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  • Author or Editor: P. A. Mandics x
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J. E. Gaynor
and
P. A. Mandics

Abstract

An acoustic sounding system placed on the NOAA Ship Oceanographer during GATE provided a unique meteorological data set. Examples of three distinct boundary-layer situations are discussed as they appear on the facsimile records of backscattered acoustic intensity: 1) ubiquitous plume echoes associated with undisturbed conditions, 2) cool-air outflows (or wakes) from either squall-line or isolated cumulonimbus activity associated with disturbed conditions and 3) “hat”- or “hummock” -shaped echoes associated with low-level cumulus clouds usually occurring during weakly disturbed conditions. Profiles of potential temperature and mixing ratio from radiosonde flights launched from the Occonograph are compared with the acoustic data. Convective plumes observed during GATE were less vigorous than those seen over land. Bulk aerodynamic fluxes of surface sensible and latent heat varied in time with the passage of thermal plumes. This indicates a minimum averaging time for valid flux estimates of about 30 min. Outflows resulted in order-of-magnitude increases in surface sensible heat flux and large increases in surface stress, but only relatively small increases in latent heat flux. The increased stability following the outflows lasted from a few hours up to 16 h, depending on the intensity of the disturbance. The rate of dissipation of turbulent kinetic energy is calculated in the upper mixed layer for the three cases using an acoustic Doppler-differencing technique. These values are intercompared and compared with those from other studies. Evidence is presented indicating that hummocky echoes were associated with low-level clouds. Plumes underneath the hummocks were characterized by larger moisture content and surface beat fluxes when compared with plumes without hummocks.

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P. A. Mandics
and
E. J. Owens

Abstract

A monostatic acoustic echo sounder with a vertically-pointed antenna was installed aboard the NOAA research vessel Oceanographer and was tested during a recent cruise in the Pacific and Caribbean Oceans. Acoustic returns produced by turbulence-induced temperature fluctuations were received from up to 300 m in height. The data revealed the existence of a rich variety in the structure of the marine atmosphere. Thermal convective plumes were usually observed when the sea water temperature exceeded the air temperature by as little as 1°C. Under more stable conditions the echo sounder detected the presence of layered structures associated with temperature inversions that were often perturbed by gravity waves or wind shear. Doppler frequency shift of the returned echo signals was used to estimate the vertical velocity of atmospheric scatterers.

The results of the tests indicate that it is possible to probe remotely the lower layers of the marine atmosphere from a moving ship using acoustic echo sounding techniques. Wind-generated noise, ambient acoustic noise, and structure-borne vibrations proved to be the major limitations on the performance of the echo sounder.

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P. A. Mandics
and
F. F. Hall Jr.

An acoustic echo sounder mounted on the NOAA ship Oceanographer during GATE proved to be a valuable tool for investigating the structure and dynamics of the tropical marine boundary layer up to 800 m in height. Under suppressed weather conditions the facsimile-recorded echo intensity returns depicted a mixed layer characterized by convective plumes rising from the surface of the water to 400 m. Disturbed weather events resulted in a substantial modification of the boundary layer; layered structures formed that at times limited the depth of the mixed layer to 100 m. The Doppler frequency shift of the acoustic returns made it possible to determine the vertical velocity field.

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R. M. Hardesty
,
P. A. Mandics
,
D. W. Beran
, and
R. G. Strauch

Wind shear has been recognized as a major aviation hazard in the airport environment. A dual, acoustic Doppler–microwave Doppler radar system has been installed at Dulles International Airport near Washington, D.C., to measure the vertical profile of wind from the surface to 510 m in 30 m height increments. The acoustic system gathers data under clear-air conditions, and the microwave radar takes over automatically when precipitation is present. System performance is being assessed by comparing its output with National Weather Service radiosondes and with balloon-borne anemometers and by intercomparing the acoustic- and microwave-measured winds under light precipitation conditions. The dual-sensor system has been operating for several months, registering the passage of fronts, some with potentially hazardous wind shears.

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