Orographically-induced stratospheric turbulence in complex upper-level jet structures

The role of jet-induced forcing on mountain-wave generated turbulence in the lower stratosphere within two case studies is explored using a new version of the Real-Time Turbulence Modeling (RTTM) system at North Carolina State University (e.g., Kaplan et al. 2006). The newest RTTM version is based on the latest release of the Advanced Research Weather, Research and Forecasting model (WRF-ARW) and WRF Pre-processing system (WPS). The RTTM uses an automatic one-way grid-nesting algorithm (i.e., autonest) based on an empirical turbulence index to build the proper location for subsequent downscale nesting on the most potentially dangerous aviation turbulence regions. In addition, the RTTM is alternatively configured with pre-determined, two-way interactive nesting with 18, 6, and 2-kilometer horizontal grid spacing. The RTTM uses 90-vertical levels with a model top of 10 hPa in order to diagnose potential turbulent regions in the lower stratosphere.

A case involving aviation turbulence experienced by the NASA ER-2 aircraft on 22 February 2006 over California, Nevada, Utah, and Wyoming is investigated with both RTTM configurations. The ER-2 aircraft experienced turbulence near the 50-millibar level along several flight legs over the 2-hour flight between 18-20 UTC 22 February. Using both one-way nesting from the RTTM and two-way nesting, the development of stratospheric turbulence is related to the juxtaposition of an exiting sub-tropical jet streak's ascending branch/thermally direct circulation and approaching polar jet streak's descending branch/thermally indirect circulation. Vertically-propagating mountain waves over northeastern Nevada, northern Utah, and southwest Wyoming occur coincident with the region where the intersection of the jet streaks produces a layer of rapidly vertically varying Richardson numbers well into the stratosphere. Subsidence in the right exit region of the approaching polar jet streak is readily apparent with significant deepening of the troposphere, indicating the region of interest is being influenced by the jet's thermally indirect circulation. Multiple upward propagating gravity waves generated by the complex topography in the region of the interest reach as high as 30 hPa into the lower stratosphere, and represent the most likely source of turbulence in the lower stratosphere that may pose a risk to high altitude flying aircraft.

The other case examined with the new RTTM is from the French Apt-OHP experiment based in southeast France between 22 November- 6 December 2004. The period of interest for the study is the 60 hr period starting 00 UTC 22 November, when a strong polar jet streak dove southeast into the Alps contained within a strong, digging long-wave trough over Central Europe. As the polar jet dives southeast, it undercuts a much weaker sub-tropical jet stream over northern Spain and northwest Italy. The intersecting jet streaks, much like the scenario described in the ER-2 aircraft case, aids in setting up a favorable shear environment for lower stratospheric wave breaking. The polar jet streak's orientation shifts from a northwest/southeast configuration to a more north/south configuration, aligning the mean flow more perpendicular to the Alps. The realignment of the polar jet allows for the kinetic energy associated with the approaching jet streak to be extracted more efficiently into the mountain waves, and significant lower stratospheric wave breaking occurs in regions coincident with complex jet-induced shear layers. Comparisons are made between an older RTTM version based on the Mesoscale Atmospheric Simulation System (MASS) and the new RTTM version, including the sensitivity of the stratospheric turbulence response to initial conditions.