Showing papers on "Thunderstorm published in 1997"

NOx from lightning 1. Global distribution based on lightning physics

Abstract: This paper begins a study on the role of lightning in maintaining the global distribution of nitrogen oxides (NOx) in the troposphere. It presents the first global and seasonal distributions of lightning-produced NOx (LNOx) based on the observed distribution of electrical storms and the physical properties of lightning strokes. We derive a global rate for cloud-to-ground (CG) flashes of 20–30 flashes/s with a mean energy per flash of 6.7×109 J. Intracloud (IC) flashes are more frequent, 50–70 flashes/s but have 10% of the energy of CG strokes and, consequently, produce significantly less NOx. It appears to us that the majority of previous studies have mistakenly assumed that all lightning flashes produce the same amount of NOx, thus overestimating the NOx production by a factor of 3. On the other hand, we feel these same studies have underestimated the energy released in CG flashes, resulting in two negating assumptions. For CG energies we adopt a production rate of 10×1016 molecules NO/J based on the current literature. Using a method to simulate global lightning frequencies from satellite-observed cloud data, we have calculated the LNOx on various spatial (regional, zonal, meridional, and global) and temporal scales (daily, monthly, seasonal, and interannual). Regionally, the production of LNOx is concentrated over tropical continental regions, predominantly in the summer hemisphere. The annual mean production rate is calculated to be 12.2 Tg N/yr, and we believe it extremely unlikely that this number is less than 5 or more than 20 Tg N/yr. Although most of LNOx, is produced in the lowest 5 km by CG lightning, convective mixing in the thunderstorms is likely to deposit large amounts of NOx, in the upper troposphere where it is important in ozone production. On an annual basis, 64% of the LNOx, is produced in the northern hemisphere, implying that the northern hemisphere should have natural ozone levels as much as 2 times greater than the southern hemisphere, even before anthropogenic influences. The amount of O3 produced from this NOx is expected to exceed the stratospheric source by a factor of 1.5, and thus the hemispheric asymmetry in LNOx would lead to a significant excess of northern hemisphere O3 even in the preindustrial troposphere. (The monthly climatologies for LNOx on a 1°×1° latitude-longitude grid can be obtained by e-mail to cprice@flash.tau.ac.il).

The global population of mesoscale convective complexes

Abstract: A global set of 714 mesoscale convective complexes is compiled and some of the common properties of the convective systems are identified and examined from a global perspective. the data set includes date of occurrence, time of first storms, initiation, maximum extent, termination, duration, cold-cloud shield areas, and tracks from initiation to termination. It is found that the typical convective complex is nocturnal, generates a cold-cloud shield area of approximately 350 000 km2, and persists for about 10 h. the largest systems and most persistent systems tend to occur near the summer solstices. For the globe, about 400 systems occur each year, primarily over land areas. Most systems develop in favoured zones, although some activity occurs over every continent (except Antarctica) and all major oceans. the concentration of activity into favoured zones indicates that there must be special dynamic and/or thermodynamic conditions necessary for convection to organize into convective complexes. Activity is strongly tied to the solar day, and shifts from 35°S in early January to about 50°N during the boreal summer and back to 35°S by December. Within the northern hemisphere there is a pronounced poleward migration as the jet stream shifts northward. Relatively little migration occurs in the ocean-dominated southern hemisphere where the subtropical jet remains quasi-stationary over the convective-complex regions. The nocturnal life cycles, copious rainfall, large cloud shields, and great frequency of mesoscale convective complexes suggest that they may be significant contributors to the global hydrologic cycle and earth-system energy budget.

The dry intrusion perspective of extra-tropical cyclone development

Abstract: The dry intrusion is a coherent region of air descending from near tropopause-level. It often has a clear signature in satellite imagery, especially in the water vapour channel, where it is seen as a ‘dark zone’. Parts of dry intrusions are characterised by high potential vorticity and, upon approaching a low-level baroclinic zone, rapid cyclogenesis may be expected to ensue. The leading edges of dry intrusions are defined by cold θw-fronts (moisture fronts). In some places the dry intrusion undercuts rearward-ascending warm air to give an ana-cold front. In other places it overruns the warm air to produce an upper cold θw-front in advance of the surface cold front. Here the dry intrusion is associated with the generation of potential instability and its eventual release as showers or thunderstorms. Identification of dry intrusions provides the forecaster with additional nowcasting evidence that is especially helpful when issuing severe weather warnings. The identification of water vapour dark zones associated with dry intrusions can also form the basis of methods for validating NWP models. Through their relationship to high potential vorticity, they can provide guidance for bogussing NWP models in situations of potentially severe weather. This article provides an introduction to the structure and behaviour of dry intrusions and their relationship to other aspects of extra-tropical cyclones.

VIL Density as a Hail Indicator

Abstract: In current severe thunderstorm warning operations, forecasters frequently use the vertically integrated liquid water content (VIL) product from the WSR-88D to estimate thunderstorm severity and, particularly, hail size. Since VIL varies greatly based on airmass characteristics, forecasters have typically determined a threshold VIL to be used for each new thunderstorm event. A product that is independent of airmass characteristics, and thus independent of season and geographic location, would be more desirable in an operational warning environment. It has been observed that high-topped thunderstorms with high VILs do not always produce large hail. It has also been observed that low-topped thunderstorms with low VILs occasionally do produce large hail. However, the maximum reflectivity in both high-topped and low-topped thunderstorms is similar when both produce similar-sized hail. From this, it was hypothesized that dividing the VIL by the echo top would “normalize” the VIL and produce a common va...

The 1993 Superstorm Cold Surge: Frontal Structure, Gap Flow, and Tropical Impact

Abstract: In the wake of the eastern United States cyclone of 12‐14 March 1993, a cold surge, originating over Alaska and western Canada, brought northerlies exceeding 20 m s21 and temperature decreases up to 158C over 24 h into Mexico and Central America. This paper addresses the multiscale aspects of the surge from the planetary scale to the mesoscale, focusing on 1) the structure and evolution of the leading edge of the cold surge, 2) the reasons for its extraordinary intensity and equatorward extent, and 3) the impact of the surge on the Tropics, specifically, on the strength of the trade winds and on the sea surface temperature in the eastern Pacific. The cold surge was initiated as a developing cyclone over the Gulf of Mexico, and an upstream anticyclone east of the Rockies caused an along-barrier pressure gradient to form, forcing topographically channeled northerlies along the Rocky and Sierra Madre Mountains to transport cold air equatorward. On the mesoscale, the leading edge of the cold surge possessed nonclassical frontal structure. For example, as the cold surge entered Mexico, the coldest air and the strongest wind arrived at about 900 hPa before affecting the surface, suggestive of a tipped-forward leading edge to the surge. Also, satellite imagery and surface observations indicate that the leading edge appeared to be successively regenerated in the warm presurge air. The cold surge had characteristics reminiscent of a Kelvin wave, a tipped-forward cold front, a pressure-jump line, a bore, and a gravity current, but none of these conceptual/dynamical models was fully applicable. Associated with the cold surge, gap winds up to 25 m s21 were observed in the Gulfs of Tehuantepec (a tehuantepecer), Fonseca, Papagayo, and Panama, owing to the strong cross-mountain pressure gradient. In the case of the tehuantepecer, a rope cloud emanated from the Isthmus of Tehuantepec and turned anticyclonically, consistent with an inertial oscillation. On the synoptic and planetary scales, the extraordinary equatorward extent of the cold surge was aided by topographic channeling similar to cold-air damming, by a low-latitude upper-tropospheric trough, and by the lower branch of the secondary circulation associated with a confluent jet-entrance region aloft. The cold surge also impacted the tropical atmosphere and ocean, by contributing to the strengthening of the northeast trade winds over the eastern Pacific Ocean and by inducing local cooling of the sea surface temperature in the Gulfs of Tehuantepec and Papagayo by about 48‐88C.

Thunderstorm Initiation, Organization, and Lifetime Associated with Florida Boundary Layer Convergence Lines

Abstract: The initiation, organization, and longevity of thunderstorms associated with boundary layer convergence lines in the Cape Canaveral, Florida, vicinity are examined using data from the Convection and Precipitation/Electrification (CaPE) experiment. The project was conducted during July and August 1991 under low vertical wind shear situations. This observational study is based on Doppler radar, mesonet, balloon sounding, and satellite data. The primary convergence lines studied were the east coast sea-breeze front (ECSBF) and a frequently occurring gust front from the west termed the west coast front (WCF) that originates with storms initiated by the west coast sea-breeze front. Significantly fewer storms were associated with the ECSBF in comparison to the WCF. This was because the convergence with the ECSBF was shallower and weaker and the updrafts were shallow and tilted. The environmental winds were generally westerly near the top of the ECSBF and at storm steering level. As a result, the low-le...

Evolution of a Florida Thunderstorm during the Convection and Precipitation/Electrification Experiment: The Case of 9 August 1991

Abstract: The relationships among kinematic, microphysical, and electric field properties within a multicell Florida thunderstorm are investigated using observations from three Doppler radars (one with multiple wavelength and polarization diversity capabilities), four instrumented penetrating aircraft, a surface-based electric field mill network, and other observation facilities. The storm was convectively active for about 1 h and at least five primary cells developed within the storm during this time, one of which went through three consecutive development cycles. The updrafts in this storm were 2–4 km wide, exhibited bubble-like evolution, and had lifetimes of 10–20 min. The maximum updraft determined by the multiple Doppler analysis was about 20 m s−1. A differential reflectivity (ZDR) “column,” indicating regions containing millimeter-size raindrops, extending above the freezing level, was associated with each cell during its developing stages. This column reached altitudes exceeding 6 km (−8°C) in the...

The Environments of Significant Convective Events in the Western Mediterranean

Abstract: The environmental characteristics associated with 313 significant convective events in the western Mediterranean are investigated using radiosonde ascents made in Mallorca (Spain). The events are separated into five groups, based on the observed event (hail, heavy rain, “dry” storms, storms with heavy rain, and tornadoes). Classic stability indices, as well as values of convective available potential energy and helicity, are considered for each group. These traditional convective indices appear not to provide good guidance for discriminating environments associated with each group of events. In order to classify the environments, each sounding is defined by means of 34 variables that describe the thermal and humidity vertical structure, instability, precipitable water, and helicity. A cluster analysis shows that four different vertical structures appear. Each kind of event shows preference for the environments defined by a cluster. A simple method is presented for sounding classification using th...

A 10-yr Monthly Lightning Climatology of Florida: 1986–95

Abstract: Average cloud-to-ground lightning flash density values for Florida have been calculated for the 10-yr period 1986–95. An annual mean map and monthly mean maps were constructed from a database exceeding 25 million flashes. These maps represent a 10-yr climatology of the geographic distribution of detected cloud-to-ground lightning flashes and provide an insight into the thunderstorm distribution in Florida. The locations of relative areas of lightning maxima and minima are strongly affected by the various combinations of synoptic and mesoscale contributions and are discussed. During the cool season, November–February, the greatest flash densities occur over the panhandle from storms mostly associated with midlatitude synoptic-scale systems. During the spring transitional period of March–May, flash densities increase over the entire state as synoptic contributions transition to mesoscale. Flash density totals in the warm season, June–August, exceed 10 flashes km−2 in the central part of Florida. Fl...

Severe Thunderstorm Development in Relation to Along-Dryline Variability: A Case Study

Abstract: Long-lived thunderstorms were initiated during the afternoon of 26 May 1991 ahead of a dryline in northwestern Oklahoma. Various reasons for initiation in this particular along-dryline location are investigated through analysis of observations collected during the Cooperative Oklahoma Profiler Studies—1991 field program. Observing systems included in situ and radar instrumentation aboard a research aircraft, soundings from mobile laboratories, a mesonetwork of surface stations, meteorological satellites, and operational networks of surface and upper-air stations. Elevated moistening east of the dryline revealed by soundings and aircraft observations in combination with thermal plume activity was apparently insufficient to promote sustained convection on this day without aid from an additional lifting mechanism. Satellite observations reveal scattered convection along the dryline by midafternoon and a convective cloud line intersecting the dryline at an angle in the area of most pronounced storm i...

Structure and Evolution of the 22 February 1993 TOGA COARE Squall Line: Organization Mechanisms Inferred from Numerical Simulation

Abstract: Mechanisms responsible for meso- and convective-scale organization within a large tropical squall line that occurred on 22 February 1993 during the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment are investigated using a three-dimensional numerical cloud model. The squall line occurred in an environment typical of fast-moving tropical squall lines, characterized by moderate convective available potential energy and moderate-to-strong vertical shear beneath a low-level jet with weak reverse vertical shear above. A well-simulated aspect of the observed squall line is the evolution of a portion of its leading convective zone from a quasi-linear to a three-dimensional bow-shaped structure over a 2-h period. This transition is accompanied by the development of both a prominent mesoscale vortex along the northern edge of the 40–60-km long bow-shaped feature and elongated bands of weaker reflectivity situated rearward and oriented transverse to the leading edge, within enha...

Initial comparison of lightning mapping with operational time‐of‐arrival and interferometric systems

Abstract: The mapping of lightning radiation sources produced by the operational Time-of-Arrival National Aeronautics and Space Administration/Lightning Detection and Ranging (NASA/LDAR) system is compared with that of the Interferometric French Office National D'Etudes et de Recherches Aerospatiales (ONERA-3D) system. The comparison comprises lightning activity in three Florida storms and also individual flashes in one of these storms. Although limited in scope, the comparison and analysis show a significant difference in the representation of lightning radiation by each mapping system. During the duration of a flash, the LDAR data show a continuity in time and a three-dimensional structure of radiation sources. The ONERA-3D radiation source data are more intermittent in time and have a more two-dimensional structure. The distinction between the radiation sources mapped by the two systems is also reflected in the difference between their propagation speeds, 104–105 m s−l, estimated by the LDAR system, and 107–108 m s−1, estimated by the ONERA-3D system. We infer that this difference occurs because most of the radiation sources mapped with LDAR are associated with virgin breakdown processes typical of slowly propagating negative leaders. On the other hand, most of the radiation sources mapped with ONERA-3D are produced by fast intermittent negative breakdown processes typical of dart leaders and K changes as they traverse the previously ionized channels. Thus each operational system may emphasize different stages of the lightning flash, but neither appears to map the entire flash.

NOx from lightning 2. Constraints from the global atmospheric electric circuit

Abstract: The global atmospheric electric circuit can be used to constrain the total amount of NOx produced by lightning (LNOx). Since the global atmospheric electric circuit is regulated by global thunderstorm activity and, more specifically, global lightning currents, we use the global electric circuit to quantify and place limits on the total amount of energy available from lightning. Using a production rate of 10×1016 molecules NO/J, we have calculated the global LNOx production on monthly, annual, and interannual timescales from 1983 to 1991. The 8-year mean production rate is found to be 13.2 Tg N/yr and agrees with the independent estimate derived in part 1 of this study (12.2 Tg N/yr), which is based on lightning physics and global lightning distributions. After considering the various uncertainties in these calculations, we conclude that the annual production rate of NOx from lightning cannot be less than 5 nor more than 25 Tg N/yr.

Discriminating between Tornadic and Nontornadic Thunderstorms Using Mesoscale Model Output

Abstract: The ability to discriminate between tornadic and nontornadic thunderstorms is investigated using a mesoscale model. Nine severe weather events are simulated: four events are tornadic supercell thunderstorm outbreaks that occur in conjunction with strong large-scale forcing for upward motion, three events are bow-echo outbreaks that also occur in conjunction with strong large-scale forcing for upward motion, and two are isolated tornadic supercell thunderstorms that occur under much weaker large-scale forcing. Examination of the mesoscale model simulations suggests that it is possible to discriminate between tornadic and nontornadic thunderstorms by using the locations of model-produced convective activity and values of convective available potential energy to highlight regions of likely thunderstorm development, and then using the values of storm-relative environmental helicity (SREH) and bulk Richardson number shear (BRNSHR) to indicate whether or not tornadic supercell thunderstorms are likely....

Midwinter Suppression of Northern Hemisphere Storm Track Activity in the Real Atmosphere and in GCM Experiments

Abstract: The seasonal cycle of the North Pacific and the North Atlantic storm track activity is investigated on the basis of daily National Meteorological Center (now known as NCEP) upper-air analyses (1946–89) and of data from the ECHAM3 T42 atmospheric general circulation model. Emphasis is put on the midwinter suppression of the Pacific storm track. This feature of seasonal variability is not sensitive to a particular definition of midlatitude synoptic wave activity, as is shown by applying a common definition of area mean storm track intensity. The suppression is reproduced by the atmospheric model with very similar characteristics. It is attributed to a negative correlation between the storm track intensity and the speed of the subtropical jet at 250 hPa for average zonal winds exceeding the threshold of approximately 45 m s−1, contrasting with a positive correlation below this value. The lack of an analogous behavior over the Atlantic may be assigned to the lower wind speeds there. In a 3·CO2 time-s...

Rainfall drop size distribution in the Western Mediterranean basin, València, Spain

Abstract: Although soil erosion processes are directly related to the raindrop sizes and the kinetic energy of the rainfall, little is known about the drop-size distribution. The paper staining technique is used to measure the size of the raindrops for natural thunderstorms with different rainfall intensities in the Western Mediterranean basin (Canals and Valencia, Spain). Drop sizes are greater during the most intense thunderstorms, ranging from 0.25 mm diameter (D50) for a 1 mm h−1 rainshower to 2.69 mm in very intense thunderstorms of about 120 mm h−1. The shape of the raindrop size distribution curve changes from lower to higher rainfall intensity thunderstorms. For the low intensity events, the raindrops are very small and most of the rain falls in four to five drop size classes (from 0 to 0.5 mm). In contrast, the raindrops are larger in the higher rainfall intensity events. For the most intense thunderstorms there are 62 size classes ranging from 0 to 3.87 mm. No raindrops larger than 4 mm were found during the seven months of measurements. In comparison with other reported measurements, the drop-sizes in the Western Mediterranean basin are smaller for rainfall intensities lower than 100 mm h'. Nevertheless, for higher rainfall intensities the raindrop size distributions are similar to those measured in Central United States since 1940.

Mesoanalysis of summertime convergence zones in central and eastern North Carolina

Abstract: It is demonstrated that it is possible to perform informative mesoanalysis of summertime convergence boundaries in the southeastern United States by combining capabilities of the new WSR-88D Doppler radar with Geostationary Operational Environmental Satellite imagery and conventional surface data. Observed phenomena are identified as thunderstorm outflow boundaries, sea-breeze fronts, horizontal convective rolls, deep synoptic-scale fronts, prefrontal troughs, shallow fronts (airmass boundaries lacking upper-level support), stationary and propagating boundaries of unknown origin, and the “Piedmont trough,” which is apparently a new feature discovered in the course of this research. The transition zone between the Piedmont and the Coastal Plain was found to be a preferred location for convergence boundaries. An unexpectedly far inland advance of the sea breeze to central North Carolina occurred in some instances. The very sensitive “clear air mode” of the WSR-88D radar, when used in combination wi...

In situ ionospheric observations of severe weather-related gravity waves and associated small-scale plasma structure

Abstract: On July 27, 1988, two sounding rockets were launched over a small thunderstorm cell which constituted the remnants of a large frontal event which had lasted for several hours over the eastern seaboard. One of the rockets was instrumented for detection of the electromagnetic impulse from lightning strikes and its subsequent interaction with the ionospheric plasma [Kelley et al., 1990]. The second had on board an absolute electron density probe, the results from which we report here. We present evidence that a gravity wave was spawned by the front and propagated nearly to the F peak in the ionosphere, where it steepened and created structure in the medium at scales much less than the vertical wavenumber of the major disturbance. The fluctuation spectrum along the rocket path was elevated for scales from 25 km down to less than 10 m. At scales between 10 km and just under 100 m, characterization of the spectrum by a power law yields a spectral index less than that displayed by such well-studied processes as bottomside spread F and barium cloud striations. Similar results have been reported for gravity wave induced intermediate scale structures at midlatitudes [Wernik et al., 1986]. The mixing theory described by Fridman [1990] may be relevant to these observations.

Total rainfall and convective rainfall in catalonia, spain

Abstract: A substantial percentage of the total rainfall amount in Catalonia (north-east of Spain) stems from convective cloud systems. To obtain a quantitative estimate, convective events were identified on the charts of a rain-rate recorder from 1960 to 1979. Events were classified into four categories: non-convective, convective with low rainfall rates, convective with moderate to high rates and thunderstorm events. The amount of rain due to each of these was computed. The ratio of convective to total rainfall amounts ranges from 70 to 80 per cent in the summer months to less than 30 per cent in winter and its pattern through the year is discussed as regards regional weather and climate. Orographical features, potential instability of the atmosphere and high sea-surface temperatures are related to high-rate events. The uncertain character of rainfall in the area is clearly linked to the large share of convective rainfall on the bulk precipitation. # 1997 Royal Meteorological Society. Int. J. Climatol., 17, 1683‐1695.

The lightning activity of a hailstorm as a function of changes in its microphysical characteristics inferred from polarimetric radar observations

Abstract: The progression of cloud-to-ground (CG) lightning activity in an Oklahoma hailstorm with well-developed supercell organization is related to changes in the microphysical characteristics of the storm as inferred from polarimetric radar observations. A decision tree based on the observed values of radar reflectivity ZH, differential reflectivity ZDR, and specific differential phase KDP is used to identify regions in the storm containing graupel, hail, supercooled liquid water, and rain. Mixing ratios of graupel/hail and liquid water are obtained in terms of ZH, ZDR, and KDP. One of the objectives of the study is to relate the development of graupel aloft, and the descent of graupel and hail, to fluctuations in CG lightning production. The results indicate that the development of graupel above the freezing level is related to the overall increase and decrease in the production of CG flashes (probably by providing the negative charge necessary for the flashes). However, it is the fluctuating descent of small hail below the height of the negatively charged graupel region that is correlated with the three or four pulses in the CG lightning activity (apparently by producing the electric bias needed for the discharge of the graupel layers aloft).

Sprites as evidence of vertical gravity wave structures above mesoscale thunderstorms

Abstract: Large area multicell thunderstorms lead to the formation of vertically oriented cylindrical structures of gravity waves at mesospheric altitudes closely resembling those observed in optical emissions associated with transient luminous glows called sprites.

Weather and climate extremes

Abstract: : This report consists of world weather and climate extremes that have been extracted from various sources. The report contains discussions covering the measurement of various meteorological elements, measurement practices, and instrumentation. All extremes are presented in terms of their location and date and, where supportive information is available in the professional literature, detailed discussions of the extreme event are provided. The elements covered in this report include surface air temperature, precipitation (rainfall, snowfall and hail), wind, thunderstorms, air pressure, solar radiation, dew point temperature and fog. The report contains five maps. There is one each for the worldwide extremes of temperature, precipitation, and other elements. The remaining two maps show U.S. state extremes for monthly temperature (absolute maximum and absolute minimum) and precipitation (maximum monthly and maximum 24-hour).

Prediction of a mesoscale convective system over catalonia (Northeastern Spain) with a nested numerical model

Abstract: A mesoscale convective system that affected Northeastern Spain on October 10, 1994, with rainfall amounts up to 400 mm, is simulated reasonably well by a nested 3-dimensional hydrostatic mesoscale model. Previous studies carried out in this region had already portrayed the main synoptic patterns that give rise to these devastating episodes. The present contribution takes a further step since it goes down to the mesoscale by means of a numerical model providing a more detailed representation not otherwise achieved by earlier analysis methods. Although the model was unable to forecast accurately the precipitation fields, it captured satisfactorily the framework in which the convective system originated and evolved.

Intense Convective Storms with Little or No Lightning over Central Arizona: A Case of Inadvertent Weather Modification?

Abstract: On 20/21 August 1993, deep convective storms occurred across much of Arizona, except for the southwestern quarter of the state. Several storms were quite severe, producing downbursts and extensive wind damage in the greater Phoenix area during the late afternoon and evening. The most severe convective storms occurred from 0000 to 0230 UTC 21 August and were noteworthy in that, except for the first reported severe thunderstorm, there was almost no cloud-to-ground (CG) lightning observed during their life cycles. Other intense storms on this day, particularly early storms to the south of Phoenix and those occurring over mountainous terrain to the north and east of Phoenix, were prolific producers of CG lightning. Radar data for an 8-h period (2000 UTC 20 August–0400 UTC 21 August) indicated that 88 convective cells having maximum reflectivities greater than 55 dBZ and persisting longer than 25 min occurred within a 200-km range of Phoenix. Of these cells, 30 were identified as “low-lightning” storm...

Uncertainties in WSR-88D Measurements and Their Impacts on Monitoring Life Cycles

Abstract: Radar measurement uncertainties associated with storm top, cloud top, and other height measurements are well recognized; however, the authors feel the resulting impacts on the trends of storm features are not as well documented or understood by some users of the WSR-88D system. Detailed examination of radar-measured life cycles of thunderstorms occurring in Arizona indicates substantial limitations in the WSR-88D’s capability to depict certain aspects of storm-height attribute evolution (i.e., life cycle) accurately. These inherent limitations are illustrated using a vertical reflectivity structure model for the life cycle of a simple, “single-pulse” thunderstorm. The life cycle of this simple storm is “scanned” at varying ranges and translation speeds. The results show that radar-determined trends are often substantially different from those of the model storm and that in extreme cases the radar-detected storm and the model storm can have trends in storm-top height of opposite sign. Caution is c...

Nocturnal Insect Migration: Effects of Local Winds

Abstract: Publisher Summary This chapter discusses the effects of local wind systems on insect migration at night. Many insect species undertake migratory flights at night. Such flights are usually dominated by the wind because the insects climb to heights where the wind speed exceeds their own flying speed through the air. Consequently, the distance moved is essentially the product of wind speed and duration of flight. The results throw light on the occurrence of clouds of flying adults and on the clumped distribution of populations of migrants on the ground. Much of the evidence comes from field studies of a few pest species, but it is likely that many other species are similarly affected. Several types of wind system are considered: low-level jet streams, coastal winds, rainstorm outflows, down-slope and down-valley winds, and a variety of local winds caused by barrier effects of mountains. Furthermore, field studies need to put emphasis on atmospheric behavior through supplementary observations of winds and temperatures in the lowest kilometer of the atmosphere. This chapter discusses coastal winds, bores and solitary waves, rainstorm outflows, and mountain winds: drainage winds and barrier winds in reference to the nocturnal insects.

Numerical Simulation of the 9 10 June 1972 Black Hills Storm Using CSU RAMS

Abstract: Strong easterly flow of low-level moist air over the eastern slopes of the Black Hills on 9-10 June 1972 generated a storm system that produced a flash flood, devastating the area. Based on observations from this storm event, and also from the similar Big Thompson 1976 storm event, conceptual models have been developed to explain the unusually high precipitation efficiency. In this study, the Black Hills storm is simulated using the Colorado State University Regional Atmospheric Modeling System. Simulations with homogeneous and inhomogeneous initializations and different grid structures are presented. The conceptual models of storm structure proposed by previous studies are examined in light of the present simulations. Both homogeneous and inhomogeneous initialization results capture the intense nature of the storm, but the inhomogeneous simulation produced a precipitation pattern closer to the observed pattern. The simulations point to stationary tilted updrafts, with precipitation falling out to the rear as the preferred storm structure. Experiments with different grid structures point to the importance of removing the lateral boundaries far from the region of activity. Overall, simulation performance in capturing the observed behavior of the storm system was enhanced by use of inhomogeneous initialization.

Vertical Velocities in a Thunderstorm Gust Front and Outflow

Abstract: Continuous vertically pointing measurements of a thunderstorm outflow, including its gust front, were obtained with a Doppler radar near New Salem, North Dakota. The measurements provide a high-resolution depiction of the vertical structure of reflectivity and vertical velocity within the gust front, the outflow, and the parent storm. Earlier gust front remote sensing studies have used Doppler observations obtained with low-elevation-angle scans to accurately measure the horizontal flow pattern from which vertical velocities were subsequently estimated by integrating the continuity equation. In contrast, the New Salem case provides direct, rather than derived, Doppler measurement of vertical velocities with better vertical resolution and vastly superior temporal resolution. The gust front’s vertical structure is in general agreement with earlier observations and numerical simulations, except that the transition from strong upward to strong downward motion was more abrupt. The maximum updraft, of ...

Electrical and chemical consequences of point discharges in a forest during a mist and a thunderstorm

Abstract: The first part of this paper is devoted to the electrical characteristics of the air below the canopy of a pine and spruce forest. In fair weather conditions, the site influence, i.e. the filtering effect of the trees on air conductivity and electric field, is evidenced. Under disturbed weather conditions, the meteorological influence is depicted to show: (i) that electric fields sufficiently high to produce `point discharges' occur not only during thunderstorms but also during mist; (ii) that, by taking into account the gaseous ions produced by the point discharges, it becomes possible to understand on the one hand the field divergence with height observed in the mist, and on the other hand the detection of alternatively positive and negative gaseous clouds of charges during the thunderstorm. The second part presents the results of chemical analysis performed below and just above the canopy on the same site and for the same period. Increased ozone and hydrogen peroxide concentrations were measured during the thunderstorm. It is shown that these chemical species were not only locally produced by photochemistry and/or transported from different (industrial and traffic) sources, but also arose from transient electrical point discharges in the forest under high electric field conditions. Their local concentration is shown to be influenced by the electrical discharge current density on the one hand, and by the local conditions of atmospheric stability and of water content, determining the evolution of the chemical products, on the other hand. Actually, after their production, the initially gaseous chemical products were shown to be involved in the local droplet chemistry. Under specific weather conditions, ionic densities as well as chemical by-products of a forest therefore depend both on the electrodynamical characteristics of the lower atmosphere and on the local environmental conditions (liquid water content of the air and atmospheric stability) associated with the two different situations investigated, a mist and a thunderstorm.