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Paul J. Neiman

An exceptional concentric halo display was observed in Boulder, Colorado, on 21 July 1986. As many as six halos were observed and photographed simultaneously between 1530 and 1945 UTC. Extensive photographic documentation (>100 photographs) captured the evolution of the display. The halos were produced by solid, pyramidal ice crystals in the upper troposphere. The pyramidal crystals possessed six different prism angles, each of which produced a halo with a different angular radius (9°, 18°, 20°, 22°, 24°, and 35° halos). The crystals may have been formed on the previous day by thunderstorms over the southwestern United States.

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Paul J. Neiman and M. A. Shapiro

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The Experiment on Rapidly Intensifying Cyclones over the Atlantic was carried out over the western North Atlantic Ocean to provide temporally continuous comprehensive datasets from which to document the life cycle of extratropical marine cyclones. The most intense cyclogenetic event occurred on 4-5 January 1989 over the warm (>20°C) Gulf Stream current; the cyclone's central sea level pressure decreased by 60 mb in 24 h, from 996 to 936 mb. This study presents the synoptic-scale and mesoscale life cycle of this cyclone in two parts. Part I, presented here, describes the 24-h frontal-cyclone evolution through 6-h analyses of observations taken by specially deployed observing systems from air, land, and sea. The analyses of temperature, wind, and pressure about the incipient cyclone first illustrate the precursor signatures to cyclogenesis. The 850- and 500-mb temperature evolutions show a significant departure from the Norwegian frontal-cyclone model. In particular, the 850-mb analyses document 1) a storm-relative westward development of the warm front as a bent-back front into the polar airstream, and 2) the formation of a warm-core frontal seclusion in the post-cold-frontal cool air at the southwestern tip of the bent-back front. Analyses of sea level pressure provide a detailed account of cyclone intensification along the bent-back front. Infrared satellite imagery shows the evolution and immense size (∼5000 km) of the cyclone's cloud signature, and a 250-km-scale comma-cloud system in the vicinity of the warm-core seclusion situated at the southwestern tip of the large-scale comma head. Thermodynamic air-sea interaction diagnostics reveal large upward fluxes of heat and moisture from the sea surface into the marine boundary layer of the evolving cyclone. The maximum of combined upward flux approached 3000 W m, several times larger than that typically observed in both extratropical and tropical cyclones. These fluxes exhibited extreme spatial variability, reflecting the mesoscale characteristics of the cyclone circulation.

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Paul J. Neiman and Joseph A. Shaw

Scattering of sunlight or moonlight by cloud particles can generate colorful optical patterns that are both scientifically and aesthetically interesting. Photographs of corona rings and iridescence are presented to demonstrate how cloud-particle distributions and meteorology combine to produce a wide variety of observed patterns. The photographs of coronas are analyzed using Fraunhofer diffraction theory to determine that these optical displays were generated by cloud particles with mean diameters ranging from 7.6 to 24.3 μm. All examples of coronas and iridescence presented in this paper were observed within mountain wave clouds along the steep lee side of the Rocky Mountains over northeastern Colorado. Such clouds, commonly observed both here and on the downstream side of many other prominent mountain ranges, tend to have small cloud particles with narrow particle-size distributions, conditions that lead to relatively frequent and vivid optical displays. The meteorology accompanying at least one-half of the displays presented here suggest that the wave cloud particles consisted of ice, whereas, at least until recently, it has been accepted that spherical liquid cloud droplets are primarily responsible for coronas and iridescence. Microphotographs of particles collected from the interior of similar mountain wave clouds show that such clouds can indeed contain quasi-spherical ice particles with effective diameters less than 25 μm, which provide a mechanism for the high-quality optical displays to be generated within wave clouds at high altitudes with temperatures below −36° to −38°C. In fact, these quasi-spherical ice particles maybe commonly associated with mountain wave clouds, thus suggesting that this type of ice particle may regularly produce coronas and iridescence.

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Paul J. Neiman and Roger M. Wakimoto

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The study presented here describes the interactions that occurred between an advancing Pacific cold front and shallow Gulf of Mexico and Arctic air masses situated east of the Rocky Mountains during the Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX) field campaign on 17–18 April over Oklahoma and adjacent states. These interactions were driven largely by the complex topography of the region. Four air masses of distinctly different origin (i.e., Pacific polar, high-altitude continental, Gulf of Mexico, and Arctic), and the boundaries that separated them (i.e., Pacific cold front, dryline, and Arctic front), were observed within the experimental domain. This event produced more than $1 million worth of damage in the experimental domain due to severe weather. A dense network of ground-based in situ and remote observing systems and two research aircraft equipped with in situ sensors and Doppler radars gathered data that allowed the authors to document the passage of a vigorous midtropospheric shortwave trough and associated Pacific cold front, and the interaction of this front with the preexisting Gulf of Mexico and Arctic air masses. The Pacific front intersected the ground to the west of the Arctic frontal boundary and dryline, and subsequently rode over the top of the Gulf of Mexico and Arctic air masses. This study also presents the detailed observational documentation of a dryline–frontal merger by showing the merging or phasing of updrafts associated with the Pacific front and dryline and the subsequent development of a squall line. The behavior of the Arctic front is also explored in detail. Its anomalous southward penetration into the VORTEX domain due to terrain-induced blocking also played a role in producing severe weather.

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Paul J. Neiman and M. A. Shapiro

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Vertical wind shears measured by the Plattevilie, Colorado wind profiler were used in conjunction with the geostrophic thermal wind equation to retrieve the horizontal thermal gradients and associated advections for a case involving an upper-tropospheric jet stream/frontal zone on 23–24 November 1986. The profiler-retrieved thermal gradients and advections and their evolutions compared favorably with those observed by the operational rawinsonde network. The retrieval of horizontal temperature gradients by a single wind profiler is generally effective in quasi-balanced flow regimes, but becomes less reliable in flow regimes dominated by nonbalanced gravity wave activity. In quasi-balanced flow regimes this simple thermal retrieval technique can aid the operational community by monitoring baroclinic features and associated temperature advections on an hourly basis, rather than on a 12-hourly basis currently available through the operational rawinsonde network.

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Paul J. Neiman, Daniel J. Gottas, and Allen B. White

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This observational study of westward-directed gap flows through the Columbia River Gorge uses three radar wind profilers during two winter seasons between October 2015 and April 2017, with a focus on the gap-exit region at Troutdale, Oregon. Of the 92 gap-flow events identified at Troutdale, the mean duration was 38.5 h, the mean gap-jet speed was 12 m s−1, and the mean gap-flow depth was 570 m MSL. The mean gap-jet height and gap-flow depth were situated below the top of the inner gorge, while a maximum depth of 1087 m MSL was contained within the gorge’s outer-wall rim. The mean gap-flow depth was deepest in the cold-air source region east of the gorge and decreased westward to the coast. Strong gap-flow events were longer lived, deeper, and capped by stronger vertical shear than their weak counterparts, and strong (weak) events were forced primarily by a cold-interior anticyclone (offshore cyclone). Deep gap-flow events were longer lived, stronger, and had weaker capping vertical shear than shallow events, and represented a combination of gap-flow and synoptic forcing. Composite temporal analysis shows that gap-flow strength (depth) was maximized midevent (early event), freezing rain was most prevalent during the second half of the event, and accumulated precipitation was greatest late-event. Gap-flow events tended to begin (end) during the evening (morning) hours and were most persistent in January. Surface wind gusts and snow occurrences around Portland, Oregon, were associated primarily with the deepest gap flows, whereas freezing rain occurred predominantly during shallow gap flows.

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Paul J. Neiman, Peter T. May, and M. A. Shapiro

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The National 0ceanic and Atmospheric Administration (NOAA) Wave Propagation Laboratory (WPL) wind profilers and accompanying radio acoustic sounding system (RASS) temperature profilers in eastern Colorado jointly measure nearly continuous (≤1 h), high vertical resolution (≤300 m) wind-velocity and virtual-temperature profiles. This study presents NOAA/WPL wind profiler and RASS observations and diagnostics of propagating lower- and midtropospheric weather systems over Colorado. The wind and temperature remote-sensing systems observed wind-velocity and virtual-temperature structures associated with a synoptic-scale trough and embedded fronts, and a propagating short-wave trough and trailing midtropospheric jet-stream-frontal-zone (jet-front) system. Single-station hourly diagnostic calculations of geopotential heights, horizontal virtual potential temperature gradients, thermal advections, vertical velocities gradient Richardson numbers, and cross-frontal isentropic potential vorticity demonstrate that dynamically consistent synoptic-scale and mesoscale signals can be obtained by combining wind profiler and RASS observations. The wind profilers and RASS documented mesoscale wind velocity and thermal features up to 400 mb that were unresolved temporally and spatially by the synoptic-scale rawinsonde network. Results demonstrate the potential for multisystem (network) applications of this revolutionary technology for describing the temporal and spatial evolution of synoptic-scale and mesoscale weather systems.

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Brooks E. Martner, Paul J. Neiman, and Allen B. White

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A strong elevated temperature inversion in a landfalling winter storm in northern California produced two simultaneous melting layers with associated radar bright bands. The storm was observed with scanning and profiling radars. Serial radiosonde launches from the scanning radar site precisely documented the evolving temperature structure of the air mass that produced the double bright band. The radiosonde and radar observations, which were coincident in location and time, clearly illustrate the cause (two melting layers) and effect (two bright bands) of this unusual phenomenon. An automated algorithm for determining the melting-layer height from profiling radar data was tested on this situation. In its operational form, the algorithm detects only the lower melting layer, but in modified form it is capable of detecting both melting layers simultaneously.

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Fang-Ching Chien, Clifford F. Mass, and Paul J. Neiman

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This paper presents an observational and numerical study of an intense wintertime cold front that made landfall along the northwest coast of the United States during IOP 2 (3–4 Dec 1993) of the Coastal Observation and Simulation with Topography experiment. Observations suggest that the offshore frontal zone was associated with two transitions: the first characterized by a substantial temperature gradient, a nearly vertically oriented wind shift from southwesterly ahead to westerly behind, and a convective rainband, while the second transition had a slantwise wind shift zone from westerly ahead to northwesterly behind. The frontal zone was quite narrow (∼5 km wide) and nearly vertical below 850 mb, and its width increased by 1–2 orders of magnitude above 850 mb.

Well before the frontal landfall, low-level flow to the west of the Olympics was associated with geostrophic balance in the cross-shore direction and downgradient acceleration in the alongshore direction, which contributed to the formation of strong coastal southerlies roughly within ∼130 km off the coast. The front started weakening approximately 80 km upstream from the coast. As the front moved closer to the coast, the westerly wind component decreased toward the coastline, which was contributed by both an offshore-directed pressure gradient force and friction as suggested by the force balance result. During landfall, the thermal evolution indicated that the low-level front was delayed by the Olympics, while it could advance farther inland to the north and to the south. Over the water of the Strait of Juan de Fuca the front maintained its integrity at low levels and its thermal gradient even increased as a result of tilting effect, in contrast to the distinct weakening over land. After the frontal landfall, strong northwesterly flow behind the front was greatly modified by the mountains: winds over the ocean were forced to turn into more westerly, and winds over the barrier were substantially disturbed.

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David E. Kingsmill, Paul J. Neiman, and Allen B. White

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This study examines the impact of microphysics regime on the relationship between orographic forcing and orographic rain in the coastal mountains of Northern California using >4000 h of data from profiling Doppler radars, rain gauges, and a GPS receiver collected over 10 cool seasons. Orographic forcing is documented by hourly upslope flow, integrated water vapor (IWV), and IWV flux observed along the coast at Bodega Bay (BBY; 15 m MSL). Microphysics regime is inferred in the coastal mountains at Cazadero (CZC; 478 m MSL), where hourly periods of brightband (BB) and nonbrightband (NBB) rain are designated. BB rain is associated with a microphysics regime dominated by the seeder–feeder process while NBB rain is associated with a microphysics regime dominated by the warm-rain process. Mean BBY upslope flow, IWV, and IWV flux are ~16%, ~5%, and ~19% larger, respectively, for NBB rain compared to BB rain, while mean CZC rain rate is ~33% larger for BB rain compared to NBB rain. The orographic enhancement ratio of CZC to BBY rain rate is 3.7 during NBB rain and 2.7 during BB rain. Rain rate at CZC increases as orographic forcing at BBY increases. For a given amount of BBY orographic forcing, mean CZC rain rates are larger for BB rain compared to NBB rain. Correlation coefficients associated with the relationship between CZC rain rate and BBY orographic forcing are smaller for NBB rain relative to BB rain, but these differences are not statistically significant.

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