The Doug Lilly Symposium


Cliff-ramp patterns and Kelvin-Helmholtz billows in stably stratified shear flow in the upper troposphere: Analsis of aircraft measurem,ents

Donald Wroblewski, Boston Univ., Boston, MA; and O. R. Coté, J. Hacker, and R. J. Dobosy

Cliff-ramp patterns (CR) are a common feature of scalar turbulence, and have been observed in a variety of turbulent shear flows. A typical CR structure is characterized by a sharp temperature increase, the cliff, followed by a more gradual temperature decrease, the ramp. The order is reversed for the Ramp-cliff (RC) structures, which are observed in stable conditions when the total vertical shear, Sz, is negative and in unstable conditions when the vertical shear is positive. Aircraft measurements obtained from NOAA "BAT" turbulence probes were used to characterize and compare CR/RC structures at three different locations in stable conditions (Ri ˜ 0.2) in the upper troposphere: an RC structure at 11.4 km. altitude over Wales on June 6, 2000 (vertical shear ,Sz ,negative) and CR structures at 8.3 km altitude over Southern Australia on Sept. 2, 2002 and 9.65 km altitude over Southern Australia on Aug. 6, 1999, the latter two in correspondence with the southern hemisphere winter subtropical jet stream. On the scale of the CR structures, horizontal velocity fluctuations were closely correlated with those of potential temperature, which suggests vertical gradient transport of buoyancy and momentum, though the vertical velocity fluctuations were not consistent with such transport. The intensity of vertical velocity fluctuations did increase noticeably when the CR patterns appeared. Two of the cases displayed anomalous scaling behavior of the temperature structure functions, consistent with previously reported results for passive scalars in laboratory flows. The temperature fronts associated with the cliffs were oriented in a direction approximately normal to the mean wind direction, while the vertical orientation angle of the cliff-front, for one of the cases, ranged from 26 to 65 degrees. Locally high values of temperature structure function, CT2, near cliff temperature fronts were shown to be associated with the steep temperature gradients in the cliff, and not small scale turbulence. These high values may be misleading when applied to electromagnetic propagation modeling, providing a "false positive" indication of high levels of small-scale turbulence that would not correspond to scintillation effects. Comparison of the observed CR/RC patterns with DNS indicated that they may be associated with Kelvin Helmholtz billows, though further investigation is needed to confirm this idea.

Poster Session 1, Doug Lilly Symposium Posters
Thursday, 2 February 2006, 9:45 AM-11:00 AM, Exhibit Hall A2

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