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R. Banta and K. R. Hanson

Abstract

The cloud model of Tripoli and Cotton was used to simulate a cumulonimbus cloud observed during the Cooperative Convective Precipitation Experiment (CCOPE). We tested the sensitivity of the precipitation pathways in the model to the initial concentration of cloud droplets above cloud base Nc (which is related to the concentration of cloud condensation nuclei). The results showed that for large Nc, Manton and Cotton's autoconversion parameterization properly suppressed supercooled rain formation via the “warm-rain” process in a cold-based, continental cloud, forcing ice processes (e.g., riming, aggregation and deposition of vapor) to produce graupel. With lower droplet concentrations, rain formed first through warm-rain processes, then graupel formed through freezing. The value of Nc, which determined the transition from graupel formation by freezing rain to graupel formation by ice processes was found to be sensitive to the parameter a cm, which represents the critical mean radius at which collision and coalescence begin.

The observed cloud was also compared with a cloud simulation which had approximately the correct initial cloud-droplet concentration (Nc). The simulated cloud base was somewhat lower than observed, indicating that the initial sounding was too moist in the subcloud layer. As a result, the modeled cloud was wetter than observed. In spite of this variation from the observed cloud base, other properties were represented rather well by the simulated cloud, including cloud top height, peak vertical velocities, and the growth stages in the development of the storm.

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R. Banta and W. R. Cotton

Abstract

In the traditional model of ridge-valley winds, there are typically two wind regimes on a dry day: a downslope, drainage wind at night due to cooling at the surface along the slopes, and an upslope wind during the day due to solar heating of the slopes. This study presents observations from South Park, a broad, flat basin in the Colorado Rockies. The observations consist of time sequences of surface observations, surface mesonet analyses, and vertical atmospheric soundings using a tethered balloon system. On a typical dry day in South Park, three wind regimes were observed: the downslope regime, the upslope regime, and a late morning or afternoon wind which corresponded in direction to the winds above the ridgetops. Because the gradient and ridgetop winds were most frequently from the west, we have called these winds the “afternoon westerues.”

The afternoon westerlies occur in conjunction with a deep (2–3 km or more) afternoon convective boundary layer in which momentum (and other properties) are well mixed all the way down to the surface. The appearance of the westerlies at the surface is thus a consequence of the strong turbulent mixing within the convective boundary layer.

Vertical tethered balloon soundings taken in mid-morning show that the upslope winds form within a shallow convective boundary layer, which develops beneath the nocturnal inversion in response to surface heating. This stable inversion layer inhibits downward mixing of the upper-level westerlies and allows easterly, upslope flow to establish itself near the surface. When the last remnant of the nocturnal inversion is erased by surface heating and other processes, the westerlies are free to mix downward, and afternoon westerlies are observed at the surface.

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A. B. White, C. J. Senff, and R. M. Banta

Abstract

The authors compare the mixing depths in the daytime convective boundary layers that were observed remotely by wind profilers and an airborne lidar during the 1995 Southern Oxidants Study. The comparison is used to determine whether the mixing depths deduced from radar reflectivity profiles measured by the wind profilers are the appropriate mixing depths to use in air pollution applications. The profiler mixing depths are based on evidence that the profile of the refractive index structure function parameter exhibits a peak at the boundary layer capping inversion. The lidar mixing depths are determined from the gradient in aerosol backscatter at the top of an aerosol mixing layer. The results of linear regression analysis show that the mixing depths measured by the wind profiler and lidar are in good agreement, particularly in the absence of scattered clouds forming at the top of the convective boundary layer. When significant cumulus convection occurs, the definition of mixing depth from both experimental and theoretical points of view is ill defined.

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R. M. Banta, L. D. Olivier, P. H. Gudiksen, and R. Lange

Abstract

Small-scale, topographically forced wind systems often have a strong influence on flow over complex terrain. A problem is that these systems are very difficult to measure, because of their limited spatial and temporal extent. They can be important, however, in the atmospheric transport of hazardous materials. For example, a nocturnal exit jet-a narrow stream of cold air-which flowed from Eldorado Canyon at the interface between the Rocky Mountains and the Colorado plains near the Rocky Flats Plant (RFP), swept over RFP for about 3 h in the middle of the night of 4–5 February 1991. It extended in depth from a few tens of meters to approximately 800 m above the ground. Because the jet was so narrow (2 km wide), it was poorly sampled by the meteorological surface mesonet, but it did prove to have an effect on the dispersion of tracer material released from RFP, producing a secondary peak in measured concentration to the southeast of RFP. The existence and behavior of the jet was documented by Environment Technology Laboratoy's Doppler lidar system, a scanning, active remote-sensing system that provides fine-resolution wind measurements. The lidar was deployed as a part of a wintertime study of flow and dispersion in the RFP vicinity during February 1993.

The MATHEW-ADPIC atmospheric dispersion model was run using the case study data from this night. It consists of three major modules: an interpolation scheme; MATHEW, a diagnostic wind-flow algorithm that calculates a mass-consistent interpolated flow; and ADPIC, a diffusion algorithm. The model did an adequate job of representing the main lobe of the tracer transport, but the secondary lobe resulting from the Eldorado Canyon exit jet was absent from the model result. Because the jet was not adequately represented in the input data, it did not appear in the modeled wind field. Thus, the effects of the jet on the transport of tracer material were not properly simulated by the diagnostic model.

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Yelena L. Pichugina, Robert M. Banta, W. Alan Brewer, Scott P. Sandberg, and R. Michael Hardesty

Abstract

Accurate measurement of wind speed profiles aloft in the marine boundary layer is a difficult challenge. The development of offshore wind energy requires accurate information on wind speeds above the surface at least at the levels occupied by turbine blades. Few measured data are available at these heights, and the temporal and spatial behavior of near-surface winds is often unrepresentative of that at the required heights. As a consequence, numerical model data, another potential source of information, are essentially unverified at these levels of the atmosphere. In this paper, a motion-compensated, high-resolution Doppler lidar–based wind measurement system that is capable of providing needed information on offshore winds at several heights is described. The system has been evaluated and verified in several ways. A sampling of data from the 2004 New England Air Quality Study shows the kind of analyses and information available. Examples include time–height cross sections, time series, profiles, and distributions of quantities such as winds and shear. These analyses show that there is strong spatial and temporal variability associated with the wind field in the marine boundary layer. Winds near the coast show diurnal variations, and frequent occurrences of low-level jets are evident, especially during nocturnal periods. Persistent patterns of spatial variability in the flow field that are due to coastal irregularities should be of particular concern for wind-energy planning, because they affect the representativeness of fixed-location measurements and imply that some areas would be favored for wind-energy production whereas others would not.

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I. G. McKendry, D. G. Steyn, R. M. Banta, W. Strapp, K. Anlauf, and J. Pottier

Abstract

Tethersonde, lidar, aircraft, and surface chemistry measurements from an intensive field campaign (Pacific’93) in the Lower Fraser Valley (LFV) demonstrate the daytime advection of pollutants into a lake-filled valley adjoining a broad urbanized coastal valley. On three separate days (immediately before, during, and after a pollutant episode), elevated concentrations of ozone (O3) in the narrow tributary valley could be attributed to the advection of pollutants northward from sources in the LFV (primarily metropolitan Vancouver). On 5 August, the highest concentrations of O3 observed in the region during the entire episode were observed over the tributary lake. Simple Lagrangian mass budget calculations suggest that the unusually high concentrations observed on 5 August over the lake were physically reasonable and consistent with the known chemistry of the air advected into the valley. They also indicate that reductions in O3 flux divergence during the overlake trajectory in the Pitt Valley, primarily as a result of reduced surface deposition, may contribute to the relatively high concentrations observed in the tributary valley. Observations immediately after the episode show that chemically aged polluted air masses can persist within the tributary valleys from the previous day. These results have implications for the understanding of air pollution in other regions of complex terrain and show that the predominance of daytime upvalley pollutant transport in such tributary valleys is likely to have significant impacts on the local ecology and visibility.

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Robert M. Banta, Yelena L. Pichugina, Neil D. Kelley, R. Michael Hardesty, and W. Alan Brewer

Addressing the need for high-quality wind information aloft in the layer occupied by turbine rotors (~30–150 m above ground level) is one of many significant challenges facing the wind energy industry. Without wind measurements at heights within the rotor sweep of the turbines, characteristics of the flow in this layer are unknown for wind energy and modeling purposes. Since flow in this layer is often decoupled from the surface, near-surface measurements are prone to errant extrapolation to these heights, and the behavior of the near-surface winds may not reflect that of the upper-level flow.

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Martin D. Weissmann, G. J. Mayr, R. M. Banta, and A. Gohm

Abstract

The investigation of gap flow in the Wipp Valley (GAP project) is one of the objectives of the Mesoscale Alpine Programme (MAP). The valley runs south–north across the Brenner Pass, from Italy to Austria. The pass is the lowest one of the main Alpine ridge and is therefore a favorable location for a gap flow, which is called foehn. Based on the extensive dataset of MAP, this study gives a detailed analysis of foehn on 2 and 3 October 1999 [intensive observation period 5 (IOP 5)]. The foehn event began as a gap flow that was separated from midlevel winds by a strong temperature inversion during the night of 1–2 October 1999. On the next night (2– 3 October) the inversion dissipated, and the gap flow was combined with strong midlevel cross-ridge flow on 3 October 1999. This study shows that the existence of a temperature inversion above the gap flow has a crucial impact on the flow structure. Another emphasis of the study was the investigation of the small-scale flow structure downstream of the gap. Jumplike features and regions with flow reversals, which were interpreted as “rollers” (reversed rotors), could be observed in the Wipp Valley. A jet layer with a wavy structure indicated a gravity wave in the southern part of the valley. In the northern part the flow showed strong asymmetry with wind speeds nearly twice as strong on the eastern side as on the western one.

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J. M. Intrieri, C. G. Little, W. J. Shaw, R. M. Banta, P. A. Durkee, and R. M. Hardesty

The Land/Sea Breeze Experiment (LASBEX) was conducted at Moss Landing, California, 15–30 September 1987. The experiment was designed to study the vertical structure and mesoscale variation of the land/sea breeze. A Doppler lidar, a triangular array of three sodars, two sounding systems (one deployed from land and one from a ship), and six surface weather stations (one shipborne) were sited around the Moss Landing area. Measurements obtained included ten sea-breeze and four land-breeze events. This paper describes the objectives and design of the experiment, as well as the observing systems that were used. Some preliminary results and selected observations are presented, called from the data collected, as well as the ensuing analysis plans.

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A. O. Langford, C. J. Senff, R. J. Alvarez II, R. M. Banta, R. M. Hardesty, D. D. Parrish, and T. B. Ryerson

Abstract

The NOAA airborne ozone lidar system [Tunable Optical Profiler for Aerosol and Ozone (TOPAZ)] is compared with the fast-response chemiluminescence sensor flown aboard the NOAA WP-3D during the 2006 Texas Air Quality Study (TexAQS). TOPAZ measurements made from the NOAA Twin Otter, flying at an altitude of ~3300 m MSL in the Houston, Texas, area on 31 August, and the Dallas, Texas, area on 13 September, show that the overall uncertainty in the 10-s (~600-m horizontal resolution) TOPAZ profiles is dominated by statistical uncertainties (1σ) of ~8 ppbv (6%–10%) at ranges of ~2300 m from the aircraft (~1000 m MSL), and ~11–27 ppbv (12%–30%) at ranges of ~2800 m (~500 m MSL). These uncertainties are substantially reduced by spatial averaging, and the averages of 11 profiles (of 110 s or 6.6-km horizontal resolution) at ~1000 m MSL are in excellent agreement (±2%) with the in situ measurements at ~500 m MSL. The TOPAZ measurements at lower altitudes on 31 August exhibit a negative bias of up to ~15%, however, when the lidar signals were strongly attenuated by very high ozone levels in the plume from the Houston Ship Channel. This bias appears to result from nonlinear behavior in the TOPAZ signal amplifiers, which is described in the companion paper by Alvarez et al. An empirical correction is presented.

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