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Rod Frehlich, Yannick Meillier, Michael L. Jensen, and Ben Balsley

Abstract

Finescale temperature and velocity measurements with multiple vertically spaced cold-wire and hot-wire sensors on the Cooperative Institute for Research in the Environmental Sciences (CIRES) tethered lifting system (TLS) were produced during the Cooperative Atmosphere–Surface Exchange Study-1999 (CASES-99). The various calibration methods are presented as well as algorithms to extract high-resolution estimates of the energy dissipation rate ϵ and the temperature structure constant C2T. The instrumentation is capable of measurements of ϵ ≈ 10−7 m2 s−3 and C2T ≈ 10−6 K2 m−2/3.

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Rod Frehlich, Yannick Meillier, Michael L. Jensen, and Ben Balsley

Abstract

The probability density function (PDF) and spatial statistics of both the energy dissipation rate ϵ and the temperature structure constant C2T are determined for the shear region of a nocturnal jet. The PDF of ϵ and C2T are approximately lognormal. In addition, the joint probability density function of ϵ and C2T is approximately a joint lognormal distribution. The one-dimensional spatial spectra of ϵ and C2T in the inertial region have a k −0.5 and k −0.6 dependence, respectively, where k is the horizontal spatial wavenumber.

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Ben B. Balsley, Rod G. Frehlich, Michael L. Jensen, Yannick Meillier, and Andreas Muschinski

Abstract

During the Cooperative Atmosphere–Surface Exchange Study-1999 (CASES-99) campaign in southeastern Kansas in the fall of 1999, the University of Colorado's Cooperative Institute for Research in Environmental Sciences (CIRES) made a series of vertical profiling measurements using the CIRES tethered lifting system (TLS). The results reported here began during a period when the nocturnal boundary layer (NBL) was characterized by a low-level jet (LLJ) peaking at 120 m and a temperature profile that increased smoothly with height to a point slightly above the height of the LLJ peak. Then, within a period of less than 30 min, the character of the NBL changed abruptly, with the breakdown of the well-defined LLJ and the appearance of a surprisingly steep temperature change of some 3.5 K around 180–190 m AGL. Part of this inversion was extremely sharp, with the steepest portion showing a temperature change of 1 K over an altitude range of only 5 cm, corresponding to a vertical temperature gradient in excess of 20 K m−1. The general shape of this steep gradient was maintained—albeit with slightly reduced values—for at least 20 min. It is understood that the magnitude of the steepest portion of this gradient exceeds all previously observed atmospheric gradients by over an order of magnitude, although comparable gradients—albeit under very disparate conditions—have been observed in the ocean.

A second surprising feature apparent in these results was the steepness of the gradients in turbulence structure at the top of the NBL and within the residual layer (RL), the region above the NBL that is usually slightly stably stratified and extends upward to the height marked by the vestiges of the previous day's capping inversion. At the NBL top, energy dissipation rates and temperature structure parameters dropped sharply by more than one order of magnitude over a distance of only a few meters. At higher altitudes within the RL, a 60-m-thick region of very weak turbulence was observed. This low-turbulence region also exhibited sharp edges, where energy dissipation rates and temperature structure parameters changed by at least an order of magnitude over vertical distances of only a few meters.

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