Regions of persistent intense turbulence in the residual layer arising from aircraft engine exhaust during CASES-99

The CASES-99 campaign in east-central Kansas included numerous measurement techniques for studying the nighttime stable boundary layer region. These techniques included the CIRES Tethered Lifting System (TLS) of the University of Colorado that profiled through the entire nighttime boundary layer (NBL) and well up into the residual layer. On at least one occasion, the TLS documented a well-defined region of surprisingly intense turbulence with observed intensities three orders of magnitude larger than the ambient background level. This layer--located well above the top of the NBL in the residual layer--was associated with a slightly enhanced (~0.4 C) temperature “step” and contained ~ 0.3 m/s wind speed fluctuations.

Examination of the flight path of the University of Wyoming’s King Air showed that, about one minute earlier than the TLS observation, the aircraft had flown an east-west track close to the same altitude and approximately one kilometer upwind of the TLS observations.

It is hard to imagine that the observed enhanced turbulence “event” could have arisen from anything but the warm aircraft engine exhaust being advected horizontally through the region occupied by the TLS sensors. This interpretation is confirmed by the observation of a second, albeit more diffuse, “event” that occurred somewhat later on a subsequent TLS profile that corresponded to another passage of the aircraft along essentially the same flight path. Further evidence is provided by concurrent sodar records.

While the above observations appear to be somewhat serendipitous, it is reasonable to predict that remotely-sensed returns from a quiescent region such as the residual layer during CASES-99 can be markedly perturbed by aircraft exhaust trails advecting through the region. Specifically, enhanced echoes from turbulent engine exhausts can be anticipated to contaminate both sodar and FMCW radar records if these instruments are examining the same heights through which the (upwind) aircraft has flown earlier.

The above observations also illustrate the potential for using this type of observation for studying the gradual evolution of a well-confined turbulent region in an unconfined atmospheric volume. Situating a suite of TLS sensors, sodars, FMCW radars, and possibly Doppler Lidars, at increasing distances downwind of an aircraft exhaust would enable a controlled temporal/spatial analysis of diffusion and transport of the turbulent trail. It would be also possible to monitor the evolution of the turbulent spectrum both within and nearby the trail.