Cloud Structures, Microphysical Processes and Synergistic Interactions between Frontal and Orographic Forcing of Precipitation

On 13–14 December 2001 a strong cyclonic storm passed over the Pacific Northwest, producing heavy orographic precipitation over the Cascade Mountains. This storm was one of several studied during the second field phase of the Improvement of Microphysical Parameterization through Observational Verification Experiment (IMPROVE). A wide variety of in situ and remotely sensed measurements were obtained as this storm passed over the Oregon Cascades. These measurements provide a comprehensive data set of meteorological state parameters (temperature, pressure, humidity, winds, and vertical air velocity), polarization Doppler radar measurements, and cloud microphysical parameters (liquid water content, and particle concentrations, size spectra, and imagery).

The 13–14 December case was characterized by the passage of a tipped-forward lower-tropospheric front that extended upward to a preceding vigorous upper cold-frontal rainband, which produced clouds up to ~ 8–9 km. An important difference between this storm and those studied previously over the Washington Cascades, was that both the pre-frontal and post-frontal low-level airflow over the Oregon Cascades were characterized by strong westerly cross-barrier flow. Consequently, as the upper-level frontal band passed over the Oregon Cascades there was both strong ice particle production aloft and significant liquid water production at lower levels in the orographic lifting zone.

Our investigation shows that there was a synergistic interaction between frontal and orographic precipitation as the broad rainband associated with the 13-14 December 2001 event progressed across the Oregon Cascade Mountains. Analyses of column integrated cloud liquid water depth and simultaneous ground-based raingauge measurements spanning the study area revealed that the combination of a deep tropospheric frontal precipitation band and lower tropospheric orographic uplift produced more precipitation than the sum of these two processes would have produced in isolation. Estimates based on radiometric measurements and raingauge measurements suggest that the synergistic effect was as much as 1 mm hr^-1 across the windward Cascade slopes.