Mountain wave structures occurring within a major orographic precipitation event: Part I. Analyses of airborne Doppler radar data

Airborne Doppler radar provides the best available means to comprehensively map detailed flow structure within precipitation-laden layers over complex terrain. IMPROVE-II is one of only two experiments (the other being MAP) in which airborne Doppler radar data have been collected during passage of precipitating cloud systems over a large mountain barrier. The 13-14 December 2001 event over west-central Oregon was particularly well-suited to such an exploration because (a) systematically designed flight tracks were repeated on both the windward and leeward sides of the Cascade crest, providing multiple views of the slowly concomitantly evolving flow/precipitation structure spanning an orographic barrier, and (b) the extension of detectable radar echo into the lee of the Cascades allows a remarkably complete illumination of both the ascending and subsiding branches of a mountain-wave circulation.

This presentation focuses on a two-hour period of heavy pre-frontal stratiform precipitation that impacted the IMPROVE study area from 2300-0100 UTC 13-14 December 2001. The extensive dual-Doppler dataset provided by the NOAA P-3 illustrates the modification of upstream kinematic flow by the barrier, and allows a detailed analysis of an episode of partial-blocking. Attention is focused on examining the amplitude and vertical extent of a standing mountain wave located over the mean crest of the Cascades, as well as multiple shorter-wavelength oscillations identified over the windward foothills. Lastly, this talk will qualitatively assess the impact of these waves upon the observed precipitation processes.