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Thomas W. Horst

Abstract

Methods of eliminating or reducing three types of errors found in the Gill UVW anemometer have been investigated by utilizing field experiments comparing this sensor with a three-component sonic anemometer. The non-cosine response of each of the three orthogonal propellers to a wind which is not parallel to the propeller axis was adequately corrected during computer processing of the data, using the manufacturer's wind-tunnel-measured calibrations. The accepted theory describing a propeller as a first-order system with a time constant τ = L/ū (where L is a distance constant characterizing the propeller inertia and ū is the mean wind) was found to be only a fair description of the frequency response, probably due to dependence of L on properties of the flow, but was used to qualitatively delineate proper applications for this sensor. The threshold response was improved for the U and V components by orienting the anemometer so that the mean wind direction bisects the angle between the horizontal axis propellers. Improvement was also achieved for the vertical component of the wind by rotating the formerly vertical W propeller 45° into the horizontal, in the plane bisecting the horizontal propeller axes. The orthogonal components of the wind must then be calculated during computer processing of the data. Since for many applications the finite, but small, response threshold of the vertical component was not found to be a serious problem, the additional complication of this modification may be unnecessary.

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Thomas W. Horst

Abstract

A comparison is made of the Gaussian diffusion-deposition models of Chamberlain (1953) and Overcamp (1976) with the exact solution of Horst (1977). Overcamp's model is found to be a useful improvement over Chamberlain's model at short distances downwind of the source. At large distances, however, Chamberlain's model is superior to that of Overcamp.

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Thomas W. Horst

Abstract

The transfer functions which describe the effect of line averaging and path separation on the response of a three-component sonic anemometer have been computed specifically for the Kaijo-Denki PAT 311. These computations differ from the previous work of Kaimal et al., by accounting for the two parallel paths which compose each measurement axis and for the vertical separation of the two horizontal axes. There is only a minor additional attenuation of response for the vertical wind component due to the measurement axis being composed of two paths rather than one. However, the differences between the horizontal transfer functions calculated here and those presented by Kaimal et al. are significant and could be important for correcting sonic spectra in the inertial subrange.

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Tammy M. Weckwerth, Thomas W. Horst, and James W. Wilson

Abstract

A comprehensive observational dataset encompassing the entire temporal evolution of horizontal convective rolls was obtained for the first time. Florida, Illinois, and Kansas measurements from preroll conditions through the development of well-defined rolls to their dissipation were utilized to determine the factors influencing roll evolution. When the buoyancy flux reached a critical value of 35–50 W m−2, the first form of boundary layer convection resolved by radar was rolls. It was noted that two-dimensional convective rolls can evolve in a convective boundary layer in the absence of significant wind speed and shear. In fact, the value of wind speed or shear in itself did not seem to determine when or if rolls would form, although it did influence roll evolution. Well-defined, two-dimensional rolls only occurred while −z i/L, where z i is the convective boundary layer depth and L is the Monin–Obukhov length, was less than ∼25, which is consistent with previous studies. As −z i/L increased throughout the day, either open cellular convection or unorganized boundary layer convection was the dominant clear-air convective mode. If the wind speed was low (mean boundary layer winds <3 m s−1 or 10-m winds <2 m s−1) during roll occurrences, rolls evolved into open cells. Alternatively, if the wind speed throughout the day was relatively high, rolls broke apart into random, unorganized convective elements. These are unprecedented observations of two-dimensional convection evolving into three-dimensional convection over land, which is analogous to laboratory convection where increased thermal forcing can produce a transition from two-dimensional to three-dimensional structures. Finally, the roll orientation was governed primarily by the mean convective boundary layer wind direction.

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Jielun Sun, Sean P. Burns, Anthony C. Delany, Steven P. Oncley, Thomas W. Horst, and Donald H. Lenschow

Abstract

A unique set of nocturnal longwave radiative and sensible heat flux divergences was obtained during the 1999 Cooperative Atmosphere–Surface Exchange Study (CASES-99). These divergences are based on upward and downward longwave radiation measurements at two levels and turbulent eddy correlation measurements at eight levels. In contrast to previous radiation divergence measurements obtained within 10 m above the ground, radiative flux divergence was measured within a deeper layer—between 2 and 48 m. Within the layer, the radiative flux divergence is, on average, comparable to or smaller than the sensible heat flux divergence. The horizontal and vertical temperature advection, derived as the residual in the heat balance using observed sensible heat and radiative fluxes, are found to be significant terms in the heat balance at night. The observations also indicate that the radiative flux divergence between 2 and 48 m was typically largest in the early evening. Its magnitude depends on how fast the ground cools and on how large the vertical temperature gradient is within the layer. A radiative flux difference of more than 10 W m−2 over 46 m of height was observed under weak-wind and clear-sky conditions after hot days. Wind speed variation can change not only the sensible heat transfer but also the surface longwave radiation because of variations of the area exposure of the warmer grass stems and soil surfaces versus the cooler grass blade tips, leading to fluctuations of the radiative flux divergence throughout the night.

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Neil P. Lareau, Erik Crosman, C. David Whiteman, John D. Horel, Sebastian W. Hoch, William O. J. Brown, and Thomas W. Horst

The Persistent Cold-Air Pool Study (PCAPS) was conducted in Utah's Salt Lake valley from 1 December 2010 to 7 February 2011. The field campaign's primary goal was to improve understanding of the physical processes governing the evolution of multiday cold-air pools (CAPs) that are common in mountain basins during the winter. Meteorological instrumentation deployed throughout the Salt Lake valley provided observations of the processes contributing to the formation, maintenance, and destruction of 10 persistent CAP episodes. The close proximity of PCAPS field sites to residences and the University of Utah campus allowed many undergraduate and graduate students to participate in the study.

Ongoing research, supported by the National Science Foundation, is using the PCAPS dataset to examine CAP evolution. Preliminary analyses reveal that variations in CAP thermodynamic structure are attributable to a multitude of physical processes affecting local static stability: for example, synoptic-scale processes impact changes in temperatures and cloudiness aloft while variations in boundary layer forcing modulate the lower levels of CAPs. During episodes of strong winds, complex interactions between the synoptic and mesoscale f lows, local thermodynamic structure, and terrain lead to both partial and complete removal of CAPs. In addition, the strength and duration of CAP events affect the local concentrations of pollutants such as PM2.5.

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Edgar L. Andreas, P. Ola G. Persson, Andrey A. Grachev, Rachel E. Jordan, Thomas W. Horst, Peter S. Guest, and Christopher W. Fairall

Abstract

The Surface Heat Budget of the Arctic Ocean (SHEBA) experiment produced 18 000 h of turbulence data from the atmospheric surface layer over sea ice while the ice camp drifted for a year in the Beaufort Gyre. Multiple sites instrumented during SHEBA suggest only two aerodynamic seasons over sea ice. In “winter” (October 1997 through 14 May 1998 and 15 September 1998 through the end of the SHEBA deployment in early October 1998), the ice was compact and snow covered, and the snow was dry enough to drift and blow. In “summer” (15 May through 14 September 1998 in this dataset), the snow melted, and melt ponds and leads appeared and covered as much as 40% of the surface with open water. This paper develops a bulk turbulent flux algorithm to explain the winter data. This algorithm predicts the surface fluxes of momentum, and sensible and latent heat from more readily measured or modeled quantities. A main result of the analysis is that the roughness length for wind speed z 0 does not depend on the friction velocity u * in the drifting snow regime (u * ≥ 0.30 m s−1) but, rather, is constant in the SHEBA dataset at about 2.3 × 10−4 m. Previous analyses that found z 0 to increase with u * during drifting snow may have suffered from fictitious correlation because u * also appears in z 0. The present analysis mitigates this fictitious correlation by plotting measured z 0 against the corresponding u * computed from the bulk flux algorithm. Such plots, created with data from six different SHEBA sites, show z 0 to be independent of the bulk u * for 0.15 < u * ≤ 0.65 m s−1. This study also evaluates the roughness lengths for temperature zT and humidity zQ, incorporates new profile stratification corrections for stable stratification, addresses the singularities that often occur in iterative flux algorithms in very light winds, and includes an extensive analysis of whether atmospheric stratification affects z 0, zT, and zQ.

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Metcrax 2006

Meteorological Experiments in Arizona's Meteor Crater

C. David Whiteman, Andreas Muschinski, Sharon Zhong, David Fritts, Sebastian W. Hoch, Maura Hahnenberger, Wenqing Yao, Vincent Hohreiter, Mario Behn, Yonghun Cheon, Craig B. Clements, Thomas W. Horst, William O. J. Brown, and Steven P. Oncley

The Meteor Crater Experiment (METCRAX 2006) was conducted in October 2006 at Arizona's Meteor Crater to investigate stable boundary layer evolution in a topographically uncomplicated basin surrounded by the nearly homogeneous plain of the Colorado Plateau. The two goals of the experiment were 1) to investigate the microscale and mesoscale structure and evolution of the stable boundary layer in the crater and its surroundings and 2) to determine whether atmospheric seiches or standing waves are produced inside the crater. This article provides an overview of the scientific goals of the experiment; summarizes the research measurements, the crater topography, and the synoptic meteorology of the study period; and presents initial analysis results. Analyses show that nighttime temperature inversions form frequently in the crater and that they are often perturbed by internal wave motions. Nighttime cooling produces a shallow (15–30 m deep) surface-based inversion that is surmounted by a horizontally homogeneous near-isothermal layer that extends all the way to the rim, where a second inversion extends above rim level. Seiches are sometimes present on the crater floor. The diurnal propagation of shadows from the crater rim produces important spatial differences in the surface radiation budget and thus the timing of the slope flow transition, and the crater atmosphere is often perturbed during nighttime by a southwesterly mesoscale drainage flow.

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Fei Chen, Kevin W. Manning, Margaret A. LeMone, Stanley B. Trier, Joseph G. Alfieri, Rita Roberts, Mukul Tewari, Dev Niyogi, Thomas W. Horst, Steven P. Oncley, Jeffrey B. Basara, and Peter D. Blanken

Abstract

This paper describes important characteristics of an uncoupled high-resolution land data assimilation system (HRLDAS) and presents a systematic evaluation of 18-month-long HRLDAS numerical experiments, conducted in two nested domains (with 12- and 4-km grid spacing) for the period from 1 January 2001 to 30 June 2002, in the context of the International H2O Project (IHOP_2002). HRLDAS was developed at the National Center for Atmospheric Research (NCAR) to initialize land-state variables of the coupled Weather Research and Forecasting (WRF)–land surface model (LSM) for high-resolution applications. Both uncoupled HRDLAS and coupled WRF are executed on the same grid, sharing the same LSM, land use, soil texture, terrain height, time-varying vegetation fields, and LSM parameters to ensure the same soil moisture climatological description between the two modeling systems so that HRLDAS soil state variables can be used to initialize WRF–LSM without conversion and interpolation. If HRLDAS is initialized with soil conditions previously spun up from other models, it requires roughly 8–10 months for HRLDAS to reach quasi equilibrium and is highly dependent on soil texture. However, the HRLDAS surface heat fluxes can reach quasi-equilibrium state within 3 months for most soil texture categories. Atmospheric forcing conditions used to drive HRLDAS were evaluated against Oklahoma Mesonet data, and the response of HRLDAS to typical errors in each atmospheric forcing variable was examined. HRLDAS-simulated finescale (4 km) soil moisture, temperature, and surface heat fluxes agreed well with the Oklahoma Mesonet and IHOP_2002 field data. One case study shows high correlation between HRLDAS evaporation and the low-level water vapor field derived from radar analysis.

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Daniel A. Rajewski, Eugene S. Takle, Julie K. Lundquist, Steven Oncley, John H. Prueger, Thomas W. Horst, Michael E. Rhodes, Richard Pfeiffer, Jerry L. Hatfield, Kristopher K. Spoth, and Russell K. Doorenbos

Perturbations of mean and turbulent wind characteristics by large wind turbines modify fluxes between the vegetated surface and the lower boundary layer. While simulations have suggested that wind farms could significantly change surface fluxes of heat, momentum, momentum, moisture, and CO2 over hundreds of square kilometers, little observational evidence exists to test these predictions. Quantifying the influences of the “turbine layer” is necessary to quantify how surface fluxes are modified and to better forecast energy production by a wind farm. Changes in fluxes are particularly important in regions of intensely managed agriculture where crop growth and yield are highly dependent on subtle changes in moisture, heat, and CO2. Furthermore, speculations abound about the possible mesoscale consequences of boundary layer changes that are produced by wind farms. To address the lack of observations to answer these questions, we developed the Crop Wind Energy Experiment (CWEX) as a multiagency, multiuniversity field program in central Iowa. Throughout the summer of 2010, surface fluxes were documented within a wind farm test site and a 2-week deployment of a vertically pointing lidar quantified wind profiles. In 2011, we expanded measurements at the site by deploying six flux stations and two wind-profiling lidars to document turbine wakes. The results provide valuable insights into the exchanges over a surface that has been modified by wind turbines and a basis for a more comprehensive measurement program planned for the summer in 2014.

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