RF07 - Mountain Lee Waves

After the passage of a cold front, the Appalachians often experience an extended period of strong flow with a westerly and/or northerly component. In cooler, more stable air this results in the formation of trapped lee waves with trains that may propagate across the mid-Atlantic Piedmont and through the SEAR-MAR domain. Since the mountains have a sharp clockwise bend in the study area, both eastward and southward traveling waves can be triggered, and there is a possibility of interference between the two.

Forecast teams identified 10 Nov, after an overnight cold front passage and ahead of an advancing 1040 hPa high pressure system (Fig. 14), as an ideal mountain wave day. Short-term model guidance suggested a midday transition from wave trains to convective cloud streets, so takeoff was scheduled for 1430 UTC.

Figure 14: WPC surface analysis for 1200 UTC 10 November 2017 showing the advancing high pressure system making for favorable conditions for the mountain wave mission RF07.

To study this phenomenon, a flight path was developed that included four vertically-stacked levels in both an N-S direction and an E-W direction (Fig. 15). Between these two stacks, in the area NE of Gettysburg, PA, a sequence of ascending legs crisscrossing the line of inflection through the Appalachians was planned, followed by a set of similar descending legs.

Figure 15: Flight path for Mountain Wave study RF07.

The N-S stacks had a well-defined wave structure at 6250’ AGL, but the lower three legs were found to all be within the turbulent boundary layer. For this reason, the flight plan was changed so that after the ascending zigzag, the return pattern was made at a constant 6250’ elevation. Then, one of the lower E-W legs was replaced by a higher one at 7750’ resulting in additional mountain wave transits.

Figure 16 is from a student presentation showing the inflight vertical velocities for the entire track. There are obvious differences between the irregular fluctuations in the lower transects and the smoother, periodic undulations along the transects at higher altitudes where the mountain waves reside.

Figure 16: Flight tracks colored with vertical velocities. The four east-west stacks are indicative of regular mountain wave undulations residing on top of a turbulent boundary layer during RF07.