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Shu-Hsien Chou and Eueng-Nan Yeh

Abstract

Airborne measurements of atmospheric turbulence spectra and cospectra made at the 50 m level above the western Atlantic Ocean during cold air outbreaks have been studied. The data cover nearshore areas of cloud streets or roll vortices. In the inertial submnge, the results are in good agreement with previous measurements made over both land and sea, except for those of temperature.

The high-frequency behavior is consistent with local isotropy. In the inertial subrange, a near 4/3 ratio is observed between velocity spectra normal to and those along the aircrat~ heading. Also, the normalized spectra and cospectra of vertical velocity, humidity, and temperature are independent of sampling directions. The normalized cospectrum of the humidity flux appears to have a -4/3 cospectral slope, while the normalized cospectrum of stress appears to have -4/3 and -5/3 cospectral slopes in the alongwind and crosswind samples, respectively.

The shapes of the spectra and cospectra vary with sampling direction. Except for crosswind velocity, the normalized spectra and cospectra appear to have more energy in the crosswind samples at the dimensionless frequency (f) ~ 0.2 and more in the alongwind samples at f< 0. I. This is mainly due to the stretching action ofthe mean wind shear on convective elements and is in good agreement with previous aircraft measurements.

For the alongwind samples, the normalized velocity spectra for f< 0.1 appear not to be in good agreement with the spectral models derived from the cloud-free, highly convective Minnesota data. According to the latter, the normalized horizontal velocity spectra are nearly equal for low frequencies, while our results show that the low-frequency convection is significantly suppressed in the alongwind direction by the vertical wind shear.

The three dissipation estimates, derived from the high-frequency part of the velocity spectra, appear to be in good agreement with a 9% mean discrepancy. The normalized dissipation is systematically smaller than those derived from the convective Kansas and Minnesota data. The turbulent kinetic energy budget is also different. For the same total energy production by wind shear and buoyancy, energy is available for exporting out of the surface layer in this case, whereas it must be imported into the Kansas and Minnesota surface layers.

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Shu-Hsien Chou, David Atlas, and Eueng-nan Yeh

Abstract

The structure and kinetic energy budget of turbulence in the convective marine atmospheric boundary layer as observed by aircraft during a cold air outbreak have been studied using mixed layer scaling. The results are significantly different from those of previous studies under conditions closer to free convection. The normalized turbulent kinetic energy and turbulent transport are about twice those found during the Air Mass Transformation EXperiment (AMTEX). This implies that, for a given surface heating, the present case is dynamically more active. The difference is mainly due to the greater importance of wind shear in the present case. This case is closer to the roll vortex regime whereas AMTEX observed mesoscale cellular convection, which is closer to free convection. Shear generation is found to provide a significant energy source in addition to buoyancy production to maintain a larger normalized turbulent kinetic energy and to balance a larger normalized dissipation. The interaction between turbulent pressure and divergence (i.e., pressure scrambling) is also found to transfer energy from the vertical to the horizontal components, and expected to be stronger in roll vortices than in mesoscale cells.

The sensible heat flux is found to fit well with a linear vertical profile in a clear or subcloud planetary boundary layer (PBL), in good agreement with that of Lenschow. The heat flux ratio between the PBL top and the surface, derived from the linear fitted curve, is approximately −0.14; this is in good agreement with that derived from the lidar data for the same case. Near the PBL top, the heat flux profiles are consistent with those of Deardorff and Deardorff et al.

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