Abstract

The influence of orographic circulations on the precipitation structure of a Wasatch Mountain winter storm is examined using observations collected during the third intensive observing period (IOP3) of the Intermountain Precipitation Experiment (IPEX). The event featured the passage of a midlevel (700–550 hPa) trough followed 3 h later by a surface trough. Prior to and during the midlevel trough passage, large-scale southwesterly flow impinged on the Wasatch Mountains. Low-level confluence was observed between this southwesterly flow and along-barrier southerly flow within 20–40 km of the Wasatch Mountains. This confluence zone, which moved toward the Wasatch Mountains during and following the passage of the midlevel trough, was accompanied by low-level convergence and precipitation enhancement over the upstream lowlands. Dual-Doppler analysis revealed the presence of a shallow along-barrier jet near the base of the Wasatch Mountains that was surmounted by southwesterly cross-barrier flow at mid- and upper-mountain levels. This cross-barrier flow produced strong (1–2 m s⁻¹) ascent as it interacted with the steep windward slopes of the Wasatch Mountains, where precipitation was roughly double that observed in the lowlands upstream. Flow deflection and splitting were also observed near the highest terrain features. A narrow region of strong subsidence, which at times exceeded 2 m s⁻¹, was found to the lee of the Wasatch and, based on radar imagery, appeared to modulate hydrometeor spillover aloft. Processes contributing to the evolution of the near-barrier flow field, including topographic blocking, diabatic effects, and surface friction contrasts, are discussed.

Despite continual increases in numerical model resolution and significant improvements in the forecasting of many meteorological parameters, progress in quantitative precipitation forecasting (QPF) has been slow. This is attributable in part to deficiencies in the bulk microphysical parameterization (BMP) schemes used in mesoscale models to simulate cloud and precipitation processes. These deficiencies have become more apparent as model resolution has increased. To address these problems requires comprehensive data that can be used to isolate errors in QPF due to BMP schemes from those due to other sources. These same data can then be used to evaluate and improve the microphysical processes and hydrometeor fields simulated by BMP schemes. In response to the need for such data, a group of researchers is collaborating on a study titled the Improvement of Microphysical Parameterization through Observational Verification Experiment (IMPROVE). IMPROVE has included two field campaigns carried out in the Pacific Northwest: an offshore frontal precipitation study off the Washington coast in January–February 2001, and an orographic precipitation study in the Oregon Cascade Mountains in November–December 2001. Twenty-eight intensive observation periods yielded a uniquely comprehensive dataset that includes in situ airborne observations of cloud and precipitation microphysical parameters; remotely sensed reflectivity, dual-Doppler, and polarimetric quantities; upper-air wind, temperature, and humidity data; and a wide variety of surface-based meteorological, precipitation, and microphysical data. These data are being used to test mesoscale model simulations of the observed storm systems and, in particular, to evaluate and improve the BMP schemes used in such models. These studies should lead to improved QPF in operational forecast models.

The Coastal Observation and Simulation with Topography (COAST) program has examined the interaction of both steady-state and transient cool-season synoptic features, such as fronts and cyclones, with the coastal terrain of western North America. Its objectives include better understanding and forecasting of landfalling weather systems and, in particular, the modification and creation of mesoscale structures by coastal orography. In addition, COAST has placed considerable emphasis on the evaluation of mesoscale models in coastal terrain. These goals have been addressed through case studies of storm and frontal landfall along the Pacific Northwest coast using special field observations from a National Oceanic and Atmospheric Administration WP-3D research aircraft and simulations from high-resolution numerical models. The field work was conducted during December 1993 and December 1995. Active weather conditions encompassing a variety of synoptic situations were sampled. This article presents an overview of the program as well as highlights from a sample of completed and ongoing case studies.

Winter storms and their prediction are of increasing importance throughout the region of the United States with the fastest growing population, the Intermountain West. Such storms can produce heavy orographic snowfall, lake-effect snowbands, and even lightning. Unfortunately, precipitation forecast skill is lower over the Intermountain West than other regions of the country because of the complex topography, the lack or limited utility of upstream and in situ data, and insufficient understanding of storm and precipitation processes.

The Intermountain Precipitation Experiment (IPEX) is a research program designed to improve the understanding, analysis, and prediction of precipitation over the complex topography of the Intermountain West. The field phase of this research program was held in northern Utah in February 2000. During this time, seven storms were observed, including the heaviest snowfall to strike the Wasatch Mountains in two years, a tornadic bow echo associated with a strong cold front, a mesoscale snowband in Tooele Valley, and three other storms with locally heavy orographic snowfall and complex mesoscale circulations. Some of these storms were electrified and produced lightning.

This paper reviews the weather of the Intermountain West, describes the experimental setup and the outreach activities of IPEX, and presents preliminary results from the field phase. Finally, lessons learned in planning and executing this field program are discussed.