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A. J. Bedard Jr.

Abstract

An array of infrasonic microphones, installed near Fraser, CO, operated from February through June 1973. This observatory, in conjunction with a similar, permanent installation at Boulder, triangulated on sources of infrasound in Colorado. We found that the lee of a group of mountains that includes Long's Peak is a preferred source region for infrasound. Heretofore, such triangulations used long propagation paths usually of hundreds of kilometers, which resulted both in poorly defined source regions and signal source characteristics. The generation of local infrasound is probable when the 500 mb winds are high. A comparison with Boulder's historical data shows that most signals occur during the winter months with durations longer than 3 h. One case study presented shows that source motion can occur (20 km over an 8 h period). The identification of a local, accessible infrasonic source region offers wide opportunities for further research (e.g., the study of an active source region using time-lapse photography, dual-Doppler radar, lidar and instrumented aircraft). Comparisons of infrasonic data with measurements of turbulence reported by aircraft suggest a temporal and spatial relation between sources of infrasound and aircraft turbulence encounters.

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A. J. Bedard Jr. and H. B. Meade

Abstract

We outline the known properties of atmospheric pressure jumps, including rise-time and pressure-amplitude statistics, and we indicate how these statistics guide the choice of pressure-jump detector components.We review design considerations and test procedures and discuss the practical constraints of inside and outside installations of such detectors. Our tentative conclusion is that a pressure switch with a threshold of 0.5mb, used with a high-pass filter with about a 3 min time constant, can detect sudden pressure increases reliably. The final choice of components and the evaluation of the sensor for thunderstorm gust-front detectionwill depend upon the accumulation of operating experience under well-defined meteorological conditions.

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A. J. Bedard Jr. and C. Ramzy

Abstract

The TOTO (Totable Tornado Observatory) device was designed as a prototype meteorological measuring instrument for use by severe storm intercept teams. Portions of the instrument evolved from our work in producing “hardened” sensors for use in networks for measuring wind shear and severe downslope winds. We describe our rationale for choices of measurement parameters, dynamic ranges and measurement resolution. TOTO measures the following: wind speed (two ranges) 0–100 mph (44.7 m s−1), 0–250 mph (111.8 m s−1); wind direction (0–360°); temperature (0–50°C); static pressure (±20 mb); and corona discharge (±1 μA). We also describe methods for rapid deployment from the back of a pickup truck. Using ramps and a winch system, the unit can be deployed in less than 15 s. Finally, we discuss plans for testing and possibilities for improving the device.

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F. Einaudi, A. J. Bedard Jr., and J. J. Finnigan

Abstract

We present a climatological study of gravity waves and other coherent disturbances at the Boulder Atmospheric Observatory, during the period mid-March-mid-April 1984. The data were collected by a network of microbarographs, and by sensors on the 300 m tower. The total observational period was divided into 522 time segments of 5120 s each. Coherent and incoherent motions were identified on the basis of a cross-correlation coefficient, calculated from the microbarograph network for each time segment and frequency band analyzed, using the assumption that the atmospheric state can be described by an equivalent plane wave. Five passbands were considered in the period range 1–20 min.

The analysis indicates that the atmospheric state at these passbands displays highly coherent structure, most of the time. During the interval from 0800 to 1800 LST, coherent motions with cross-correlation coefficient larger than 0.5 are present about 25% of the time for periods between 1 and 5 min and more than 80% of the time for periods between 10 and 20 min. In the remaining hours of the day, the percentages rise to more than 40% and 95% of the time, respectively.

A relationship is illustrated between the turbulent kinetic energy measured on the tower and the amplitude of the rms pressure field at the ground for disturbances having up to 5 min periods. For longer periods, such a relationship appears to be absent, indicating that the longer the scales, the deeper the atmospheric zone important to the dynamics of the pressure fluctuations.

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S. J. Caplan, A. J. Bedard Jr., and M. T. Decker

Abstract

This study validates the predicted association between frequency of dry microburst occurrence and large temperature lapse rate. In applying lapse rate trend data and high time resolution data from remote sensors, we first compared lapse rates from the Denver rawinsonde with the thermodynamic profiler and obtained linear correlation coefficients ranging from .63 to .94. Continuous 20-minute radiometer samples of lapse rate were available throughout the experiment period. The data indicate a critical value of 700–500 mb lapse rate ≥8°C km−1 for dry microburst occurrence. Also, we found dry microburst occurrence in the Denver area better correlated with late afternoon lapse rates than with early morning lapse rates: 67% of dry microbursts occurred with 1200 UTC lapse rates ≥8°C km−1, while 89% of dry microbursts occurred with 2200 UTC lapse rates ≥8°C km−1. We recommend that remote sensor temperature retrievals such as with Radio Acoustic Sounding Systems (RASS) extend to at least 3 km AGL to aid dry microburst nowcasting and forecast verification.

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J. M. Intrieri, A. J. Bedard Jr., and R. M. Hardesty

Abstract

Three cases of colliding outflow boundaries are examined using data collected from the NOAA Doppler lidar and a meteorological tower during the summer of 1986 near Boulder, Colorado. The data are unique because the lidar and the 300 m tower were colocated, providing measurements of both kinematic and thermodynamic properties. Lidar data reveal small-scale vortex roll instabilities within the leading edge of the outflow. Observations of the post-collision interactions showed that the warmer of the two outflows was deflected upward by the colder outflow to heights of 2 km. In all cases, this forced mechanical lifting was sufficient to produce convection. A simple model of two colliding density currents also suggests that deeper outflows are more efficient in initiating convection.

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A. J. Bedard Jr., F. Canavero, and F. Einaudi

Abstract

We describe aircraft turbulence-atmospheric gravity wave events which occurred during a 2-day period over the Continental Divide. The waves are observed by two microbarograph networks an each side of the divide and last for several hours at a time. We show them to be unstable modes of the jet stream, corresponding to propagating internal gravity waves. We also show that the position of aircraft-reported turbulence coincides with the critical levels of the waves.

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A. J . Bedard Jr., W. H. Hooke, and D. W. Beran

Wind shear has long been recognized as one of the major aviation hazards in the airport environment. A principal source of dangerous wind shear is the thunderstorm gust front, a cold air outflow from the thunderstorm downdraft. The gust front is particularly hazardous not only because of the large surface wind shears associated with it, but also because of its highly localized character. Often the downdraft or downburst region producing such fronts is of the order of a few kilometers or less in dimension.

As a result, vertically profiling wind shear detection techniques such as the hybrid acoustic–microwave radar system described in the companion paper by Hardesty et al 1977 do not provide adequate total protection. In this paper we describe an array of pressure sensors installed at Dulles International Airport in Washington, D.C., to detect and monitor gust fronts that could endanger aircraft operations. The pressure sensors (designed to respond only to sudden pressure increase) are so inexpensive that they can be used in dense networks of large spatial extent to monitor in detail the gust front progress as it approaches the airport. Some 125 sensors have been installed at Dulles. Indeed, a major cost of the installation is the cost of the phone lines required to return the information to a central data-processing location.

The system has been operating unattended for extended periods of time, registering the frontal passages that have also been detected by the acoustic–microwave radar system as they pass overhead. Results are presented showing the complementary nature of the two monitoring methods.

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David A. Schecter, Melville E. Nicholls, John Persing, Alfred J. Bedard Jr., and Roger A. Pielke Sr.

Abstract

This paper addresses the physics and numerical simulation of the adiabatic generation of infrasound by tornadoes. Classical analytical results regarding the production of infrasound by vortex Rossby waves and by corotating “suction vortices” are reviewed. Conditions are derived for which critical layers damp vortex Rossby waves that would otherwise grow and continually produce acoustic radiation. These conditions are similar to those that theoretically suppress gravity wave radiation from larger mesoscale cyclones, such as hurricanes. To gain perspective, the Regional Atmospheric Modeling System (RAMS) is used to simulate the infrasound that radiates from a single-cell thunderstorm in a shear-free environment. In this simulation, the dominant infrasound in the 0.1–10-Hz frequency band appears to radiate from the vicinity of the melting level, where diabatic processes involving hail are active. It is shown that the 3D Rossby waves of a tornado-like vortex (simulated with RAMS) can generate stronger infrasound if the maximum wind speed of the vortex exceeds a modest threshold. Technical issues regarding the numerical simulation of tornado infrasound are also addressed. Most importantly, it is shown that simulating tornado infrasound likely requires a spatial resolution that is an order of magnitude finer than the current practical limit (10-m grid spacing) for modeling thunderstorms.

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