An F5 tornado that devastated Plainfield, Illinois, and environs on 28 August 1990, killing 29 people, is shown to be produced by a thunderstorm characterized by highly anomalous cloud-to-ground (CG) lightning activity. Unlike typical summertime convection in which the majority of CG flashes lower negative charge to ground, the Plainfield storm produced predominantly positive-polarity CG flashes during development. Changes in storm structure revealed by radar imagery appear tied to distinct patterns in the CG flash parameters of polarity, flash frequency, first stroke peak current, flash multiplicity, and flash location relative to the parent cumulonimbus. The primary findings are 1) the anomalous predominance (91%) of positive-polarity CG flashes during development; 2) positive CG flashes anomalously occurring mainly within the region of the storm's radar reflectivity core; 3) the onset of a major downburst coinciding with a sudden increase in CG flash rate, from 4 to 17 flashes min-1, and positive percentage, from 91% to 100%; 4) a reduction in flash rate from 17 to 3 flashes min⁻¹ in 3 min, coinciding with the rapid development of a front-flank mesocyclone; 5) a 20-min span of reduced CG activity (1–2 flashes min⁻¹) coinciding with tornado formation and intensification; 6) the reversal in dominant CG flash polarity from positive to negative over the entire thunderstorm domain at the time of tornado touchdown, an occurrence previously undocumented in any other tornadic thunderstorm; 7) steadily weakening mean peak current values (from +100 to +38 kA) leading to the reversal, and steadily strengthening values (from −20 to − 40 kA) following the reversal; and 8) the temporary clustering of all CG flashes within 10 km of the tornadic mesocyclone at maximum (F5) tornadic intensity. These findings suggest the possibility of a relationship between this tornadic thunderstorm's dynamics and electrical activity.

Abstract

A remarkable long-lived, large-amplitude gravity wave in the Carolinas and Virginia on 27 February 1984 is investigated by means of a subsynoptic-scale case study. The wave was characterized by a minor-wave of elevation followed by a sharp wave of depression with a period of 2–3 h, a horizontal wavelength of 100–150 km and surface pressure perturbation amplitudes of 3–14 mb. The wave propagated toward the east-southeast at 1 5 m s⁻¹, accelerating to more than 20 m s⁻¹ after it crossed the Atlantic coast. Wave passage was accompanied by gusty surface easterly winds reaching 30 m s⁻¹ and an abrupt termination of precipitation with the rapid surface pressure fall. The synoptic criteria identified by Uccellini and Koch as common to many cases of large amplitude gravity waves were present in this case.

Gravity waves were first detected across western Tennessee and northern Mississippi in a wake depression region to the rear of an advancing squall line. An amplifying wave emerged out of the wave packet across the southern Appalachians as the downstream squall line was most intense. The wave, once organized, amplified still further in the cold air damming region east of the Appalachian mountains and followed the back edge of the precipitation shield to the coast. Meanwhile, a second gravity wave formed just to the north of the primary wave in southeastern Kentucky around 1300 UTC 27 February. It propagated rapidly northeastward at ∼30 m s⁻¹ as a zone of enhanced pressure fails superimposed on a broader region of synoptic pressure falls.

Geostrophic adjustment appeared to play a prominent role in the organization and intensification of the primary gravity wave, whereas both shearing instability and geostrophic adjustment contributed to the genesis of the second wave. A particularly important aspect of this case was the juxtaposition of prominent jets in the upper and lower troposphere in the region of wave formation and amplification. The low-level jet carded a plume of warm, moist unstable air northward over cooler, stable boundary layer air and helped to trigger a line of active convection in northern Georgia. The gravity wave, which organized and intensified to the rear of the convective line, appeared as a prominent wake depression in the mean sea level isobaric pattern. Forced subsidence to the rear of the convective line in the presence of a deep, cold and stable boundary layer may have contributed to wave amplification. East of the mountains the prominent stable layer or wave duct was capped by a deep layer of weak stability and strong vertical wind shear containing a critical layer, conditions favorable for wave trapping and wave reflectance. Wave propagation and maintenance was in excellent agreement with the Lindzen and Tung ducted gravity wave model. Dissipation occurred as the wave approached and crossed the coastal front boundary.

Abstract

The 31 May 2013 El Reno, Oklahoma, tornado is used to demonstrate how a video imagery database crowdsourced from storm chasers can be time-corrected and georeferenced to inform severe storm research. The tornado’s exceptional magnitude (∼4.3-km diameter and ∼135 m s⁻¹ winds) and the wealth of observational data highlight this storm as a subject for scientific investigation. The storm was documented by mobile research and fixed-base radars, lightning detection networks, and poststorm damage surveys. In addition, more than 250 individuals and groups of storm chasers navigating the tornado captured imagery, constituting a largely untapped resource for scientific investigation.

The El Reno Survey was created to crowdsource imagery from storm chasers and to compile submitted materials in a quality-controlled, open-access research database. Solicitations to storm chasers via social media and e-mail yielded 93 registrants, each contributing still and/or video imagery and metadata. Lightning flash interval is used for precise time calibration of contributed video imagery; when combined with georeferencing from open-source geographical information software, this enables detailed mapping of storm phenomena. A representative set of examples is presented to illustrate how this standardized database and a web-based visualization tool can inform research on tornadoes, lightning, and hail. The project database offers the largest archive of visual material compiled for a single storm event, accessible to the scientific community through a registration process. This approach also offers a new model for poststorm data collection, with instructional materials created to facilitate replication for research into both past and future storm events.

Abstract

An analysis is presented of prominent mesoscale structure in a moderately intense cyclone with emphasis on a long-lived, large-amplitude inertia–gravity wave (IGW) that moved through the northeastern United States on 4 January 1994. Available National Weather Service WSR-88D Doppler radar and wind profiler observations are employed to illustrate the rich, time-dependent, three-dimensional structure of the IGW. As the IGW amplified [peak crest-to-trough pressure falls exceeded 13 hPa (30 min)⁻¹], it also accelerated away from the cyclone, reaching a peak forward speed of 35–40 m s⁻¹ across eastern New England. The IGW was one of three prominent mesoscale features associated with the cyclone, the others being a weak offshore precursor warm-frontal wave and an onshore band of heavy snow (“snow bomb”) in which peak hourly snowfalls of 10–15 cm were observed. None of these three prominent mesoscale features were well forecast by existing operational prediction models, particularly with regard to precipitation amount, onset, and duration. The observed precipitation discrepancies illustrate the subtle but important effects of subsynoptic-scale disturbances embedded within the larger-scale cyclonic circulation. The precursor offshore warm-frontal wave was instrumental in reinforcing the wave duct preceding the IGW. The snow bomb was an indication of vigorous ascent, large upper- (lower-) level divergence (convergence), unbalanced flow, and associated large parcel accelerations, environmental conditions known to be favorable for IGW formation.

Small-amplitude IGWs (<1 hPa) are first detected over the southeastern United States from surface microbarogram records and are confirmed independently by the presence of organized and persistent mesoscale cloud bands oriented approximately along the wave fronts. The area of IGW genesis is situated poleward of a weak surface frontal boundary where there is a weak wave duct (stable layer) present in the lower troposphere. In the upper troposphere the region of IGW genesis is situated on the forward side of a deep trough where there is significant cyclonic vorticity advection by the thermal wind. Diagnostic evidence supports the importance of shearing instability and/or unbalanced flow in IGW genesis.

The large-amplitude IGW originates on the downstream edge of the northeastward-advancing packet of small-amplitude IGWs. Wave amplification occurs near the upshear edge of a high, cold cloud shield that generally marks the warm conveyor belt. Although it is not possible to conclusively state whether the amplifying IGW forms in situ or grows from a predecessor weaker (<1 hPa) disturbance, rapid amplification occurs 1) as the wave encounters an increasingly deeper and stronger wave duct, possibly permitting wave overreflection, in the cold air damming region east of the Appalachians, and 2) downshear of an area of significantly positive unbalanced divergence and parcel divergence tendency. The authors raise the possibility that IGW amplification can be associated with the penetration and perturbation of the wave duct by vigorous subsynoptic-scale vertical motions whose vigor is increased by wave-induced latent heat release.

Abstract

The process of tornadogenesis in complex terrain environments has received relatively little research attention to date. Here, an analysis is presented of a long-lived supercell that became tornadic over complex terrain in association with the Great Barrington, Massachusetts (GBR), F3 tornado of 29 May 1995. The GBR tornado left an almost continuous 50–1000-m-wide damage path that stretched for ∼50 km. The apparent rarity of significant tornadogenesis in rough terrain from a supercell well documented in operational Doppler radar motivated this case study. Doppler radar observations showed that the GBR supercell possessed a midlevel mesocyclone well prior to tornadogenesis and that the mesocyclone intensified as it crossed the eastern edge of New York’s Catskill Mountains and entered the Hudson Valley. Tornadogenesis occurred as the GBR mesocyclone crossed the Hudson Valley and ascended the highlands to the east. Subsequently, the mesocyclone weakened as it approached the Taconic Range in western Massachusetts before it intensified again as it moved downslope into the Housatonic Valley where it was associated with the GBR tornado. Because of a dearth of significant mesoscale surface and upper-air observations, the conclusions and inferences presented in this paper must be necessarily limited and speculative. What data were available suggested that on a day when the mesoscale environment was supportive of supercell thunderstorm development, according to conventional indicators of wind shear and atmospheric stability, topographic configurations and the associated channeling of ambient low-level flows conspired to create local orographic enhancements to tornadogenesis potential. Numerical experimentation is needed to address these inferences, speculative points, and related issues raised by the GBR case study.

Abstract

The tropical Andes of southern Peru and northern Bolivia have several major mountain summits suitable for ice core paleoclimatic investigations. However, incomplete understanding of the controls on the isotopic (δD, δ ¹⁸O) composition of precipitation and a paucity of field observations in this region continue to limit ice-core-based paleoclimate reconstructions. This study examines four years of daily observations of δD and δ ¹⁸O in precipitation from a citizen scientist network on the northeastern margin of the Altiplano, to identify controls on the subseasonal spatiotemporal variability in δ ¹⁸O during the wet season (November–April). These data provide new insights into modern δ ¹⁸O variability at high spatial and temporal scales. We identify a regionally coherent subseasonal signal in precipitation δ ¹⁸O featuring alternating periods of high and low δ ¹⁸O of 9–27-day duration. This signal reflects variability in precipitation delivery driven by synoptic conditions and closely relates to variations in the strength of the South American low-level jet and moisture availability over the study area. The annual layer of snowpack on the Quelccaya Ice Cap observed in the subsequent dry season retains this subseasonal signal, allowing the development of a snow-pit age model based on precipitation δ ¹⁸O measurements, and demonstrating how synoptic variability is transmitted from the atmosphere to mountaintop snowpacks along the Altiplano’s eastern margin. This result improves our understanding of the hydrometeorological processes governing δ ¹⁸O and δD in tropical Andean precipitation and has implications for improving paleoclimate reconstructions from tropical Andean ice cores and other paleoclimate records.

Abstract

Over the course of his career, Fuqing Zhang drew vital new insights into the dynamics of meteorologically significant mesoscale gravity waves (MGWs), including their generation by unbalanced jet streaks, their interaction with fronts and organized precipitation, and their importance in midlatitude weather and predictability. Zhang was the first to deeply examine “spontaneous balance adjustment”—the process by which MGWs are continuously emitted as baroclinic growth drives the upper-level flow out of balance. Through his pioneering numerical model investigation of the large-amplitude MGW event of 4 January 1994, he additionally demonstrated the critical role of MGW–moist convection interaction in wave amplification. Zhang’s curiosity-turned-passion in atmospheric science covered a vast range of topics and led to the birth of new branches of research in mesoscale meteorology and numerical weather prediction. Yet, it was his earliest studies into midlatitude MGWs and their significant impacts on hazardous weather that first inspired him. Such MGWs serve as the focus of this review, wherein we seek to pay tribute to his groundbreaking contributions, review our current understanding, and highlight critical open science issues. Chief among such issues is the nature of MGW amplification through feedback with moist convection, which continues to elude a complete understanding. The pressing nature of this subject is underscored by the continued failure of operational numerical forecast models to adequately predict most large-amplitude MGW events. Further research into such issues therefore presents a valuable opportunity to improve the understanding and forecasting of this high-impact weather phenomenon, and in turn, to preserve the spirit of Zhang’s dedication to this subject.

Abstract

Rapid-scan radar observations of a supercell that produced near-record size hail in Oklahoma are examined. Data from the National Weather Radar Testbed Phased Array Radar (PAR) in Norman, Oklahoma, are used to study the overall character and evolution of the storm. Data from the nearby polarimetric KOUN WSR-88D and rapid-scanning X-band polarimetric (RaXPol) mobile radar are used to study the evolution of low- to midaltitude dual-polarization parameters above two locations where giant hailstones up to 16 cm in diameter were observed. The PAR observation of the supercell’s maximum storm-top divergent outflow is similar to the strongest previously documented value. The storm’s mesocyclone rotational velocity at midaltitudes reached a maximum that is more than double the median value for similar observations from other storms producing giant hail. For the two storm-relative areas where giant hail was observed, noteworthy findings include 1) the giant hail occurred outside the main precipitation core, in areas with low-altitude reflectivities of 40–50 dBZ; 2) the giant hail was associated with dual-polarization signatures consistent with past observations of large hail at 10-cm wavelength, namely, low Z _DR, low ρ _HV, and low K _DP; 3) the giant hail fell along both the northeast and southwest edges of the primary updraft at ranges of 6–10 km from the updraft center; and 4) with the exception of one isolated report, the giant hail fell to the northeast and northwest of the large tornado and the parent mesocyclone.

Abstract

This study used the first detailed radar measurements of the vertical structure of precipitation obtained in the central Andes of southern Peru and Bolivia to investigate the diurnal cycle and vertical structure of precipitation and melting-layer heights in the tropical Andes. Vertically pointing 24.1-GHz Micro Rain Radars in Cusco, Peru (3350 m MSL, August 2014–February 2015), and La Paz, Bolivia (3440 m MSL, October 2015–February 2017), provided continuous 1-min profiles of reflectivity and Doppler velocity. The time–height data enabled the determination of precipitation timing, melting-layer heights, and the identification of convective and stratiform precipitation features. Rawinsonde data, hourly observations of meteorological variables, and satellite and reanalysis data provided additional insight into the characteristics of these precipitation events. The radar data revealed a diurnal cycle with frequent precipitation and higher rain rates in the afternoon and overnight. Short periods with strong convective cells occurred in several storms. Longer-duration events with stratiform precipitation structures were more common at night than in the afternoon. Backward air trajectories confirmed previous work indicating an Amazon basin origin of storm moisture. For the entire dataset, median melting-layer heights were above the altitude of nearby glacier termini approximately 17% of the time in Cusco and 30% of the time in La Paz, indicating that some precipitation was falling as rain rather than snow on nearby glacier surfaces. During the 2015–16 El Niño, almost half of storms in La Paz had melting layers above 5000 m MSL.

Abstract

The predictability of the weather on Mount Everest’s upper slopes can be a matter of life or death for those trying to climb the world’s highest mountain, yet the performance of forecasts has been almost unknown due to a lack of surface observations. The extent to which climate change may be affecting this iconic location is also uncertain for the same reason. To address this data limitation, the National Geographic and Rolex Perpetual Planet Expedition installed the world’s highest weather station network (reaching within 420 m of the summit) on the Nepal side of Mount Everest in 2019. Its observations have already generated considerable advances in understanding the meteorological environment on the mountain’s upper slopes, but the network was compromised by damage to the highest stations in recent years. Here, we describe the expedition that upgraded the network and took it to new heights, focusing on the installation at the Bishop Rock (8,810 m MSL), just below the summit. Almost 70 years after Everest was first climbed successfully, we can now provide open access data to illuminate conditions at Earth’s highest climate frontier.