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David C. Powell and C. E. Elderkin


The application of Taylor's hypothesis to atmospheric boundary layer turbulence at heights of 15, 30 and 58 m has been investigated by correlation analysis and phase-spectral analysis of data taken from lines of meteorological towers when the mean wind direction was essentially along the tower line. The time lag of maximum cross correlation between parallel components measured at different alongwind locations indicates that the eddy structure travels slightly faster than the mean value of the longitudinal wind at the same height. The autocorrelation functions in space and in time for the longitudinal and vertical components show good agreement, as predicted by Talyor's hypothesis, for space lags up to 252 m. The agreement for the lateral component is inferior for lags ≥32 m.

According to Lin, Taylor's hypothesis holds in boundary layer shear flow for those wavenumbers κ such that κŪ/dz. Phase spectral analysis of parallel wind component data at different alongwind locations shows Taylor's hypothesis to hold for κŪ≥4/dz for near-neutral turbulence.

Preliminary coherency analysis of the same data indicates that the coherency (not squared) for parallel components falls to a value of less than e −1 for wavelengths less than three-halves the separation distance between measuring points.

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S-K. Kao and David C. Powell


The characteristics of the large-scale relative particle displacement tensor, the correlation functions, and spectra of the relative particle velocities at 10-, 30-, 50- and 100-mb levels are investigated; pertinent results concerning relative turbulence and diffusion at various levels in both troposphere and stratosphere are discussed and summarized. It is found that a quasi-stationary process exists in the large-scale turbulence diffusion in both the troposphere and stratosphere, the rate of relative particle dispersion being greatest in the tropopause level and generally proportional to the variance of the relative velocity. In general, the auto-correlation functions for the relative zonal velocities in both the troposphere and stratosphere behave like an exponentially decreasing function, whereas those for the relative meridional velocities shows a combination of an exponential function and a cosine function with a damping amplitude. The power spectra of the relative zonal velocities at all levels show the similar characteristics of increasing kinetic energy with decreasing frequency, whereas those of the relative meridional velocities show an energy peak near the frequency of 10−2 cycles hr−1. The high frequency portion of the power spectra of both the zonal and meridional components of the relative velocities at all levels is found to be proportional to the minus third power of the frequency. The principal axis of the large-scale turbulent diffusion in the stratosphere is generally oriented ENE-WSW, whereas in the troposphere it is ESE-WNW.

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Ángel F. Adames, Scott W. Powell, Fiaz Ahmed, Víctor C. Mayta, and J. David Neelin


Observations have shown that tropical convection is influenced by fluctuations in temperature and moisture in the lower free troposphere (LFT; 600–850 hPa), as well as moist enthalpy (ME) fluctuations beneath the 850 hPa level, referred to as the deep boundary layer (DBL; 850–1000 hPa). A framework is developed that consolidates these three quantities within the context of the buoyancy of an entraining plume. A “plume buoyancy equation” is derived based on a relaxed version of the weak temperature gradient (WTG) approximation. Analysis of this equation using quantities derived from the Dynamics of the Madden–Julian Oscillation (DYNAMO) sounding array data reveals that processes occurring within the DBL and the LFT contribute nearly equally to the evolution of plume buoyancy, indicating that processes that occur in both layers are critical to the evolution of tropical convection. Adiabatic motions play an important role in the evolution of buoyancy both at the daily and longer time scales and are comparable in magnitude to horizontal moisture advection and vertical moist static energy advection by convection. The plume buoyancy equation may explain convective coupling at short time scales in both temperature and moisture fluctuations and can be used to complement the commonly used moist static energy budget, which emphasizes the slower evolution of the convective envelope in tropical motion systems.

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