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  • Author or Editor: Monique Y. Leclerc x
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Anandakumar Karipot, Monique Y. Leclerc, and Gengsheng Zhang

Abstract

The seasonal and interannual variability of the nocturnal low-level jets over the north Florida region are investigated using sodar measurements spanning 540 nights. On average, jets are present in 62% of the nocturnal periods examined. The observed jet speeds range between 3 and 21 m s−1 and heights are between 80 and 700 m. Observations show that the low-level jet occurs more frequently (70% of the nocturnal periods) during the colder months November–February in contrast with the warmer months June–August (∼47%). The presence of southerly jets dominates the summer months, whereas northerly jets are more frequent during winter. Colder months frequently exhibit jets with speeds exceeding 14 m s−1, often associated with the passage of frontal systems. The interannual variability observed using the North American Regional Reanalysis (NARR) wind profile data during a 4-yr period shows only minimal differences in jet characteristics. A comparison of jet heights with NARR planetary boundary layer heights suggests that jets at the north Florida location frequently occur within the planetary boundary layer. The occurrence and speed of observed low-level jets are linked to both the land–ocean temperature contrast and to the strength and orientation of surface pressure gradients over the region. A high occurrence of large-amplitude oscillations with approximately a 24-h period near the jet height is shown using the Hilbert–Huang transform analysis, suggesting that inertial oscillations are one possible cause of jet formation in north Florida.

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Szymon P. Malinowski, Monique Y. Leclerc, and Darrel G. Baumgardner

Abstract

Fractal analyses of individual cloud droplet distributions using aircraft measurements along one-dimensional horizontal cross sections through clouds are performed. Box counting and cluster analyses are used to determine spatial scales of inhomogeneity of cloud droplet spacing. These analyses reveal that droplet spatial distributions do not exhibit a fractal behavior. A high variability in local droplet concentration in cloud volumes undergoing mixing was found. In these regions, thin filaments of cloudy air with droplet concentration close to those observed in cloud cores were found. Results suggest that these filaments may be anisotropic. Additional box counting analyses performed for various classes of cloud droplet diameters indicate that large and small droplets are similarly distributed, except for the larger characteristic spacing of large droplets.

A cloud-clear air interface defined by a certain threshold of total droplet count (TDC) was investigated. There are indications that this interface is a convoluted surface of a fractal nature, at least in actively developing cumuliform clouds. In contrast, TDC in the cloud interior does not have fractal or multifractal properties. Finally a random Cantor set (RCS) was introduced as a model of a fractal process with an ill-defined internal scale. A uniform measure associated with the RCS after several generations was introduced to simulate the TDC records. Comparison of the model with real TDC records indicates similar properties of both types of data series.

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Thara V. Prabha, Monique Y. Leclerc, Anandakumar Karipot, and David Y. Hollinger

Abstract

Turbulent bursts observed over a tall forest canopy during the initiation of a nocturnal low-level jet (LLJ) are studied with the help of wavelet analysis. The burst of turbulence is observed in response to a shear instability associated with the initiation of LLJ. Turbulent kinetic energy, momentum, and CO2-rich cold air are transferred downward by large eddies with length scales that are higher than the LLJ height. Microfronts are observed over the canopy as a secondary instability that enhances the mixing processes within and above the canopy. The scale-dependent wavelet correlation analysis reveals that countergradient fluxes result from low frequencies, whereas cogradient flux is associated with high-frequency turbulent motions. The countergradient flux is initially noted at low frequencies, and, through coherent motions, it is transferred to smaller scales with a nearly 20-min delay. The countergradient flux dominates at the initiation of the event and reduces net flux, whereas enhanced cogradient flux at the decay of the event increases the net flux. The wavelet correlation coefficient corresponding to cogradient and countergradient fluxes is applied to segregate three regions of the spectra corresponding to “turbulent,” “coherent,” and “noncoherent” large scales. These findings are used to examine the implications on eddy covariance flux measurements.

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Thomas Foken, Marc Aubinet, John J. Finnigan, Monique Y. Leclerc, Mattthias Mauder, and Kyaw Tha Paw U

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