Modulation of Precipitation in the Olympic Mountains by Trapped Gravity Waves During the OLYMPEX/RADEX Field Campaigns

Precipitation over the Olympic Mountains was studied intensely with multiple aircraft and ground-based measurements during the Olympic Mountains Experiment (OLYMPEX) during the fall-winter season 2015-2016 as part of validation for the Global Precipitation Mission (GPM) (Houze et al. 2017). The Radar Definition Experiment (RADEX) supported by the Aerosol Chemistry, Ecosystem (ACE) NASA Decadal Mission formulation joined efforts with OLYMPEX in an effort to understand algorithms and science associated with the proposed ACE dual-frequency (Ka-band, W-band) radar, polarimeter, and lidar. ACE provided mainly the NASA ER-2 high-altitude remote sensing aircraft as well as partial support for in situ University of North Dakota Citation aircraft. This presentation focuses on observations of a broad frontal cloud system with strong flow over the mountains on 5 December 2015. Unique observations of trapped waves were obtained with in the three Goddard Space Flight Center nadir-looking, X- through W-band, Doppler radars on the NASA high-altitude ER-2: the High-altitude Wind and Rain Airborne Profiler (HIWRAP) at Ku and Ka-band, the W-band Cloud Radar System (CRS), and the ER-2 X-band Radar (EXRAD). Deep trapped gravity waves on a scale ranging from 10-25 km were very prominent at all frequencies in the nadir-looking Doppler and reflectivity observations on 5 December 2015 as well as for other cases during the RADEX flights. These waves cause localized vertical up/down motions on the order of 1-2 ms^-1 and they are superimposed on the widespread south-southwest flow over the Olympic Mountains. While much of this widespread flow over the mountains produces copious amounts of snowfall, the gravity waves play an important role in role in modulating this precipitation indirectly through microphysical processes at higher levels. The wave processes are difficult to extract exclusively from the radar observations so we will discuss other remote sensing and surface observations to support the wave observations.