Timothy J. Lang

Environmental controls on storm intensity and charge structure in multiple regions of the continental United States

A database consisting of approximately 4000 storm observations has been objectively analyzed to determine environmental characteristics that produce high radar reflectivities above the freezing level, large total lightning flash rates on the order of 10 flashes per minute, and anomalous vertical charge structures (most notably, dominant midlevel positive charge). The storm database is drawn from four regions of the United States featuring distinct environments, each with coinciding Lightning Mapping Array (LMA) network data. LMAs are able to infer total lightning flash rates using flash clustering algorithms, such as the one implemented in this study. Results show that anomalous charge structures inferred from LMA data, significant lightning flash rates, and increased radar reflectivities above the freezing level tend to be associated with environments that have high cloud base heights (approximately 3 km above ground level) and large atmospheric instability, quantified by normalized convective available potential energy (NCAPE) near 0.2 m s−2. Additionally, we infer that aerosols may affect storm intensity. Maximum flash rates were observed in storms with attributed aerosol concentrations near 1000 cm−3, while total flash rates decrease when aerosol concentrations exceed 1500 cm−3, consistent with previous studies. However, this effect is more pronounced in regions where the NCAPE and cloud base height are low. The dearth of storms with estimated aerosol concentrations less than 700 cm−3 (approximately 1% of total sample) does not provide a complete depiction of aerosol invigoration.

Synoptic scale outbreak of self‐initiated upward lightning (SIUL) from tall structures during the central U.S. blizzard of 1–2 February 2011

A major central U.S. winter cyclone on 1–2 February 2011 produced a band of high winds, up to 75 cm of snow, and numerous reports of thundersnow from Oklahoma into Ontario over a 26 h period. The National Lightning Detection Network (NLDN) recorded 282 flashes comprised of 1153 events which were >96% negative polarity. Hopes of imaging winter sprites associated with energetic positive cloud-to-ground events that sometimes accompany such winter storms did not materialize. However, the lack of lightning over the Great Lakes waters, plus media reports of numerous thundersnow events in downtown Chicago, prompted a detailed analysis of the NLDN data. This revealed that >93% of all lightning in the snow band was likely or possibly associated with self-initiated upward lightning (SIUL) events from a variety of tall, and some not so tall, structures. In addition to 43 events from two Chicago skyscrapers, many shorter structures were involved, including wind turbines (13.1% of the total) and transmission line towers (6.7%). Wind speeds for all events exceeded the 8 m s−1 minimum threshold associated with SIULs in Japanese winter lightning storms. Radar reflectivities at the event locations had a mean of 28 dBZ and were almost always <35 dBZ. While conventional radar displays suggested stratiform precipitation in the thundersnow region, detailed analysis of 3-D-gridded NMQ (National Mosaic and Multi-Sensor Quantitative Precipitation Estimation) radar reflectivity data confirmed elevated embedded cellular convection spanning the −10°C region associated with isentropic lifting above a frontal surface, evidence of noninductive charge generation sufficient to allow upward leader initiation from tall objects.

Diurnal Characteristics of Lightning Flashes Detected Over the São Paulo Lightning Mapping Array

This study examines diurnal variations of lightning flash characteristics observed by the Lightning Mapping Array in Sao Paulo, Brazil. The diurnal flash counts exhibit the typical afternoon convective maximum. The mean source altitude demonstrates a discrete increase that is temporally coincident with the local sunrise. The mean horizontal and vertical flash extents each attain a maximum (minimum) around local sunrise (afternoon, i.e., 13:00–17:00 local solar time). In addition, joint histograms of flash horizontal and vertical extents show that the majority of the flashes occurring during the afternoon convection are shorter and more comparable in size, and the differences between the horizontal and vertical extents are reduced. Conversely, flashes preceding and following the peak in afternoon convection are less symmetric, with larger horizontal than vertical extents. We discuss whether these observations could be partially explained by the diurnal variations in the convectively induced mixing regimes that control thundercloud charge regions and associated charge separation distances. The documented diurnal flash characteristics closely match recently published findings on the diurnal variation of the peak currents of cloud-to-ground flashes. Possible physical mechanisms for these observations are discussed.

Using a multiwavelength suite of microwave instruments to investigate the microphysical structure of deep convective cores

Abstract Due to the large natural variability of its microphysical properties, the characterization of solid precipitation is a longstanding problem. Since in situ observations are unavailable in severe convective systems, innovative remote sensing retrievals are needed to extend our understanding of such systems. This study presents a novel technique able to retrieve the density, mass, and effective diameter of graupel and hail in severe convection through the combination of airborne microwave remote sensing instruments. The retrieval is applied to measure solid precipitation properties within two convective cells observed on 23–24 May 2014 over North Carolina during the IPHEx campaign by the NASA ER‐2 instrument suite. Between 30 and 40 degrees of freedom of signal are associated with the measurements, which is insufficient to provide full microphysics profiling. The measurements have the largest impact on the retrieval of ice particle sizes, followed by ice water contents. Ice densities are mainly driven by a priori assumptions, though low relative errors in ice densities suggest that in extensive regions of the convective system, only particles with densities larger than 0.4 g/cm3 are compatible with the observations. This is in agreement with reports of large hail on the ground and with hydrometeor classification derived from ground‐based polarimetric radars observations. This work confirms that multiple scattering generated by large ice hydrometeors in deep convection is relevant for airborne radar systems already at Ku band. A fortiori, multiple scattering will play a pivotal role in such conditions also for Ku band spaceborne radars (e.g., the GPM Dual Precipitation Radar).

Hail-Detection Algorithm for the GPM Core Observatory Satellite Sensors

AbstractBy exploiting an abundant number of extreme storms observed simultaneously by the Global Precipitation Measurement (GPM) mission Core Observatory satellite’s suite of sensors and by the ground-based S-band Next Generation Weather Radar (NEXRAD) network over the continental United States, proxies for the identification of hail are developed from the GPM Core Observatory satellite observables. The full capabilities of the GPM Core Observatory are tested by analyzing more than 20 observables and adopting the hydrometeor classification on the basis of ground-based polarimetric measurements being truth. The proxies have been tested using the critical success index (CSI) as a verification measure. The hail-detection algorithm that is based on the mean Ku-band reflectivity in the mixed-phase layer performs the best of all considered proxies (CSI of 45%). Outside the dual-frequency precipitation radar swath, the polarization-corrected temperature at 18.7 GHz shows the greatest potential for hail detection...

Observations of two sprite-producing storms in Colorado

Two sprite-producing thunderstorms were observed on 8 and 25 June 2012 in northeastern Colorado by a combination of low-light cameras, a lightning mapping array, polarimetric and Doppler radars, the National Lightning Detection Network, and charge moment change measurements. The 8 June event evolved from a tornadic hailstorm to a larger multicellular system that produced 21 observed positive sprites in 2 h. The majority of sprites occurred during a lull in convective strength, as measured by total flash rate, flash energy, and radar echo volume. Mean flash area spiked multiple times during this period; however, total flash rates still exceeded 60 min(sup 1), and portions of the storm featured a complex anomalous charge structure, with midlevel positive charge near 20degC. The storm produced predominantly positive cloud-to-ground lightning. All sprite-parent flashes occurred on the northeastern flank of the storm, where strong westerly upper level flow was consistent with advection of charged precipitation away from convection, providing a pathway for stratiform lightning. The 25 June event was another multicellular hailstorm with an anomalous charge structure that produced 26 positive sprites in less than 1 h. The sprites again occurred during a convective lull, with relatively weaker reflectivity and lower total flash rate but relatively larger mean flash area. However, all sprite parents occurred in or near convection and tapped charge layers in adjacent anvil cloud. The results demonstrate the sprite production by convective ground strokes in anomalously charged storms and also indicate that sprite production and convective vigor are inversely related in mature storms.

Large charge moment change lightning on 31 May to 1 June 2013, including the El Reno tornadic storm

On 31 May 2013, a line of severe tornadic thunderstorms (the El Reno event) developed during the local afternoon in central Oklahoma, USA. Within range of the Oklahoma Lightning Mapping Array, the evolution of the event can be separated into three distinct periods: an Early period (before 02:00 UTC on 1 June) when the storm consisted of discrete supercells, a Middle period (02:00–05:00 UTC) when the convection began merging into a linear feature and stratiform precipitation developed, and a Late period (after 05:00 UTC) featuring a mature mesoscale convective system (MCS). Each of these periods demonstrated distinct patterns in the large (>100 C km) charge moment change (CMC) lightning that was produced. During the Early period, large-CMC positive cloud-to-ground (+CG) lightning was produced in the convective cores of supercells. These flashes were small in area (typically 500 km2, >300 C km) in the developing stratiform, similar to typical sprite-parent lightning in MCSs. During the Late period, convective large CMC +CGs ceased and instead large-CMC negative CGs were produced in and near the MCS convection. These flashes neutralized charge both in convection as well as in adjacent stratiform and anvil precipitation. The results suggest that the CMC metric has potential applications for studying tropospheric weather.

A New Paradigm in Earth Environmental Monitoring with the CYGNSS Small Satellite Constellation

A constellation of small, low-cost satellites is able to make scientifically valuable measurements of the Earth which can be used for weather forecasting, disaster monitoring, and climate studies. Eight CYGNSS satellites were launched into low Earth orbit on December 15, 2016. Each satellite carries a science radar receiver which measures GPS signals reflected from the Earth surface. The signals contain information about the surface, including wind speed over ocean, and soil moisture and flooding over land. The satellites are distributed around their orbit plane so that measurements can be made more often to capture extreme weather events. Innovative engineering approaches are used to reduce per satellite cost, increase the number in the constellation, and improve temporal sampling. These include the use of differential drag rather than propulsion to adjust the spacing between satellites and the use of existing GPS signals as the science radars’ transmitter. Initial on-orbit results demonstrate the scientific utility of the CYGNSS observations, and suggest that a new paradigm in spaceborne Earth environmental monitoring is possible.

Investigating the Seasonal and Diurnal Cycles of Ocean Vector Winds Near the Philippines Using RapidScat and CCMP

The seasonal and diurnal cycles of ocean vector winds in the domain of the South China Sea are characterized and compared using RapidScat and the Cross-Calibrated Multi-Platform (CCMP) data sets. Broad agreement in seasonal flow patterns exists between these data sets during the year 2015. Both observe the dramatic reversal from wintertime trade winds (November-April) to westerly flow associated with the summer monsoon (May-October). These seasonal changes have strong but not equivalent effects on mean wind divergence patterns in both data sets. Specifically near the Philippines, the data sets agree on several aspects of the seasonal mean and diurnal cycle of near-surface vector winds and divergence. In particular, RapidScat and CCMP agree that daytime onshore and nocturnal offshore flow patterns affect the diurnal cycle of winds up to ~200 km west of Luzon, Philippines. Observed disagreements over the diurnal cycle are explainable by measurement uncertainty, as well as shortcomings in both data sets.

Characteristics of Lightning Within Electrified Snowfall Events Using Lightning Mapping Arrays

This study examined 34 lightning flashes within four separate thundersnow events derived from lightning mapping arrays (LMAs) in northern Alabama, central Oklahoma, and Washington DC. The goals were to characterize the in-cloud component of each lightning flash, as well as the correspondence between the LMA observations and lightning data taken from national lightning networks like the National Lightning Detection Network (NLDN). Individual flashes were examined in detail to highlight several observations within the dataset. The study results demonstrated that the structures of these flashes were primarily normal polarity. The mean area encompassed by this set of flashes is 375 km2, with a maximum flash extent of 2300 km2, a minimum of 3 km2, and a median of 128 km2. An average of 2.29 NLDN flashes were recorded per LMA-derived lightning flash. A maximum of 11 NLDN flashes were recorded in association with a single LMA-derived flash on 10 January 2011. Additionally, seven of the 34 flashes in the study contain zero NLDN identified flashes. Eleven of the 34 flashes initiated from tall human-made objects (e.g., communication towers). In at least six lightning flashes, the NLDN detected a return stroke from the cloud back to the tower and not the initial upward leader. This study also discusses lightnings interaction with the human built environment and provides an example of lightning within heavy snowfall observed by GOES-16s Geostationary Lightning Mapper.