During the spring of 1991, scientists from the National Severe Storms Laboratory conducted a field observational program to obtain a better understanding of the processes responsible for organizing and maintaining the dynamical and electrical structure of mesoscale convective systems (MCSs), as well as mechanisms acting to organize and propagate the dryline. Extensive use was made of a relatively new observing tool, the airborne Doppler radar installed on one of the NOAA P-3 research aircraft, to map the precipitation and kinematic structure of large mesoscale convective systems. The radar was operated in an innovative scanning mode in order to collect pseudo-dual-Doppler wind data from a straight-line flight path. This scanning method, termed the fore/aft scanning technique (FAST), effectively maps out the three-dimensional wind field over mesoscale domains (e.g., 80 km × 100 km) in ~15 min with horizontal data spacing of 1–2 km. Several MCSs were observed over central Oklahoma during May and June of 1991, and one such system exhibiting a “bow-echo” structure is described. Many observed features of this MCS correspond to structures seen in nonhydrostatic numerical simulations. These features include a pronounced bulge or “bow” in the convective line (convex toward the storm's direction of propagation), a strong descending rear inflow jet whose axis is aligned with the apex of the bow, and a cyclonic vortex (most pronounced at heights of 2–3 km) situated in the trailing stratiform region lateral to the axis of strongest rear inflow. Dopplerderived wind analyses reveal the likely role played by the mesoscale circulation in twisting environmental vertical shear and converging ambient vertical vorticity in maintaining and amplifying the vortex. The relatively detailed yet horizontally extensive airflow analyses also reveal the utility and advantages of airborne Doppler radar in the study of large convective systems.

Shipborne Doppler radar operations were conducted over the western Pacific warm pool during TOGA COARE using the Massachusetts Institute of Technology and NOAA TOGA C-band Doppler radars. Occasionally the ships carrying these radars were brought to within 50 km of each other to conduct coordinated dual-Doppler scanning. The dual-Doppler operations were considered a test of the logistical and engineering constraints associated with establishing a seagoing dual-Doppler configuration. A very successful dual-Doppler data collection period took place on 9 February 1993 when an oceanic squall line developed, intensified, and propagated through the shipborne dual-Doppler lobes. Later on the same day, NOAA P-3 aircraft sampled a more intense squall line located approximately 400 km to the southeast of the shipborne operations. This study provides an overview of the shipborne dual-Doppler operations, followed by a comparison of the kinematic and precipitation structures of the convective systems sampled by the ships and aircraft. Special emphasis is placed on interpretation of the results relative to the electrical characteristics of each system.

Soundings taken in the vicinity of the ship and aircraft cases exhibited similar thermodynamic instability and shear. Yet Doppler radar analyses suggest that the aircraft case exhibited a larger degree of low-level forcing, stronger updrafts, more precipitation mass in the mixed-phase region of the clouds, and a relatively higher degree of electrification as evidenced by lightning observations. Conversely, convection in the ship case, while producing maximum cloud-top heights of 16 km, was associated with relatively weaker low-level forcing, weaker vertical development above the −5°C level, moderate electric fields at the surface, and little detectable lightning. Differences in the kinematic and precipitation structures were further manifested in composite vertical profiles of mean convective precipitation and vertical motion. When considered relative to the electrical properties of the two systems, the results provide further circumstantial evidence to support previously hypothesized vertical velocity and radar reflectivity thresholds that must be exceeded in the 0° to −20°C regions of tropical cumulonimbi prior to the occurrence of lightning.

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.