Analysis of the Tropical Marine Boundary Layer during GATE Using Acoustic Sounder Data

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  • 1 Wave Propagation Laboratory, Environmental Research Laboratories, NOAA, Boulder, Colo. 80302
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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.

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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