Much of the early work related to orographic precipitation over the western United States was driven by weather-modification efforts in the 1970s and 80s [see Marwitz (1986) and Rangno (1986) for a review]. Marwitz (1986) compares orographic storms over the Sierra Nevada to those of the San Juan Mountains. Significant differences in flow dynamics and cloud microphysics were found over the two mountain ranges. These include contrasts in the strength of the windward barrier jet and associated low-level moisture transport during stable flow, and the development of unique local circulations during unstable flow, such as a convergence zone over the foothills of the San Juans. Compared to storms of the Sierra Nevada, those of the San Juans featured colder cloud bases and tops, higher CCN concentrations, and higher cloud droplet concentrations. As a result, the Hallett-Mossop secondary ice crystal production process (Hallett and Mossop 1974) and riming growth were important in Sierra Nevada storms, whereas over the San Juans, the former was not observed and the latter was relatively inefficient. More recent studies over the Tushar range of southern Utah have illustrated changes in the character of storm dynamics and microphysics with stability, and the interactions between orographic cloud structures and mesoscale precipitation bands (Long et al. 1990; Sassen et al. 1990). These studies, however, featured relatively limited observations to document the three-dimensional, orographically-induced flow field and its interaction with precipitation microphysics.

More recent field programs such as COAST (Bond et al. 1997) and CAL-JET (Ralph et al. 1998) have examined orographic-precipitation dynamics along the west coast of the United States. Colle and Mass (1996) described the three-dimensional kinematic and precipitation structure of an orographic precipitation event around the quasi-three-dimensional Olympic Mountains. This study also utilized tail-Doppler radar from the NOAA P-3 for the purposes of mesoscale model verification. Braun et al. (1997) described the interaction of a narrow cold-frontal rainband as it approached the Oregon coast. Dual-doppler analyses from the NOAA P-3 were used to document significant modifications to the front upstream of the coastal topography. Modification of precipitation structures and dynamics upstream of coastal topography during COAST and CAL-JET have also been documented by Yu et al. (1998), Bond and Doyle (1998), Doyle and Bond (1998), and Ralph et al. (1998). These studies illustrate the value of multisensor observations and numerical simulations in understanding precipitation dynamics in complex terrain.

Despite the success of programs such as COAST and CALJET, and presumed future success of the Mesoscale Alpine Programme (MAP), additional studies are needed in regions with differing topography and climatological conditions to sample the spectrum of circulations and precipitation processes that occur in complex terrain. The topography of the Intermountain West is characterized by a quasiperiodic series of narrow, steeply-sloped mountain ranges that are 10 to 20 km in width and separated by broad lowland valleys that are several tens of km across. One of the more dramatic ranges of the Intermountain West is the Wasatch Mountains, which rise 1200-2000 m in ~5 km on their western slope and 1000-1500 m in ~10 km on their eastern slope. As suggested by Marwitz (1986), precipitation microphysics over Intermountain West ranges, such as the Wasatch Mountains, likely differs from that of coastal ranges due to colder, mixed-phase cloud conditions and large CCN and cloud droplet concentrations. Additionally, the relatively steep and narrow profile of the Wasatch Mountains may result in a precipitation distribution that differs from the broader coastal mountain ranges of the western United States, with the heaviest precipitation found near the crest of the barrier instead of 10-20 km upstream (e.g., Rauber 1992; Sinclair et al. 1997; Westrick and Mass 1998; Colle and Mass 1999).

IPEX will examine questions and test hypotheses related to the dynamics and microphysics of orographic precipitation produced by the Wasatch Mountains and similar mountain ranges of the Great Basin. For example, what mesoscale circulations and vertical-motion patterns are induced by this narrow, steeply sloped mountain range and how do such circulations vary with changes in the large-scale flow and stability? We hypothesize that, similar to studies in coastal regions, stable flow regimes will be characterized by upstream blocking and the development of orographically-induced ascent and precipitation upstream of the topographic barrier. However, this blocking front may be more diffuse than that found upstream of coastal mountain ranges due to the influence of upstream topographic features. During unstable flow regimes, we anticipate that orographically-induced ascent will be quite intense due to the steepness of the windward slopes and confined to near the barrier. A related question concerns how such orographic circulations influence cloud microphysics and precipitation rate. We hypothesize that, similar to studies over the Tushar and San Juan mountain ranges, the magnitude of liquid water content, its breadth and depth, and associated relative importance of accretional versus diffusional growth, will increase as the atmospheric stability decreases. However, orographic precipitation over the Wasatch is distinct from that over the Tushars and San Juans in two important respects: the relative steepness of the Wasatch and the urban character of the cloud condensation nuclei distribution upwind of the Wasatch, due to the close proximity of Salt Lake City and Ogden. We hypothesize that the former factor will lead to stronger upward motion during neutral and unstable conditions, which will produce larger liquid water contents, enhance accretional growth, and increase precipitation rates. In contrast, we hypothesize that the latter factor will cause a pronounced shift of this liquid water to smaller cloud droplet sizes, which will result in a decreased precipitation rate due to inhibition of the accretional growth process (Borys et al. 1995). This effect should be strongest in the lower portion of the cloud as CCN are entrained into the orographic updraft. The relative significance of these competing factors in the orographic precipitation process over the Wasatch will be examined with IPEX datasets.

Finally, there are a number of significant scientific issues related to distribution, dynamics, and microphysics of precipitation "spillover" to the lee of mountain ranges (e.g., Fraser et al. 1973; Hobbs et al. 1973). Based on rain gauge observations, Sinclair et al. (1997) suggest that spillover is greatest during periods of (1) low static stability when orographic ascent is located near the barrier, and (2) large cross-barrier wind speeds that allow for downwind transport of hydrometeors. Under such conditions, however, one might also expect enhanced terminal fall speeds due to accretional growth during unstable orographic ascent, which would limit leeside hydrometeor transport. We hypothesize that significant leeside spillover occurs during neutral or unstable conditions, moderate to large cross-barrier wind speeds, and periods of lower cloud liquid water content that result in less accretional growth. IPEX datasets will be used to test this hypothesis and examine the factors that control the spillover of precipitation to the lee of narrow, steeply sloped mountain barriers.