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Suhas U. Pol and Michael J. Brown

Abstract

During the Joint Urban 2003 experiment held in Oklahoma City, Oklahoma, an east–west-running street canyon was heavily instrumented with wind sensors. In this paper, the flow patterns at the street canyon ends are investigated by looking at sonic anemometers placed near ground level and tethersonde wind vane systems operated in “ladder” mode that were suspended over the sides of the buildings on each side of the street. For southerly flow conditions, the street-level wind sensors often showed what appeared to be a horizontally rotating “corner” or “end” vortex existing at each end of the street canyon near the intersections. It was found that this vortex flow pattern appeared for a wide range of upper-level wind directions but then changed to purely unidirectional flow for wind directions that were outside this range. The tethersonde wind vane measurements show that this vortexlike flow regime occasionally existed through the entire depth of the street canyon. The horizontal extent of the end vortex into the street canyon was found to be different at each end of the street. Under high-wind conditions, the mean wind patterns in the street did not vary appreciably during the day and night. The end vortex may be important in the dispersal of airborne contaminants, acting to enhance lateral and vertical mixing.

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Michael G. Brown and Kevin B. Smith

Abstract

When particle trajectories in a fluid are chaotic, the fluid is stirred efficiently, which, in turn, enhances diffusive mixing within the fluid. Regular (nonchaotic) particle trajectories on the other hand, are not associated with efficient stirring. The question of whether or not SOFAR float trajectories are chaotic is, then, central to understanding the process by which passive tracers are laterally mixed in the ocean. Two techniques commonly used to investigate the behavior of computer generated trajectories, the construction of Poincaré sections and the calculation of Lyapunov exponents, cannot be applied to analyze isolated float trajectories in an unknown velocity field. Power spectra of both meridional and zonal components of float trajectories contain structure on all resolvable scales, suggesting chaotic motion. Attempts to estimate K2, a lower bound on the Kolmogorov entropy, using the Grassberger-Procaccia algorithm failed when individual trajectories were used as input. Using a multiple trajectory embedding technique, however, K2. was estimated using the same algorithm to be approximately (140 day)−1. This also suggests that the motion is chaotic. While our analysis does not unambiguously identify SOFAR float trajectories as being chaotic, it provides no evidence that the trajectories are not chaotic.

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T. Michael Duncan and Robert C. Brown

A new data acquisition system designed for use in meteorological research on small aircraft is described. Compact in size, weight, and power, the system uses a 16-bit microcomputer and 57 000 bytes of memory and provides improved real-time computations, flexibility, and ease of operation and maintenance for machines of this class. Modular hardware and extensive use of a high-level language make adapting the system to other aircraft or nonaircraft environments straight-forward.

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Gilbert S. Raynor, Paul Michael, Robert M. Brown, and S. Sethuraman

Abstract

A research program is in progress at Brookhaven National Laboratory to determine the nature of atmospheric diffusion from a representative oceanic site, to relate observed diffusion patterns to meteorological and oceanographic variables, and to develop models to describe such diffusion. The program was initiated in response to plans for construction of offshore nuclear power plants.

Tracer experiments are conducted utilizing oil-fog smoke released from a boat stationed from 1–3 mi off-shore during onshore flows. The smoke is photographed from above and from the side to document lateral and vertical spread. The crosswind concentration distribution is measured by vehicle- and boat-mounted densitometers during successive traverses across the plume. Wind, turbulence and temperature at several levels are measured on the beach by tower-mounted instruments. Temperature profiles at greater heights are measured by kytoon- and aircraft-borne sensors. Water temperatures are also measured. Winds aloft are determined by pibal ascents and turbulence at various altitudes is sampled by an aircraft-mounted variometer.

Preliminary results show that diffusion is governed primarily by water and air temperature differences. With colder water, low-level air is very stable and diffusion minimal but water warmer than the air induces vigorous diffusion. Measurements of plume width and height have been obtained which are smaller and of normalized concentration which are larger than those predicted for the Pasquill F category. Measured values of plume width can be predicted from Eulerian measurements at the beach.

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Irina I. Rypina, Michael G. Brown, and Huseyin Koçak

Abstract

Motivated by observations of surface drifters in the Adriatic Sea, transport in a three-gyre system is studied with the aid of dynamical systems techniques. Particular attention is paid to the issue of intergyre transport. The velocity field is assumed to be two-dimensional and incompressible and composed of a steady three-gyre background flow on which a time-dependent perturbation is superimposed. Two systems of this type are considered: 1) an observationally motivated, analytically prescribed model consisting of a steady background on which a multiperiodic time-dependent perturbation is superimposed, and 2) an observationally based model of the Adriatic Sea consisting of the mean surface circulation derived from surface drifter trajectories on which a time-dependent altimetry-based perturbation velocity field is superimposed. It is shown that for a small perturbation to the steady three-gyre background, two of the gyres exchange no fluid with the third gyre. When the perturbation strength exceeds a certain threshold, transport between all three gyres occurs. This behavior is described theoretically, illustrated using the analytic model and shown to be consistent with the observationally based model of the Adriatic. The relevance of the work presented to more complicated multiple-gyre problems is discussed.

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Michael J. Naughton, Gerald L. Browning, and William Bourke

Abstract

The convergence of spectral model numerical solutions of the global shallow-water equations is examined as a function of the time step and the spectral truncation. The contributions to the errors due to the spatial and temporal discretizations are separately identified and compared. Numerical convergence experiments are performed with the inviscid equations from smooth (Rossby-Haurwitz wave) and observed (R45 atmospheric analysis) initial conditions, and also with the diffusive shallow-water equations. Results are compared with the forced inviscid shallow-water equations case studied by Browning et at. Reduction of the time discretization error by the removal of fast waves from the solution using initialization is shown. The effects of forcing and diffusion on the convergence are discussed. Time truncation errors are found to dominate when a feature is large scale and well resolved; spatial truncation errors dominate-for small-scale features and also for large scales after the small scales have affected them. Possible implications of these results for global atmospheric modeling are discussed.

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Michael J. Brown, S. Pal Arya, and William H. Snyder

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The vertical diffusion of a passive tracer released from surface and elevated sources in a neutrally stratified boundary layer has been studied by comparing field and laboratory experiments with a non-Gaussian K-theory model that assumes power-law profiles for the mean velocity and vertical eddy diffusivity. Several important differences between model predictions and experimental data were discovered: 1) the model overestimated ground-level concentrations from surface and elevated releases at distances beyond the peak concentration; 2) the model overpredicted vertical mixing near elevated sources, especially in the upward direction; 3) the model-predicted exponent α in the exponential vertical concentration profile for a surface release [C̄(z) exp(−z α)] was smaller than the experimentally measured exponent. Model closure assumptions and experimental shortcomings are discussed in relation to their probable effect on model predictions and experimental measurements.

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Craig D. Croskery, Kathleen Sherman-Morris, and Michael E. Brown

Abstract

The coronavirus disease 2019 (COVID-19) pandemic resulted in unprecedented challenges that dramatically affected the way of life in the United States and globally in 2020. The pandemic also made the process of protecting individuals from tornadoes more challenging, especially when their personal residence lacks suitable shelter, and particularly for residents of mobile homes. The necessity of having to shelter with other families—either in a public shelter or at another residence—to protect themselves from a tornado threat conflicted with the advice of public health officials who recommended avoiding public places and limiting contact with the public to minimize the spread of COVID-19. There was also a perception that protecting against one threat could amplify the other threat. A survey was undertaken with the public to determine the general viewpoint to see if that was indeed the case. The results found that it was possible to attenuate both threats provided that careful planning and actions were undertaken. Understanding how emergency managers should react and plan for such dual threats is important to minimize the spread of COVID-19 while also maintaining the safety of the public. Because there was no precedence for tornado protection scenarios amid a pandemic, both short-term and long-term recommendations were suggested that may also be useful in future pandemic situations.

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Barrett F. Gutter, Kathleen Sherman-Morris, and Michael E. Brown

Abstract

A great deal of research has been conducted regarding tornado warnings and protective actions taken, including some studies in which respondents were presented with hypothetical tornado warning scenarios. Much less research has been conducted in which respondents were presented with tornado watch scenarios, even though they cover a larger area and longer time period, thus potentially disrupting a far greater number of people. To address this lack of research, surveys were used to determine the influence of severe weather watches on planned Saturday afternoon and evening activities away from the immediate vicinity of the respondent’s home. Respondents were presented a hypothetical watch scenario, in which they had some activity planned for later that afternoon or evening. Each respondent rated his or her likelihood to continue an activity depending on the severity of the watch and the length of the activity. Respondents were provided information about each hypothetical watch including duration and primary threats. Responses from the survey indicated that as the severity of the watch or the length of the activity increased, the likelihood of the respondent continuing the activity decreased. For a severe thunderstorm watch, a tornado watch, and a particularly dangerous situation (PDS) tornado watch, 36.1%, 51.2%, and 80.2% of the respondents, respectively, would not continue an activity lasting 30 min or longer.

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Benjamin C. Trabing, Michael M. Bell, and Bonnie R. Brown

Abstract

Potential intensity theory predicts that the upper-tropospheric temperature acts as an important constraint on tropical cyclone (TC) intensity. The physical mechanisms through which the upper troposphere impacts TC intensity and structure have not been fully explored, however, due in part to limited observations and the complex interactions between clouds, radiation, and TC dynamics. In this study, idealized Weather Research and Forecasting Model ensembles initialized with a combination of three different tropopause temperatures and with no radiation, longwave radiation only, and full diurnal radiation are used to examine the physical mechanisms in the TC–upper-tropospheric temperature relationship on weather time scales. Simulated TC intensity and structure are strongly sensitive to colder tropopause temperatures using only longwave radiation, but are less sensitive using full radiation and no radiation. Colder tropopause temperatures result in deeper convection and increased ice mass aloft in all cases, but are more intense only when radiation was included. Deeper convection leads to increased local longwave cooling rates but reduced top-of-the-atmosphere outgoing longwave radiation, such that the total radiative heat sink is reduced from a Carnot engine perspective in stronger storms. We hypothesize that a balanced response in the secondary circulation described by the Eliassen equation arises from upper-troposphere radiative cooling anomalies that lead to stronger tangential winds. The results of this study further suggest that radiation and cloud–radiative feedbacks have important impacts on weather time scales.

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