Total Number Of Flashes Research Articles

Lightning Evolution In Two North Central Florida Summer Multicell Storms and Three Winter/Spring Frontal Storms

AbstractWe present the first lightning evolution studies, via the Lightning Mapping Array (LMA) and radar, performed in North Central Florida. Parts of three winter/spring frontal storms (cold season) and two complete summer (warm season) multicell storms are studied. Storm parameters measured are as follows: total number of flashes, flash‐type classification, first flashes, flash initiation altitude, flash initiation power, flash rate (flashes per minute), charge structure, altitude and temperature ranges of the inferred charge regions, atmospheric isotherm altitude, radar base reflectivity (dBZ), and radar echo tops (EET). Several differences were found between summer multicell and winter/spring frontal storms in North Central Florida: (1) in winter/spring storms, the range of altitudes that all charge regions occupy is up to 1 km lower in altitude than in summer storms, as are the 0°C, −10°C, and −20°C isotherms; (2) lightning activity in summer storms is highly correlated with changes in radar signatures, in particular, echo tops; and (3) the LMA average initiation power of all flash types in winter/frontal storms is about an order of magnitude larger than that for summer storms. In relation to storms in other geographical locations, North Central Florida seasonal storms were found to have similarities in most parameters studied with a few differences, examples in Florida being (1) colder initiation altitudes for intracloud flashes, (2) charge regions occupying larger ranges of atmospheric temperatures, and (3) winter/spring frontal storms not having much lightning activity in the stratiform region.

Observation and analysis of electrical structure change and diversity in thunderstorms on the Qinghai-Tibet Plateau

A comprehensive observation on thunderstorms was conducted in the Qinghai area by using a very high frequency three-dimensional lightning mapping system and Doppler radar. The spatio-temporal evolution of the charge structure of the isolated thunderstorm was analyzed according to the developing process of thunderstorm, and the reasons for the change in charge structure diversity were studied. During the initial developing and mature stages of the thunderstorm, the charge structure was a steady negative dipole polarity, i.e., the negative charge region was above the positive charge region. Furthermore, the total number of flashes was lower during these two stages. During the thunderstorm's dissipation stage, the charge structure was varied and complicated, with a positive dipole, negative dipole, and a tripole charge structure changing and coexisting during this stage. This charge structure diversity was primarily caused by the collision and merging of two local thunderstorm cells, leading to a charge rearrangement and distribution and the formation of a new charge structure. The frequency of the negative cloud-to-ground and intracloud flashes increased sharply in the dissipation stage, reaching a maximum value. The increase in frequency of negative cloud-to-ground was mainly caused by the lower positive charge weakening during the dissipation stage. In addition, the relationship between charging regions and temperature layers was analyzed by combining sounding temperature data with the theory of a non-inductive charging mechanism.

Intense pulsed light annealing of copper zinc tin sulfide nanocrystal coatings

A promising method for forming the absorber layer in copper zinc tin sulfide [Cu2ZnSnS4 (CZTS)] thin film solar cells is thermal annealing of coatings cast from dispersions of CZTS nanocrystals. Intense pulsed light (IPL) annealing utilizing xenon flash lamps is a potential high-throughput, low-cost, roll-to-roll manufacturing compatible alternative to thermal annealing in conventional furnaces. The authors studied the effects of flash energy density (3.9–11.6 J/cm2) and number of flashes (1–400) during IPL annealing on the microstructure of CZTS nanocrystal coatings cast on molybdenum-coated soda lime glass substrates (Mo-coated SLG). The annealed coatings exhibited cracks with two distinct linear crack densities, 0.01 and 0.2 μm−1, depending on the flash intensity and total number of flashes. Low density cracking (0.01 μm−1, ∼1 crack per 100 μm) is caused by decomposition of CZTS at the Mo-coating interface. Vapor decomposition products at the interface cause blisters as they escape the coating. Residual decomposition products within the blisters were imaged using confocal Raman spectroscopy. In support of this hypothesis, replacing the Mo-coated SLG substrate with quartz eliminated blistering and low-density cracking. High density cracking is caused by rapid thermal expansion and contraction of the coating constricted on the substrate as it is heated and cooled during IPL annealing. Finite element modeling showed that CZTS coatings on low thermal diffusivity materials (i.e., SLG) underwent significant differential heating with respect to the substrate with rapid rises and falls of the coating temperature as the flash is turned on and off, possibly causing a build-up of tensile stress within the coating prompting cracking. Use of a high thermal diffusivity substrate, such as a molybdenum foil (Mo foil), reduces this differential heating and eliminates the high-density cracking. IPL annealing in presence of sulfur vapor prevented both low- and high-density cracking as well as blistering. However, grain growth was limited even after annealing with 400 flashes. This lack of grain growth is attributed to a difficulty of maintaining high sulfur vapor pressure and absence of alkali metal impurities when Mo foil substrates are used.

Variability of lightning activity in South/Southeast Asia during 1997–98 and 2002–03 El Nino/La Nina events

Impacts of the 1997–98 and 2002–03 ENSOs on total number of flashes and number of flash days from the LIS (Lightning Imaging Sensor) on board the Tropical Rainfall Measuring Mission satellite and on the average flash rate from the combined OTD (Optical Transient Detector) / LIS data are studied in South/Southeast Asia (8° –30°N, 60°–120°E) for the period 1998–2005. Also examined is the difference between the geographical distributions of these parameters in the El Nino period in 1998 and in the corresponding normal period in 1999. During the 1997–98 ENSO event, the number of flashes and average flash rate increase by 12% and 36%, respectively, during the El Nino period and during the La Nina period decrease by 22% and 5%, respectively, as compared to the corresponding normal periods. Similarly, during the 2002–2003 ENSO event, the number of flashes and average flash rate increase by 28% and 5% during the El Nino period in 2002 as compared to the corresponding normal period. The contrast in lightning activity between the El Nino and La Nina periods is more pronounced in Southeast Asia than in South Asia because of the southeastward shift in the convection activity during the ENSO events. In both regions, however, monthly variations of both total number of flashes and average flash rate are almost parallel to that of surface temperature. Although, the inverse relationship between the Indian Summer Monsoon Rainfall (ISMR) and El Nino broke down in 1997–98, the relationship between the El Nino and lightning maintains its normal increasing trend, indicating that the lightning activity is more sensitive to the convective activity than to rainfall on seasonal time scale. The total number of flashes and aerosol loading increase in the break‐period regime as compared to the monsoonal regime. The number of flash days in both ENSO events is higher in the EL Nino phase and lower in the La Nina phase as compared to their normal values in the subsequent years. Enhancement of lightning activity is mostly concentrated over land areas. Our results show that although diurnal variations in lightning activity can be related to the variations in surface temperature, convective adjustment induced by the corresponding ENSO phase needs to be considered for explaining the lightning activity variations on these longer time scales.

Atmospheric factors governing winter thunderstorms in the coastal region of the eastern Mediterranean

The factors controlling lightning activity over central Israel and the adjacent Mediterranean Sea were studied. Potential predictors were correlated at 12-h intervals with total number of flashes. Since during the winter season lightning is generated in this region by Cyprus Lows, the data includes 283 observations on days in which this system prevailed for December to February, which covered four winters. The average lightning rate was 26.8 h-1, with a high standard deviation of 55.2 h-1. The total number of flashes at night exceeded the daytime number by 35%, in agreement with previous studies. The CAPE values were on the order of hundreds of J kg-1. A statistical linear multi-regression model was developed for the number of lightning flashes based on 35 atmospheric variables. The correlation between the modeled and the observed number of lightning flashes was 0.67, and 0.81 for the logarithm of the number of lightning flashes (log-lightning). This suggests that the lightning intensity responds exponentially to its governing factors. A linear multi-regression stepwise model for the log-lightning selected seven predictors as significant and yielded a correlation of 0.74. This model was validated by three holdout and three leave-one-out validation experiments. The composition and hierarchy of the significant predictors reflect the dominance of the thermodynamic factors, in particular instability, in determining lightning activity. Though thunderstorms are local or meso-scale phenomena, the synoptic-scale atmospheric variables were found to be powerful predictors for their intensity.

Contribution to the climatological study of lightning

In this work, a new parameterization of the thunderstorm cloud electrical flash frequency is obtained on the basis of simple dimensional arguments. The flash frequency is expressed as a power function of both the cloud top height and the cloud droplet concentration, thus partly taking into account the role of the microphysical cloud characteristics. Since droplet concentrations are not predicted by General Circulation Models (GCMs) and further show great variability, two different “standard” droplet concentrations, respectively valid for continental and maritime thunderstorms, are used. A numerical experiment has been made by using the METEO‐France GCM “ARPEGE” in relation to both, this new parameterization and the Price and Rind (1992) parameterization, in order to determine the global characteristics of electrical flashes. The analysis of the results reveals that the mean annual global flash frequency as well as the total number of flashes that strike annually the globe are higher when the new parameterization is used but are in better agreement with the Optical Transient Detector (OTD) (Christian et al., 1996) observations, thus indirectly attesting for its validity. Further, the good agreement between monthly and diurnal global flash numbers and the corresponding OTD observations confirms the ability of “ARPEGE” of correctly representing the global thunderstorm development. Finally, the new parameterization has been used in a second numerical experiment in order to infer the role of a doubling of CO2 on the global distribution of electrical flashes. A 10% increase of the mean global annual flash frequency is obtained for a surface warming of roughly 2°C.

The gap-width distribution in lightning flashes

At high frequencies of the order of 10 MHz or more gaps in the noise radiation from lightning discharges follow a different distribution law dependent on whether or not the radiation emanates from a cloud or a ground flash. A detailed investigation has shown that the gap-width distribution for cloud flashes can be expressed byn(w) c = A exp (−0.575w) while the corresponding expression for ground flashes isn(w) g = B exp (−0.102w), wheren(w) c andn(w) g are the number of gaps occurring for a particular gap-widthw with cloud respectively ground flashes. The constantsA andB depend merely on the total number of flashes under consideration. For a given number of thunderstorms the two distribution laws intersect at a fairly well defined value ofw=11 ms. Gap-widths greater than this value can be attributed to ground strokes while shorter gaps seem to be connected with the noise radiated from cloud discharges. Based on this “critical” gap-width, ground flash counters have been designed that show good discrimination against cloud flashes.