Mountain Waves entering the Stratosphere: New aircraft data analysis techniques from T-Rex

During the T-Rex project in March and April 2006, twelve flights of the new NCAR GV aircraft observed Sierra Nevada mountain waves in the stratosphere. The vertical velocity amplitudes ranged from 1.5 to 10 meters per second. Three particular aspects of the GV observing strategy allowed some innovative analysis of wave properties. First, the measurement of aircraft geometric altitude using differential GPS allowed the mechanical form of the Bernoulli function to be determined as a function of potential temperature. The Bernoulli function is particularly helpful in distinguishing wind variations associated with vertical shear from those caused by the wave-induced pressure fields. The Bernoulli function was found to be layered in the stratosphere, with a typical layer thickness of 100meters. This “Bernoulli layering” of the stratosphere appears to be correlated with the fine-scale ozone layering, suggesting an isentropic interleaving of air masses from different latitudes. For the upper troposphere levels, water vapor is also a useful tracer of wave motion. The GPS altitude also allowed the direct computation of vertical energy flux for the first time in a gravity wave project.

Second, the flight track design in the form of a wind-aligned 46km–wide “racetrack” allowed some three dimensional aspects of the wave field to be determined, such as the lee wave alignment and lateral amplitude heterogeneity. The length of the racetrack (~180km) allows massif scale flow structures to be distinguished from shorter lee waves associated with atmospheric resonances. Also, the combined use of Bernoulli's and Crocco's theorems allowed the potential vorticity to be determined.

The third important aspect of the GV campaign in T-Rex was the accurate repeatability of the tracks during each mission and from flight to flight. Our primary focus will be on the eight missions (87 racetracks) that used the so-called Track B, with a cross-Sierra WSW-ENE orientation. This standardization allowed the dynamic differences between legs to be attributed to real atmospheric differences, rather than to an altered location relative to the complex Sierra Nevada terrain. By using racetrack data from three altitudes (~9, 11 and 13km), variation in wave properties between the troposphere and stratosphere can be seen. The properties of each wave encounter are summarized in a table, including indices describing the degree of non-hydrostatic and non-linear wave dynamics. Some statistical relationships regarding wave propagation properties are discussed.