Alfred J. Bedard

A Climatology of Gravity Waves and Other Coherent Disturbances at the Boulder Atmospheric Observatory during March–April 1984

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...

Low-Frequency Atmospheric Acoustic Energy Associated with Vortices Produced by Thunderstorms

An infrasonic observatory collocated with the Colorado State University CHILL radar during the summer of 1995 permitted unique comparisons between severe storm kinematics and detected acoustic energy at subaudible frequencies near 1 Hz. Radar observations of a velocity couplet aloft (evolving into a tornado) showed a circulation maximum descending for about 30 min while moving to the east. The detected infrasound followed the trend of these observations. A model of sound radiated from vortex systems predicts frequencies in the range observed. These data are interpreted in the context of past infrasonic observations. An ongoing study comparing regional tornado and funnel sightings with archived infrasonic data has identified over 100 cases to date where the infrasonic signals occurred at the time of, and from the direction of, the vortices. For some of these cases, the distances were greater than 100 km. The author and his associates continue to collect datasets to permit further evaluation of infrasonic detection methods.

Details of Colliding Thunderstorm Outflows as Observed by Doppler Lidar

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.

Highly directional sound projector and receiver apparatus

An acoustic apparatus (10) adapted for use as either a sound projector or aound receiver. The apparatus (10) comprises a bundle (20) of individual sound tubes (21, 22) disposed in a tube array unit (14) which rests on a support unit (13) operatively engaged with an acoustic unit (11). Only the ends of the individual sound tubes (21, 22) are provided with a plurality of spaced discrete sound ports (25).

Atmospheric gravity waves and aircraft turbulence encounters

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.

Infrasound Originating Near Mountainous Regions in Colorado

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 Longs 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 Boulders 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-laps...

The 700–500 mb Lapse Rate as an Index of Microburst Probability: An Application for Thermodynamic Profilers

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 dr...

Sources and Detection of Atmospheric Wind Shear

This paper outlines a range of phenomena producing significant atmospheric wind shears, providing more details on the shears related to thunderstorm gust fronts. A case study from a wintertime Colorado front-range experiment during 1980 documents strong wind gradients that can occur in the lees of mountains. Strengths and weaknesses of some detection methods are discussed, and evaluations of both remote-sensor and in situ sensor approaches for detecting atmospheric wind shear are emphasized.

Ground-Based Microwave Radiometric Observations of the Temporal Vanation of Atmospheric Geopotential Height and Thickness

Since 1981, the Wave Propagation Laboratory of NOAA has operated a ground-based zenith-viewing microwave radiometer. This radiometer, designed to measure precipitable water vapor, cloud liquid, and temperature profiles, has two moisture-sensing channels and four temperature-sounding channels. Data from this system, taken at Denver, Colorado, are used to derive geopotential heights and thicknesses from the surface (about 830 mbar) to 300 mbar. Time series and spectra of several directly measured and inferred quantities are analyzed for different meteorological situations: a period of unusual calm in surface pressure, a frontal passage, and a gravity wave event. The three cases presented illustrate how rapid variations in meteorological variables can be studied using ground-based radiometers. These radiometers provide temporal continuity not hitherto available. The performance of the radiometer, both in observing a blackbody target and during an unusually calm pressure event, shows high sensitivity to changes in geopotential height and thickness and to integrated water vapor. Consequently, the combination of high temporal resolution and high sensitivity allows unique monitoring of rapidly changing conditions, such as frontal passages and gravity wave events. Comparisons of these data with various sources of ground truth, including radiosondes, satellite cloud observations, and arrays of microbarographs, show excellent agreement.

A review of the evidence for strong, small-scale vortical flows during downslope windstorms

Abstract During downslope windstroms, damage can occur on quite small scales. In the foothills near Boulder, Colorado, individual trees or lines of trees may be uprooted or snapped off with little or no damage to surrounding vegetation. Also, considerable damage can occur to individual structures with no primary damage occurring nearby. Often debris will be scattered in directions different from that of the mean flow. This paper reviews evidence for the existence of vortical motions and other small-scale features associated with downslope windstorms. Possible mechanisms for producing such organized regions of small-scale vorticity are suggested.