusually strong and deep downvalley jet, exceeding 15 m s−1. The analysis is based on the T-REX measurement data and the output of high-resolution large-eddy simulations using the Advanced Regional Prediction System (ARPS). Using horizontal grid spacings of 1 km and 350 m, ARPS reproduces the observed flow features for these two cases very well. It is found that the low-level along-valley forcing of the valley wind is the result of a superposition of the local thermal forcing and a midlevel (2–2.5 km MSL) along-valley pressure forcing. The analysis shows that the large difference in valley flow evolution derives primarily from differences in the midlevel pressure forcing, and that the Owens Valley is particularly susceptible to these midlevel external influences because of its specific geometry. The results demonstrate the delicate interplay of forces that can combine to determine the valley flow structure on any given night.

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Daran L. Rife, Emilie Vanvyve, James O. Pinto, Andrew J. Monaghan, Christopher A. Davis, and Gregory S. Poulos

Abstract

This paper describes a new computationally efficient and statistically robust sampling method for generating dynamically downscaled climatologies. It is based on a Monte Carlo method coupled with stratified sampling. A small yet representative set of “case days” is selected with guidance from a large-scale reanalysis. When downscaled, the sample closely approximates the long-term meteorological record at a location, in terms of the probability density function. The method is demonstrated for the creation of wind maps to help determine the suitability of potential sites for wind energy farms. Turbine hub-height measurements at five U.S. and European tall tower sites are used as a proxy for regional climate model (RCM) downscaled winds to validate the technique. The tower-measured winds provide an independent test of the technique, since RCM-based downscaled winds exhibit an inherent dependence upon the large-scale reanalysis fields from which the case days are sampled; these same reanalysis fields would provide the boundary conditions to the RCM. The new sampling method is compared with the current approach widely used within the wind energy industry for creating wind resource maps, which is to randomly select 365 case days for downscaling, with each day in the calendar year being represented. The new method provides a more accurate and repeatable estimate of the long-term record of winds at each tower location. Additionally, the new method can closely approximate the accuracy of the current (365 day) industry approach using only a 180-day sample, which may render climate downscaling more tractable for those with limited computing resources.

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