IMPROVE-2

The goal of the 2nd Improvement of Microphysical PaRameterization through Observational Verification Experiment (IMPROVE-2) study was to focus on orographic lifting as the primary precipitation generating mechanism. This required a mountain range that was wide and high enough to perturb the cross-barrier flow to an extent that significant orographic enhancement of precipitation occurred, but that was also relatively isolated and 2D, so that complicated 3D flow regimes and interactions between flows generated by multiple ridges were minimized. Additionally, such a mountain range needed to experience a high frequency of synoptic weather patterns provided a moist cross-barrier flow and copious orographically enhanced precipitation during the wintertime.

A location that satisfied these requirements was an approximately 55 km-long segment of the Cascade Mountains between Mount Jefferson and North Sister Mountain in west central Oregon (see map below).  It consists of essentially one north/south ridge, approximately 2000 m high. Other than minor foothills on either side, the range is reasonably 2D.  The location was also sufficiently close to Seattle so that the University of Washington (UW)’s Convair-580 research aircraft could operate out of its home airport in the Seattle area, which is a 50-minute flight from the study area. The site had good road access for deployment of ground-based observations, but was sufficiently distant from major commercial flight paths to permit good flexibility in deploying research aircraft flight tracks.

In terms of orographic weather systems, situations were sought that provided strong, moist cross-mountain flow for a period of several hours, that minimized embedded deep convection, and that provided a variety of precipitation growth conditions for exercising the MM5 model.  With regard to the best time period for the orographic field study, the climatological precipitation record at Santiam Pass, Oregon (located roughly in the middle of the proposed ridge—see map) indicated a strong precipitation maximum (>10 mm precipitation per day) from late November through early January.  With regard to the frequency of targeted weather situations, the precipitation record at Santiam Pass showed that daily precipitation in excess of 10 mm could be expected on 2.7 days per week, roughly consistent with the passage of frontal systems through the area during that time of year (2.8 per week, based on a survey of 3-hourly surface maps from 5 different years). Therefore, over a period of four weeks, from 26 November - 22 December 2001, 10-12 observable weather systems were expected to pass through the study area.  As it turned out, the winter of 2001/2002 was wetter than average, and fifteen weather systems were studied with flight operations, even with self-imposed restriction of operating only between 7 AM and 10 PM.