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John D. Locatelli, Mark T. Stoelinga, Peter V. Hobbs, and Jim Johnson

On 18 September 1992 a series of thunderstorms in Nebraska and eastern Colorado, which formed south of a synopticscale cold front and north of a Rocky Mountain lee trough, produced a cold outflow gust front that moved southeastward into Kansas, southeastern Colorado, and Oklahoma around sunset. When this cold outflow reached the vicinity of the lee trough, an undular bore developed on a nocturnally produced stable layer and moved through the range of the Dodge City WSR-88D Doppler radar. The radar data revealed that the undular bore, in the leading portion of a region of northwesterly winds about 45 km wide by 4 km high directly abutting the cold outflow, developed five undulations over the course of 3 h. Contrary to laboratory tank experiments, observations indicated that the solitary waves that composed the bore probably did not form from the enveloping of the head of the cold air outflow by the stable layer and the breaking off of the head of the cold air outflow. The synoptic-scale cold front subsequently intruded on the surface layer of air produced by the cold outflow, but there was no evidence for the formation of another bore.

Profiler winds, in the region affected by the cold air outflow and the undular bore, contained signals from nocturnally, southward-migrating birds (most likely waterfowl) that took off in nonfavorable southerly winds and remained aloft for several hours longer than usual, thereby staying ahead of the turbulence associated with the undular bore.

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Peter V. Hobbs, Thomas J. Matejka, Paul H. Herzegh, John D. Locatelli, and Robert A. Houze Jr.

Abstract

Detailed information is deduced on the mesoscale organization of precipitation, the structures of the clouds, the air flows associated with mesoscale rainbands, and the precipitation efficiencies and the mechanisms producing precipitation in the rainbands associated with a cold front. Measurements were obtained with quantitative reflectivity and Doppler radars, two instrumented aircraft, serial rawinsondes and a network of ground stations.

The regions of heaviest precipitation were organized into a complex mesoscale rainband in the warm-sector air ahead of the front, a narrow band of precipitation at the surface cold front, and four wide cold-frontal rainbands. The wide cold-frontal rainbands and the smaller mesoscale areas of precipitation within them moved with the velocities of the winds between ∼3—6 km. The narrow rainband, which was produced by strong convergence and convection in the boundary layer, moved with the speed of the cold front at the surface. A coupled updraft and downdraft was probably responsible for the heavy precipitation on the cold front being organized, on the small mesoscale, into ellipsoidal areas with similar orientations.

The precipitation efficiencies in the warm-sector and narrow cold-frontal rainbands were ∼40–50% and ∼30–50%, respectively. One of the wide cold-frontal rainbands, in which there was a steady production of ice panicles in the main updraft, had a precipitation efficiency of at least 80%, whereas another wide cold-frontal band, in which some precipitation evaporated before reaching the surface, had a precipitation efficiency of ∼20%.

Ice particles from shallow convective cells aloft played important roles in the production of precipitation in the wide cold-frontal rainbands and in some regions of the warm-sector rainband. These “seed” ice particles grew by aggregation and by the deposition of vapor as they fell through lower level “feeder” clouds. About 20% of the mass of the precipitation reaching the ground in the wide cold-frontal rainbands originated in the upper level “seeder” zones and ∼80% in the “feeder” zones.

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Mark T. Stoelinga, Peter V. Hobbs, Clifford F. Mass, John D. Locatelli, Brian A. Colle, Robert A. Houze Jr., Arthur L. Rangno, Nicholas A. Bond, Bradley F. Smull, Roy M. Rasmussen, Gregory Thompson, and Bradley R. Colman

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.

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