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.

Abstract

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.

Abstract

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.

Abstract

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.

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.

Abstract

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.

Abstract

This study uses a unique combination of airborne and satellite observations to characterize narrow regions of strong horizontal water vapor flux associated with polar cold fronts that occurred over the eastern North Pacific Ocean during the winter of 1997/98. Observations of these “atmospheric rivers” are compared with past numerical modeling studies to confirm that such narrow features account for most of the instantaneous meridional water vapor transport at midlatitudes.

Wind and water vapor profiles observed by dropsondes deployed on 25–26 January 1998 during the California Land-falling Jets Experiment (CALJET) were used to document the structure of a modest frontal system. The horizontal water vapor flux was focused at low altitudes in a narrow region ahead of the cold front where the combination of strong winds and large water vapor content were found as part of a low-level jet. A close correlation was found between these fluxes and the integrated water vapor (IWV) content. In this case, 75% of the observed flux through a 1000-km cross-front baseline was within a 565-km-wide zone roughly 4 km deep. This zone contained 1.5 × 10⁸ kg s⁻¹ of meridional water vapor flux, the equivalent of ∼20% of the global average at 35°N.

By compositing polar-orbiting satellite Special Sensor Microwave Imager (SSM/I) data from 46 dates containing long, narrow zones of large IWV, it was determined that the single detailed case was representative of the composite in terms of both the IWV amplitude (3.09 cm vs 2.81 cm) and the width of the area where IWV ≥ 2 cm (424 km vs 388 km). The SSM/I composites also showed that the width scales (defined by the 75% cumulative fraction along a 1500-km cross-plume baseline) for cloud liquid water and rain rate were 176 and 141 km, respectively, which are narrower than the 417 km for IWV. Examination of coincident Geostationary Operational Environmental Satellite (GOES) and SSM/I satellite data revealed that GOES cloud-top temperatures were coldest and cloud-top pressures were lowest in the core of the IWV plumes, and that the core cloud tops became substantially colder and deeper for larger IWV. A strong latitudinal dependence of the satellite-derived cross-river characteristics was also found.

Atmospheric rivers form a critical link between weather and climate scales. They strongly influence both short-term weather and flood prediction, as well as seasonal climate anomalies and the global water cycle, through their cumulative effects. However, the rivers remain poorly observed by the existing global atmospheric observing system in terms of their horizontal water vapor fluxes.

Abstract

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.

Abstract

Dropsonde observations are used to document the mean vertical profiles of kinematic and thermodynamic conditions in the pre-cold-frontal low-level-jet (LLJ) region of extratropical cyclones over the eastern Pacific Ocean. This is the region within storms that is responsible not only for the majority of heavy rainfall induced by orography when such storms strike the coast, but also for almost all meridional water vapor transport at midlatitudes. The data were collected from NOAA’s P-3 aircraft in 10 storms during the California Land-falling Jets Experiment (CALJET) of 1998 and in 7 storms during the Pacific Land-falling Jets Experiment (PACJET) of 2001. The mean position of the dropsondes was 500 km offshore, well upstream of orographic influences. The availability of data from two winters that were characterized by very different synoptic regimes and by differing phases of ENSO—that is, El Niño in 1998 and La Niña in 2001—allowed examination of interannual variability.

The composite pre-cold-frontal profiles reveal a well-defined LLJ at 1.0-km altitude with a wind speed of 23.4 m s⁻¹ and a wind direction of 216.7°, as well as vertical shear characteristic of warm advection. The composite thermodynamic conditions were also documented, with special attention given to moist static stability due to the nearly saturated conditions that were prevalent. Although the dry static stability indicated very stable conditions (4.5 K km⁻¹), the moist static stability was approximately zero up to 2.8-km altitude. Although the composite winds, temperatures, and water vapor mixing ratios in 2001 differed markedly from 1998, the moist static stability remained near zero from the surface up to 2.8–3.0-km altitude for both seasons. Hence, orographic precipitation enhancement is favored in this sector of the storm, regardless of the phase of ENSO.

The dropsonde data were also used to characterize the depth and strength of atmospheric rivers, which are responsible for most of the meridional water vapor transport at midlatitudes. The vertically integrated along-river water vapor fluxes averaged 525 × 10⁵ kg s⁻¹ (assuming a 100-km-wide swath), while the meridional and zonal components were 387 × 10⁵ kg s⁻¹ and 302 × 10⁵ kg s⁻¹, respectively. Although the composite meridional transport in 2001 was less than half that in 1998 (230 × 10⁵ kg s⁻¹ versus 497 × 10⁵ kg s⁻¹), the characteristic scale height of the meridional water vapor transport remained constant; that is, 75% of the transport occurred below 2.25-km altitude.

Abstract

This is the second of two articles describing the evolving structure and selected physical processes within an intense extratropical marine cyclone observed during the Experiment on Rapidly Intensifying Cyclones over the Atlantic (ERICA) field program. Part I describes the 24-h frontal-cyclone evolution through 6-h horizontal analyses of observations taken by specially deployed observing systems from air, land, and sea. Part II presents frontal-scale and precipitation structures and physical processes from analyses based primarily on research aircraft observations taken during three phases of the cyclone's life cycle. Horizontal analyses at 350 m above ground level describe the cyclone's mesoscale frontal baroclinic structure and associated flow patterns. The vertical structure and evolution of the cyclone's cold front, warm front, and bent-back front are illustrated in cross-sectional analyses of potential temperature, wind velocity, potential vorticity (PV), front-relative transverse flow vectors, diabatic heating, and PV tendencies. Of particular interest are the lower-tropospheric positive PV anomalies within the warm front and within its bent-back extension westward into the polar airstream. Airborne radar reflectivities and Doppler velocities provide a detailed account of the precipitation elements and associated wind flow patterns in the vicinity of the fronts, including mesoconvective radar reflectivities of greater than 50 dBZ and cross-frontal convergent flow exceeding −20 × 10⁻⁴ s⁻¹. Time series traces of the 1-s aircraft observations show large and rapid changes in meteorological variables as the aircraft crossed the narrow frontal zones.