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E. S. Takle

Abstract

Temperature measurements taken over a 6-year period from a 32 m tower are used to study the climatology of low-level superadiabatic conditions in a rural area. Plots of monthly distributions of event commencement times and durations display a high degree of symmetry compared to the distributions for inversions previously published. This climatology can be used to better identify periods when spores, pollens, insects, smoke or agricultural chemicals introduced at the surface will quickly become available for long-range transport by the upper level winds.

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Hao Wang and E. S. Takle

Abstract

The authors report results of a numerical model used to simulate wind and turbulence fields for porous, living shelterbelts with seven different cross-sectional shapes. The simulations are consistent with results of Woodruff and Zingg whose wind-tunnel study demonstrated that all shelterbelts with very different shapes have nearly identical reduction of wind and turbulence. The simulations also showed that the pressure-loss (resistance) coefficient for smooth-shaped or streamlined shelterbelts is significantly smaller than that for rectangle-shaped or triangle-shaped shelterbelts with a windward vertical side. However, the shelter effects are not proportional to the pressure-loss coefficient (drag). Analysis of the momentum budget demonstrated that in the near lee and in the far lee, both vertical advection and pressure gradient have opposite roles in the recovery of wind speed. This behavior, combined with differences in permeability, is the likely cause of reduced sensitivity of shelter effects to shelterbelt shape.

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W. E. Heilman and E. S. Takle

Abstract

A two4Mensional second-order turbulence-closure model based on Mellor-Yamada level 3 is used to examine the nocturnal turbulence characteristics over Rattlesnake Mountain in Washington. Simulations of mean horizontal velocities and potential temperatures agree well with data. The equations for the components of the turbulent kinetic energy (TKE) show that anisotropy contributes in ways that are counter to our intuition developed from mean now considerations: shear production under stable conditions forces the suppression of the vertical component proportion of loud TKE, while potential-temperature variance under stable conditions leads to a positive (countergradient) contribution to the heat flux that increases the vertical component proportion of total TKE. This paper provides a qualitative analysis of simulated turbulence fields, which indicates significant variation over the windward and leeward slopes. From the simulation results, turbulence anisotropy is seen to develop in the katabatic flow region where vertical wind shears and atmospheric stability are large. An enhancement of the vertical component proportion of the total TKE takes place over the leeward slope as the downslope distance increases. The countergradient portion of the turbulent heat flux plays an important role in producing regions of anisotropy.

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E. S. Takle and J. M. Brown

Abstract

A hybrid density function is given for describing wind-speed distributions having nonzero probability of “calm.” A Weibull probability graph paper designed specifically for plotting wind-speed distributions is used to determine distribution parameters to within a few percent of values obtained by the maximum likelihood technique. Data from the National Weather Service are used to demonstrate the use of the hybrid density function and the Weibull graph paper.

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E. S. Takle, R. H. Shaw, and H. C. Vaughan

Abstract

Temperature and wind speed measurements over a 6-year period from a 32 m tower located in a primarily rural area are used to assess the pollutant-dispersive characteristics of a rural site. A monthly comparison of a crude pollution-trapping index shows July through September the most favorable, and December through February the least favorable, months for the trapping of contaminants emitted from ground-based sources in rural areas.

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L. P. Chang, E. S. Takle, and R. L. Sani

Abstract

We have developed a two-dimensional finite-element model for simulating atmospheric flow in the planetary boundary layer (PBL) of the earth. The finite-element method provides a useful alternative to the conventional finite-difference method in studying Bow phenomena that involve graded meshes and (or) irregular computational domains. It also provides a more natural way of incorporating Dirichlet-type boundary conditions. These properties make the finite-element method especially suitable for studying PBL flows. With the Deardorff-O'Brien turbulence scheme, the model was able to generate reasonable results in the simulations of a neutral PBL wind profile and a sea-breeze circulation.

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M. Segal, Z. Pan, R. W. Turner, and E. S. Takle

Abstract

The potential impact of the increase in irrigated areas in North America during the past 100 years on summer rainfall associated with medium- to large-scale precipitation systems is evaluated conceptually and by several illustrative numerical model simulations. The model results for the simulated cases suggest a tendency toward some increase in the continental-average rainfall for the present irrigation conditions compared with those of past irrigation. The maximum increase obtained for several studied cases of 6-day duration each was 1.7%. Rainfall increases typically occur in the location of existing rainfall areas, and the main effect of irrigation is to redistribute rainfall in those preexisting precipitation regions.

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M. A. Lind, J. S. Hartman, E. S. Takle, and J. L. Stanford

Abstract

The purpose of this note is to show that not all tornadic storms yield the same radio-frequency noise behavior. One funnel cloud observed gave significant radio noise over a frequency range from 2.5 to 144 MHz.

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Brian J. Vanderwende, Julie K. Lundquist, Michael E. Rhodes, Eugene S. Takle, and Samantha L. Irvin

Abstract

In the U.S. state of Iowa, the increase in wind power production has motivated interest into the impacts of low-level jets on turbine performance. In this study, two commercial lidar systems were used to sample wind profiles in August 2013. Jets were systematically detected and assigned an intensity rating from 0 (weak) to 3 (strong). Many similarities were found between observed jets and the well-studied Great Plains low-level jet in summer, including average jet heights between 300 and 500 m above ground level, a preference for southerly wind directions, and a nighttime bias for stronger jets. Strong vertical wind shear and veer were observed, as well as veering over time associated with the LLJs. Speed, shear, and veer increases extended into the turbine-rotor layer during intense jets. Ramp events, in which winds rapidly increase or decrease in the rotor layer, were also commonly observed during jet formation periods. The lidar data were also used to evaluate various configurations of the Weather Research and Forecasting Model. Jet occurrence exhibited a stronger dependence on the choice of initial and boundary condition data, while reproduction of the strongest jets was influenced more strongly by the choice of planetary boundary layer scheme. A decomposition of mean model winds suggested that the main forcing mechanism for observed jets was the inertial oscillation. These results have implications for wind energy forecasting and site assessment in the Midwest.

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Kenneth E. Kunkel, Karen Andsager, Xin-Zhong Liang, Raymond W. Arritt, Eugene S. Takle, William J. Gutowski Jr., and Zaitao Pan

Abstract

A regional climate model simulation of the period of 1979–88 over the contiguous United States, driven by lateral boundary conditions from the National Centers for Environmental Prediction–National Center for Atmospheric Research reanalysis, was analyzed to assess the ability of the model to simulate heavy precipitation events and seasonal precipitation anomalies. Heavy events were defined by precipitation totals that exceed the threshold value for a specified return period and duration. The model magnitudes of the thresholds for 1-day heavy precipitation events were in good agreement with observed thresholds for much of the central United States. Model thresholds were greater than observed for the eastern and intermountain western portions of the region and were smaller than observed for the lower Mississippi River basin. For 7-day events, model thresholds were in good agreement with observed thresholds for the eastern United States and Great Plains, were less than observed for the most of the Mississippi River valley, and were greater than observed for the intermountain western region. The interannual variability in frequency of heavy events in the model simulation exhibited similar behavior to that of the observed variability in the South, Southwest, West, and North-Central study regions. The agreement was poorer for the Midwest and Northeast, although the magnitude of variability was similar for both model and observations. There was good agreement between the model and observational data in the seasonal distribution of extreme events for the West and North-Central study regions; in the Southwest, Midwest, and Northeast, there were general similarities but some differences in the details of the distributions. The most notable differences occurred for the southern Gulf Coast region, for which the model produced a summer peak that is not present in the observational data. There was not a very high correlation in the timing of individual heavy events between the model and observations, reflecting differences between model and observations in the speed and path of many of the synoptic-scale events triggering the precipitation.

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