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Peter F. Lester

Abstract

In order to study the physical aspects of intermittent turbulence in the free atmosphere, it is suggested that terms of an “instantaneous” turbulent energy budget be computed from data collected by aircraft. This can be accomplished through the application of numerical filters to specify mean and turbulence parameters as a function of distance along the aircraft track.

The proposed analysis is illustrated with data from a highly intermittent turbulence record derived from a flight through a rotor. Results are compatible with the known characteristics of the rotor, thus suggesting the feasibility of the technique. Some difficulties are encountered in the satisfaction of the Reynolds averaging rules because of the requirement for a gap in the velocity spectrum and the existence of a finite cutoff interval for a practical numerical filter.

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Peter F. Lester

During his tenure in the Meteorology Department at San Jose State University (1961–1978), Professor Albert Miller conducted extensive field investigations of the marine inversion over the San Francisco Bay Area. Measurements were made with instrumented light aircraft, radar-tracked tetroons and slow-rise balloons carrying modified radiosondes. Later, Mount Sutro Tower in San Francisco was utilized for microscale studies of the inversion. Miller's field work culminated with an extensive measurement program over the coastal waters of central California. Results of his work included 1) a detailed description of the spatial and temporal variations of the structure of the marine inversion over the Bay Area, 2) the documentation of the characteristics of gravity waves and turbulence near the base of the inversion, 3) a variety of evidence that strongly supports significant cross-inversion mass flux, and 4) a hypothesis that explains the latter as the result of small-amplitude gravity waves in a tilted, strongly sheared inversion layer.

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Peter F. Lester
and
William A. Fingerhut

Abstract

The characteristics of the lower turbulent zone (LTZ) which is associated with mountain lee waves have been investigated through the analysis of aircraft observations made near Boulder, Colo. Numerical filters and statistical analysis techniques have been applied to the data from six cases to yield vertical sections of potential temperature, horizontal wind, and turbulence intensity along the aircraft paths. One case study is presented to illustrate the structure of the LTZ and its changes during a frontal passage. In addition, the main features of all the analyses are summarized in the form of a schematic vertical section.

The horizontal dimension of the LTZ varied between 25 and more than 65 km downstream of the first lee wave trough. The vertical dimensions ranged from a few hundred meters AGL at the lee wave troughs to 3 km AGL at the wave crests. Turbulence levels were light, moderate or severe over more than 90% of the total distance flown in the LTZ (nearly 1100 km). Severe turbulence was commonly encountered near the upstream side of the rotor, where the largest horizontal and vertical wind and temperature gradients were also found.

The kinetic energy dissipation rate for a large, long-lived LTZ, such as occurs under hydraulic jump conditions, was estimated to be 20–100 W m−2; thus, the LTZ may play an important role in the large-scale energetics of the atmosphere.

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K. C. Pantley
and
Peter F. Lester

Abstract

Data derived from the flight tapes of two airliners that experienced severe turbulence near thunderstorm tops are used to produce quantitative descriptions of the turbulence and its environment. The likely turbulence-producing processes include a three-dimensional turbulent wake in the lee of a squall line and an updraft in the top of a thunderstorm. Results suggest that current procedures for using surface and airborne weather radar for routing aircraft near thunderstorm tops should be reexamined. Also, although useful rules for safe flight near thunderstorm tops already exist, there is evidence that they are not universally applied.

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D. K. Lilly
and
Peter F. Lester

Abstract

Detailed stratospheric wind and temperature data were gathered by aircraft over the mountains of southern Colorado on 1 March 1970. In a unique operation, two instrumented RB-57F aircraft flew a total of twelve upwind and downwind legs at altitudes of 13 to 20km,.with an average separation of 600 m.

The observational data show that the stratosphere was disturbed by sporadically-turbulent gravity waves of length 20–30 km, which were apparently generated directly or indirectly by the mountainous terrain. Wave amplitudes and turbulence frequency showed a general increase with height up to about 17 km, where they reached a maximum. The maximum amplitude layer was also characterized by a minimum in the mean wind speed.

The horizontal and vertical velocity and temperature variances and covariances were evaluated and found to be generally consistent with predictions of linear gravity wave theory. The spectra of horizontal kinetic and potential energy were also calculated and appear to follow a −3 law over about a decade in wavenumber space. The covariance of the horizontal and vertical velocity perturbations is everywhere negative, with a mean downward momentum flux of about ∼2 dyn cm−2. The correlation spectra show that most of this momentum flux is contributed by the long waves.

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Alison F. C. Bridger
,
William C. Brick
, and
Peter F. Lester

Abstract

Data collected on board an instrumented aircraft during the Marine Atmospheric Boundary Layer Experiments, West Coast (MABLES WC) field study are examined. The data were collected at several levels in the marine and inversion layers approximately 200 km west of the central California coast during August 1978. Composite sounding and cross sections of wind and temperature variables are presented to provide an overview of the mesoscale structure of the offshore marine and inversion layers at the time of MABLES WC.

Under typical summertime synoptic conditions, which prevailed early in the study period (three flights examined), a strong inversion is found. Base heights and temperature increases across the inversion base are comparable to values observed in the San Francisco Bay area. The topography of the inversion base shows a slope downward toward the east during all three of these flights and also suggests the presence of convective activity, gravity-wave activity, or both, during two of the three flights. The inversion weakened later in the study period when atypical synoptic conditions prevailed.

During typical conditions, wind-speed minima were found near the base of the inversion, and jets with speeds as high as 17–18 m−1 were found within the inversion layer. These observations of the vertical wind profile agree with those made in the San Francisco Bay area. Further, in two of the three cases, a well-organized horizontal structure of the jet was apparent. Data gathered during a flight later in the study period (“atypical” synoptic conditions) are also presented and show a particularly good example of the jet in the offshore region.

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Peter F. Lester
,
Orhan Sen
, and
R. E. Bach Jr.

Abstract

Digital flight data recorder (DFDR) tapes from commercial aircraft can provide useful information about the mesoscale environment of severe turbulence incidents. Air motion computations from these data and their errors are briefly described. An example of mesoscale meteorological information available from DFDR tapes is presented for a case of turbulence in mountain waves over the Greenland icecap.

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