IMPROVE will focus on precipitation systems that are relatively simple (in terms of dynamical mechanisms) and predictable, but which also exhibit a variety of cloud microphysical processes that will permit testing of a wide spectrum of forcing terms in the BMP. Furthermore, the systems we have chosen for study have a high likelihood of being observed in limited, pre-specified time periods and geographic locations.
Orographic precipitation situations are also good candidates for study because much of the forcing is tied to the terrain, and the terrain is precisely known. Thus, in situations where essentially steady flow impinges on a topographic barrier and the upstream conditions are known, the dynamical response to that flow is highly deterministic, assuming the forecast model can properly resolve the key terrain-forced dynamics (Colle and Mass 1996). In addition, terrain-forced flow produces large gradients in microphysical variables and processes, providing a good test bed for the model microphysics.
The orographic study requires a mountain range that is wide and high enough to perturb the cross-barrier flow to an extent that significant orographic enhancement of precipitation occurs, but that is also relatively isolated and 2D, so that complicated 3D flow regimes and interactions between flows generated by multiple ridges are minimized. Additionally, the range must experience a high frequency of synoptic weather patterns that provide moist cross-barrier flow and copious orographically enhanced precipitation during the wintertime.
A location that satisfies these requirements is an approximately 55 km-long segment of the Cascade Mountains between Mount Jefferson and North Sister Mountain in west central Oregon. We will refer to the region surrounding this ridge (see map) as the orographic study area (OSA). It consists of essentially one north/south ridge, approximately 2000 m high. Other than minor foothills on either side, the OSA is reasonably 2D. In terms of existing observational platforms, there are two well situated NWS rawinsonde sites (Salem and Medford, Oregon) one NOAA wind profiler (Newport, Oregon) upstream of the OSA, and there is a high density of precipitation measuring sites within the OSA. The location is also sufficiently close to Seattle that the UW?s Convair-580 research aircraft can operate out of its home airport in the Seattle area, which is a 50-minute flight from the OSA. The site has good road access for deployment of ground-based observations, but is sufficiently distant from major commercial flight paths to permit good flexibility in deploying research aircraft.
In terms of orographic weather systems, we seek situations that will provide essentially uniform moist cross-mountain flow for a period of several hours, that minimize embedded deep convection, and that provide a variety of precipitation growth conditions for exercising the MM5 model. Such situations occur in either pre-frontal or post-frontal conditions, or during the passage of a weak or slow-moving front-in other words, any precipitation event that is not complicated by the transient effects of a strong surface frontal passage. Some of the characteristic meteorological and microphysical aspects of orographic precipitation in the Cascade Mountains were reported by Hobbs et al. (1971), based on findings in the Cascade Project. This study found that pre-frontal (including warm-frontal overrunning) precipitation situations in the Cascade Mountains are characterized by layered non-convective clouds distributed through the depth of the troposphere, in which ice crystals that grow at temperatures below -20 ºC are common. There is little riming except near the surface, with an absence of liquid water at temperatures below -10 ºC. While there are higher precipitation rates on the western than eastern slopes, precipitation falls on both sides of the crest. Winds have a westerly component aloft, but may be easterly at or below crest level if there is a sufficient cross-mountain pressure gradient. In contrast, post-frontal conditions are commonly characterized by winds with a westerly component at all levels producing wide-spread shallow convective clouds with tops below 5 km, that are embedded in stratiform orographic clouds. Most ice crystals grow at temperatures above -15 ºC and are heavily rimed. Precipitation falls primarily on the western slopes, with nearly none on the eastern slopes.
With regard to the best time period for the orographic field study, an examination of the climatological precipitation record at Santiam Pass, Oregon (located roughly in the middle of the OSA-see map), shows that the period from 10 November through 25 January receives an average of around 10 mm of precipitation per day, with a brief period of around 13 mm per day during the second week of December. Another consideration is minimizing the possibility of embedded deep convection. Lapse rates with onshore flow tend to become less stable as the winter progresses, due to decreasing temperature throughout troposphere without equal decreases in the underlying sea surface temperature. Thus, a consideration of minimizing embedded convection favors early winter over mid- or late winter. With regard to the frequency of targeted weather situations, the precipitation record at Santiam Pass shows that daily precipitation in excess of 10 mm can be expected on 2.7 days per week, roughly consistent with the passage of frontal systems through the area during this time of year (2.8 per week, based on a survey of 3-hourly surface maps from 5 different years). Experience from the COAST project, which examined frontal systems moving onshore in the Pacific Northwest, suggested that around 75% of frontal weather systems provided a useful period of roughly uniform flow and steady precipitation for several hours. Therefore, a period of four weeks, from late November through late December, should provide about eight observable cases, which would be sufficient to address the goals of IMPROVE.
As in the frontal study, the primary microphysical observation platform that will be required for the orographic study is the UW Convair-580 research aircraft. Unlike the frontal study, the orographic study will make use of both a ground-based radar (the NCAR S-Pol) and airborne dual-Doppler radar. The S-Pol radar is necessary for weather surveillance in the upstream environment, and also for polarimetric measurements of cloud microphysics. It will be positioned in the Willamette Valley approximately 75 km west of the OSA ridge crest (see map). When studying airflow in the vicinity of orographic barriers, it is necessary to have an aircraft equipped with dual-Doppler capability instead of relying on ground-based dual-Doppler radar, because it is difficult to position ground-based dual-Doppler radars without significant blocking by terrain features. To this end, IMPROVE plans to deploy an aircraft equipped with airborne dual-Doppler radar instrumentation during the orographic study. The radar aircraft will fly regular, pre-determined flight tracks in coordination with the Convair-580, remotely sampling the same volume of the atmosphere that the Convair-580 samples directly. The airborne dual-Doppler radar data will provide the necessary dynamical context over mountainous terrain for interpretation of the cloud microphysical parameters measured by the Convair-580.
Soundings are crucial for obtaining temperature, humidity, and wind profiles throughout the depth of the troposphere in the upstream environment. The location of the OSA can take advantage of two operational rawinsonde sites at Salem and Medford, Oregon (see map). The National Weather Service will launch 3-hourly rawinsondes at Salem and Medford during OSA observing periods. Additional sondes will be launched at other locations (yet to be determined) upstream of the OSA. Finally, IMPROVE will make use of a profiler currently operated by NOAA at Newport, Oregon (see map), which is in a good upstream position relative to the OSA.
Key additional microphysical observations can be provided by a ground observer on the mountain ridge. By examining ice crystal structures with a microscope and mounted camera at a fixed ground location during aircraft flight periods, a ground observer can obtain valuable information on the growth history of precipitation (see, for example, Hobbs 1975). In addition, the precipitation observations taken at the ground will help fill gaps in the observations taken by the Convair-580, which is required to fly no less than 600 m (or in some cases 300 m with FAA approval) above the highest terrain within an 8-km radius. Road access to the FSA is good: State Highway 20, between Corvallis and Bend, Oregon, crosses the middle of the OSA at Santiam Pass (1468 m) and provides relatively easy access to all points along the cross-mountain profile. The ground observer (probably a student) will be stationed in either Sweet Home or Bend, Oregon, both just a 1-h drive from Santiam Pass.
Measurement of precipitation accumulation at the ground is necessary. We will make use of two operational networks that have several observing stations distributed throughout the OSA (see inset in map). These are the Snowpack Telemetry (SNOTEL) network operated by Natural Resources Conservation Service (7 sites within the OSA), which measures hourly liquid equivalent precipitation and water equivalent of the snowpack; and the Cooperative Observer Network (Coop Network, 25 sites within the OSA), which measures hourly liquid equivalent precipitation.
For the orographic study, the flight strategy is simplified because the locations of flight tracks are determined by fixed topographic features rather than by mobile weather systems. Therefore, precise flight tracks can be predetermined, and once the aircraft have taken off, a need for deviation from those tracks is not anticipated. This implies that ground-to-air communication and weather-based flight guidance are not crucial to the success of the orographic study, simplifying coordination between the Convair-580 and the radar aircraft. The strategy for the radar aircraft is that it will fly parallel to the Convair-580 at altitudes of 1000-2500 m (depending on local terrain height), in a manner that keeps the Convair-580 within a volume recently sampled (or about to be sampled) by the airborne dual-Doppler scans. Other observational activities that will occur during an IOP are an increase of rawinsonde frequency to 3-h at Medford and Salem, Oregon; the dispatching of an ice crystal observer to Santiam Pass; and periodic RHI scans to produce cross sections of polarimetrically derived microphysical information.