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Nimal Gamage and Carl Hagelberg

Abstract

A general localized transforms is used to study the characteristics of conditional sampling techniques. A covariance transform is defined that measures the covariance between the signal and a dilated and translated generating function which has compact support. It is shown to be superior as a method of detecting events containing sharp edges such as fronts or microfronts when compared with energy-preserving transforms such as the Fourier and wavelet transform. A method of finding the most coherent scale with respect to dilation of the generating function is presented. A sample analysis using sonic data measured in shear driven boundary-layer atmospheric turbulence is presented.

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Nimal Gamage and William Blumen

Abstract

Atmospheric cold fronts observed in the boundary layer represent relatively sharp transition zones between air masses of disparate physical characteristics. Further, wavelike features and/or eddy structures are often observed in conjunction with the passage of a frontal zone. The relative merits of using both global and local (with respect to the span of a basis element) transforms to depict cold-frontal features are explored. The data represent both tower and aircraft observations of cold fronts. An antisymmetric wavelet basis set is shown to resolve the characteristics of the transition zone, and associated wave and/or eddy activity, with a relatively small number of members of the basis set. In contrast, the Fourier transformation assigns a significant amplitude to a large number of members of the basis set to resolve a frontal-type feature. In principle, empirical orthogonal functions provide an optimal decomposition of the variance. The observed transition zone, however, has to be phase aligned and centered to yield optimal results, and variance may not be the optimum norm to depict a front. It is concluded that the wavelet or local transform provides a superior representation of frontal phenomena when compared with global transform methods. Further, the local transform offers the potential to provide some physical insight into wave and/or eddy structures revealed by the data.

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William Blumen, Nimal Gamage, Robert L. Grossman, Margaret A. LeMone, and L. Jay Miller

Abstract

This investigation examines the meso- and microscale aspects of the 9 March 1992 cold front that passed through Kansas during the daylight hours. The principal feature of this front is the relatively rapid frontogenesis that occurred. The total change in the cross-frontal temperature is about 6 K, with most of the change occurring between about 0820 and 1400 local time and over a relatively small subsection of the total frontal width. The surface data are able to resolve a sharp horizontal transition zone of 1–2 km. The principal physical processes that produce this frontogenesis are shown to be the cross-frontal differential sensible heating, associated with differential cloud cover, and the convergence of warm and cold air toward the front. The former process is responsible for an increase in the magnitude of the differential temperature change across the front; the latter process concentrates the existing temperature differential across an ever-decreasing transitional zone until a near discontinuity in the horizontal temperature distribution is essentially established during the period of a few hours. Two approaches are taken to demonstrate that these processes control the observed frontogenesis. First, surface data from an enhanced array, set up during the Storm-scale Operational and Research Meteorology Fronts Experiment System Test, are used to evaluate the terms that contribute to the time rate of change of the gradient of potential temperature, d|∇θ| / dt, following the motion of the front. Then, the processes of differential sensible heating and convergence are incorporated into a simple two-dimensional nonlinear model that serves to provide a forecast of the surface temperature and velocity fields from given initial conditions that are appropriate at the onset of the surface heating. Verification of the model predictions by observed data confirms that both processes contribute to the observed daytime frontogenesis on 9 March 1992. A critique of the model does. however, suggest that the accuracy of some quantitative evaluations could be improved.

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L. Jay Miller, Margaret A. LeMone, William Blumen, Robert L. Grossman, Nimal Gamage, and Robert J. Zamora

Abstract

Observations taken over the period 8–10 March 1992 during the Storm-scale Operational and Research Meteorology Fronts Experiment Systems Test in the central United States are used to document the detailed low-level structure and evolution of a shallow, dry arctic front. The front was characterized by cloudy skies to its north side and clear skies to its south side. It was essentially two-dimensional in the zone of intense observations.

There was a significant diurnal cycle in the magnitude of the potential temperature gradient across both the subsynoptic and mesoscale frontal zones, but imposed upon an underlying, more gradual, increase over the three days. On the warm (cloudless) side., the temperature increased and decreased in response to the diurnal heating cycle, while on the cold (cloudy) side the shape of the temperature decrease from its warm-side value (first dropping rapidly and then slowly in an exponential-like manner) remained fairly steady. The authors attribute the strong diurnal variation in potential temperature gradient mostly to the effects of differential diabatic heating across the front due to differential cloud cover.

The front is described in terms of three scales: 1) a broad, subsynoptic frontal zone (∼250–300 km wide) of modest temperature and wind gradients; 2) a narrower mesoscale zone (∼15–20 km wide) with much larger gradients; and 3) a microscale zone of near-zero-order discontinuity (≤1–2 km wide). There was some narrowing (≲50 km) of the subsynoptic frontal zone, but the authors found no evidence for any significant contraction of this zone down to much smaller mesoscale sizes. In response to the differential diabatic heating, the strongest evolution occurred in the micro-mesoscale zone, where dual-Doppler radar and aircraft measurements revealed the development of a density-current-like structure in and behind the leading edge of cold air. Here the steepest gradients developed shortly after sunrise and then increased by an order of magnitude during the day, with leading-edge vorticity, divergence, and temperature gradients reaching maximum values of 10−2 s−1 and 8 K km−1. A narrow updraft, marked by cumulus clouds, grew in intensity above the leading edge through the day to a maximum of 5–8 m s−1. Stratus clouds lay in the cold air, their leading edge receding by noon to 10–20 km behind the cumulus line.

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