An investigation of a midlatitude lower stratospheric gravity wave “valve layer”

The Deep Propagating Gravity Wave Experiment (DEEPWAVE) field campaign was conducted over the New Zealand region to study gravity waves from tropospheric origins to their dissipation at very high altitudes. Preliminary analysis from a season long 6-km resolution WRF simulation suggests about half of New Zealand mountain wave events are strongly attenuated in a lower stratosphere “valve layer” near 15 km. How are mountain waves attenuated in this layer? How are waves modified as they pass through this layer? Does mountain wave attenuation violate PV conservation? These questions are investigated with NCAR Gulfstream V (NGV) aircraft observations and high-resolution simulations of such a valve layer attenuation event that occurred on 24 May 2014 over the South Island of New Zealand. The NGV provided in situ measurements at 12 and 13.5 km and sampled wave breaking along higher flight legs. A microwave temperature profiler onboard the NGV provided remotely sensed 2-D cross sections of temperature 5 km above and below the aircraft. High-resolution (2-km) WRF simulations are compared with the aircraft and AIRS observations and used to study the dynamics of propagation and attenuation through the valve layer. A new 2-D spectral filtering method is used to quantify resolved gravity wave momentum flux and drag within the simulated fields, which suggest the strongest attenuation occurred near 14 km resulting in gravity wave drag of roughly 25 m s-1 day-1.