Showing papers on "Thunderstorm published in 1988"

A Satellite Infrared Technique to Estimate Tropical Convective and Stratiform Rainfall

Abstract: This paper describes a new method of estimating both tropical convective precipitation and stratiform precipitation (produced under the anvils of mature and decaying convective systems) from satellite infrared data. The method, denoted CST (Convective-stratiform Technique) locates, in an array of infrared data, all local minima in the brightness temperature field (Tmin. After an empirical screening to eliminate cirrus, these points are assumed to be convective centers. Rainrate and rain area are assigned to each minimum point as a function of its Tmin, based on one-dimensional cloud model results. A stratiform rain algorithm, using a brightness temperature threshold based on the mode temperature of thunderstorm anvils, completes the convective/stratiform rain estimation. Individual CST rain fields wore spatially most similar to the radar for young, isolated storms, and most dissimilar in capturing linear features such as squall lines. Some convective features were missed, while others (generally ...

Lightning and precipitation history of a microburst‐producing storm

Abstract: Quantitative measurements of the lightning and precipitation life cycle of a microburst-producing storm are discussed. The storm, which occurred on July 20, 1986 at Huntsville, Alabama, was studied using Doppler radar data. The storm produced 116 flashes, 6 of which were discharges to the ground. It is suggested that an abrupt decrease in the total flash rates is associated with storm collapse, and serves as a precursor to the arrival of the maximum microburst outflows at the surface. Ice-phase precipitation is shown to be an important factor in both the formation of the strong downdraft and the electrification of the storm.

Forcing Mechanisms and Other Characteristics of Significant Summertime Precipitation

Abstract: Satellite, radar, surface, and upper-air data from the June–August periods of 1982–83 are examined to determine the mechanisms, and their relative contributions, for producing warm-season precipitation in the United States. Only areas where rainfall equaled or exceeded 12.7 mm during the 24-h period ending at 1200 UTC were considered. Rainfall associated with extratropical cyclones accounted for about half of the warm-season precipitation. Most of the remaining precipitation was produced by mesoscale forcing mechanisms acting independently from the traveling extratropical cyclones. Nearly all the warm-season precipitation was convective; over 80% of the total was directly or indirectly associated with thunderstorms. Nearly three-fourths of the precipitation that occurred between the Rockies and the Mississippi Valley was nocturnal. Conversely, about three-fourths of the precipitation east of the Mississippi Valley fell during the daylight hours. During the 1983 July-August drought, the area of pr...

Estimates of Thunderstorm Precipitation Efficiency from Field Measurements in CCOPE

Abstract: The precipitation efficiency (the ratio of surface rainfall to water vapor inflow) of a small sample of thunderstorms observed in the Cooperative Convective Precipitation Experiment is calculated using surface and cloud-base airflow and moisture measurements and subcloud rainout based on radar reflectivity factor. Highly-resolved vertical flux measurements from aircraft indicate that a significant amount of water vapor inflow may have been overlooked in past work of this kind, resulting in overestimates in precipitation efficiency. Trends in the mass of water vapor influx resolved at intervals of 10 to 30 min are corroborated by the evolution of water vapor flux convergence computed at 5-min intervals from objectively analyzed surface mesonetwork observations. Fluxes of water vapor inflow and precipitated rainwater are integrated over periods exceeding an hour to obtain precipitation efficiencies applicable to the mature storm phase. Precipitated rainwater estimates from radar reflectivity-rainra...

A Comparison of the Optical Pulse Characteristics of Intracloud and Cloud-to-Ground Lightning as Observed above Clouds.

Abstract: The optical-pulse characteristics of intracloud (IC) and cloud-to-ground (CG) lightning flashes were investigated. The time-resolved optical waveforms at 777.4 nm and electric-field changes produced by lightning flashes were measured aboard a U2 aircraft flying above clouds at the same time that ground-based lightning measurements were carried out. The pulse shapes and intensities of IC and CG flashes, as viewed from above cloud, were found to exhibit remarkably similar waveshapes, radiances, and radiant energy densities. The median radiance at cloud top was found to be about 0.007 W/sq m per sr, and the median energy density about 0.000003 J/sq m per sr.

Mobile Sounding Observations of a Tornadic Storm near the Dryline: The Canadian, Texas Storm of 7 May 1986

Abstract: On 7 May 1986 thunderstorms formed during the afternoon near a dryline in the Texas Panhandle under weak synoptic-scale forcing. Five tornadoes and large hail were produced by one storm near Canadian, Texas. The focus of the paper is the analysis of soundings obtained by a storm-intercept crew. A sounding launched into the wall cloud of the storm just after the fourth tornado indicated a nearly pseudomoist adiabatic lapse rate, a temperature excess of 10°C over the environment at 500 mb, and an updraft speed of almost 50 m s−1 near 6 km AGL, in reasonable agreement with parcel theory.

Satellite-observed characteristics of midwest severe thunderstorm anvils

Abstract: The cloud top and anvil structure of severe thunderstorms observed by the GOES satellite are analyzed for five SESAME cases in 1979 and four non-SESAME cases in 1980-1982. The data is compared with previous models and hypotheses, paying particular attention to the V feature and thermal couplets in the IR observations. The characteristics of the cases are examined and related to the upper-level temperature and wind conditions. It is found that the warm points downwind of the cloud top are due to subsidence. The anaylsis suggests the presence of subsidence due to mountainlike waves. A model in which the close-in warm point is produced by both internal cloud air motions and stratospheric flow around and over the cloud top. It is suggested that the distant warm point is due to either a wave perturbation from air flowing over the cloud top, or air flowing horizonatlly around the elevated portion of the cloud top and anvil.

A mesoscale gravity wave event observed during CCOPE. II: Interactions between mesoscale convective systems and the antecedent waves

Abstract: The interactions between preexisting gravity waves and convective systems were investigated using data obtained by the Cooperative Convection Precipitation Experiment observational network in Montana on July 11-12, 1981. The results indicate that strong convection substantially affects gravity waves locally by augmenting the wave amplitude, reducing its wavelength, distorting the wave shape, altering the wave phase velocity, and greatly weakening the in-phase covariance between the perturbation wind and pressure fields. These convective effects upon gravity waves are explained in terms of hydrostatic and nonhydrostatic pressure forces and gust front processes associated with thunderstorms.

The generation of electric charges and fields in thunderstorms

Abstract: The generation and separation of electric charge and the growth of electric fields in thunderstorms are accounted for by the rebound of a small fraction of the cloud droplets impacting on the undersides of small pellets of soft hail, thereby carrying away some of the polarized charge induced by the prevailing vertical electric field. It is shown that just those droplets making grazing incidence with the millimetre-sized hail pellets are able to separate sufficient charge to create large-scale fields of 4000 V cm -1 within 10 min if, during this time, the precipitation intensity builds up from zero to 20 mm h -1 to produce a total precipitation of about 2 mm. The growth rate of the electric field is determined largely by the precipitation intensity, which is related to the size, density and hence the falling speed of the hail pellets. The field, starting from the initial fine-weather value, grows exponentially but slowly at first, then more rapidly as the hail pellets grow and because the mechanism is self-accelerating, but eventually slows down as the electrical forces on the charged hail pellets and rebounding droplets counteract the gravitational forces and slow down their rate of separation. Calculation of the trajectories of the droplets that make grazing contact with the hail pellets of different sizes and densities, ρ i , allow the collision cross sections and impact angles, θ , to be determined. Charge separation is proportional to cos θ and is most effective when 0.1 ρ i ≤ 0.5 g cm -3 . Very light hail pellets with ρ i -3 become ‘suspended’ in the electric field before this reaches breakdown strength, whereas the faster-falling particles of ρ i > 0.5 g m -3 force the droplets to rebound very close to the electrical equator of the hail pellet where they carry away little charge. It is shown that thunderstorm cells of radius 2 km would be able to produce a succession of lightning flashes at intervals of about 30 s as long as the updraught and precipitation rate are maintained. Very intense lightning activity, with flashes at intervals of less than 10 s, would require storm cells exceeding 5 km in radius, but is more likely to be produced by large multicellular storms.

Diurnal variation of precipitation and thunderstorm frequency in Japan in the warm season

Abstract: Diurnal variation of precipitation in the warm season (June-September) in Japan is described by using data at 1,234 stations on an automated rain-gauge network Local precipitation and widespread precipitation are separately analyzed Diurnal variation of thunderstorm frequency is also described by use of data at 85 observatories Local precipitation and thunderstorms exhibit a pronounced maximum between 1500 and 1800 local time in the inland areas Time of maximum differs by a few hours according to regions and local topography On the other hand, 0300-0600 maxima prevail at maritime stations on islands and peninsulas, except for the Nansei Islands where a 1200-1500 maximum occurs at relatively large islands Widespread precipitation shows a pervasive signal of weak maximum in early to middle morning, except that a maximum tends to occur around noon over the Nansei Islands

Mesoscale and Synoptic Scale Interactions Leading to Intense Convection: The Case of 7 June 1982

Abstract: A case study is presented involving an unusually intense mesoscale convective system (MCS) which produced extensive hail and wind damage in northeast Kansas and northern Missouri, including the Kansas City metropolitan area, during the predawn hours of 7 June 1982. The study emphasizes the preconvective period and examines interactions between mesoscale processes and the synoptic scale environment that led to thunderstorm development. The initial storms formed after dark over the western High Plains, in an area characterized by relatively weak convective potential making the thunderstorm difficult to forecast. Near midnight the convection rapidly intensified into an MCS as it progressed eastward into a much more convectively unstable environment. Several meteorological scenarios that might have led to the initiation and intensification of convection are proposed and examined. These scenarios consider lower tropospheric convergence within an air mass which initially had weak convective potential, ...

Comparison of High-Altitude Remote Aircraft Measurements with the Radar Structure of an Oklahoma Thunderstorm: Implications for Precipitation Estimation from Space

Abstract: The present NASA ER-2 aircraft observations of an isolated group of thunderstorms over Oklahoma, encompassing passive radiometry in the visible, IR, and microwave portions of the spectrum obtained above the storm top, is compared with coincident data from two Doppler radars as well as aircraft-gathered in situ cloud top particle data. Reflectivity cores are found to be nearly colocated with cold anomalies in the microwave brightness temperature field. Theoretical considerations suggesting that microwave frequencies are sensitive to the deeper layer of large ice particles in the storm's convective region are supported.

Thunderstorm-producing terrain features

Abstract: Thunderstorms were traced back to their initiation sites to determine areas of repeated thunderstorm genesis over the Sangre de Cristo Mountains of Colorado and New Mexico. Using three summers of GOES data it was found that genesis-zone activity depended on the direction of the winds above the ridgetops, indicating upper-level wind direction to be a likely “necessary” (but not “sufficient”) predictor of the location of mountain-thunderstorm initiation. Some individual topographic features associated with each genesis zone can be identified.

Convective Tendency Images Derived from a Combination of Lightning and Satellite Data

Abstract: A technique is presented for generating convective tendency products by combining satellite images with observations of cloud-to-ground lightning activity. Rapid scan (5-min) infrared satellite images are used to define the areal distribution of convection. Lightning flash rate trends provide diagnostic and predictive information pertaining to the growth and decay of the thunderstorms. A single derived product from these data can show the location of the lightning activity and convective cores, the spatial distribution of convective rainfall, the remaining cloudy and statiform rain areas, and the growing and decaying storms. Examples are given to illustrate how the flash rate trend may produce a much different and more useful portrayal of storm evolution than the time rate-of-change change of cloud-top blackbody temperatures. This difference can be exacerbated in mesoscale convective weather systems where the cirrus debris can mask the life history of the embedded convective elements.

Water Vapor and ice Mass Transported into the Anvils Of CCOPE Thunderstorms: Comparison with Storm Influx and Rainout

Abstract: The transport of water substance (ice and vapor) into the anvils of midlatitude continental thunderstorms is examined. Doppler radar reflectivity fields and horizontal and vertical windfields, in situ anvil measurements, and environmental soundings were used to estimate the anvil water mass flux for approximately five-minute intervals over one hour periods in six moderate to severe storms. Vapor and ice mass fluxes into the anvil time-averaged for the study periods are about equal. Ratios of the time-averaged sums of these fluxes (A¯) to aircraft-derived cloud base influx (from Fankhauser) range from 18% to greater than 100%. Estimated accuracies are ±30 to 40%. Anvil fluxes exceed rainout at cloud base level as derived from radar reflectivity data by Fankhauser for half the storms. It is shown that influx values alone are not reliable predictors of total storm condensation rates. The water mass storage term is evaluated and is found to be unimportant in relation to influx for all but one storm s...

Aircraft jolts from lightning bolts

Abstract: The trend toward the use of composite materials and digital electronics has renewed the need to quantify the effects of lightning strikes to airplanes, since composite structures do not provide shielding equivalent to that of metal aircraft, and digital systems are potentially more susceptible to upset by electrical transients than are analog electronic systems. A research program, called the Storm Hazards Program, has been run by the National Aeronautics and Space Administration (NASA) for the past eight years and has provided the first statistically significant measurements of the electromagnetic interaction between lightning and aircraft. A NASA-owned F-106B airplane has been flown through thunderstorms about 1500 times at altitudes between 5,000 and 40,000 feet (1,500 to 12,000 meters). The airplane, lightning-hardened and outfitted with special instruments, was hit by lightning 714 times. The types of measurements made and the results are described.

Aircraft jolts from lightning bolts (electronic systems protection)

Abstract: The trend toward the use of composite materials and digital electronics has renewed the need to quantify the effects of lightning strikes to airplanes, since composite structures do not provide shielding equivalent to that of metal aircraft, and digital systems are potentially more susceptible to upset by electrical transients than are analog electronic systems. A research program, called the Storm Hazards Program, has been run by the National Aeronautics and Space Administration (NASA) for the past eight years and has provided the first statistically significant measurements of the electromagnetic interaction between lightning and aircraft. A NASA-owned F-106B airplane has been flown through thunderstorms about 1500 times at altitudes between 5000 and 40000 feet (1500 to 12000 meters). The airplane, lightning-hardened and outfitted with special instruments, was hit by lightning 714 times. The types of measurements made and the results are described. The Storm Hazards Program has concluded, and interpretation of the data gathered is underway. >

A Comparison of Three Radar-Based Severe-Storm-Detection Algorithms on Colorado High Plains Thunderstorms

Abstract: Volumetric reflectivity data collected with the National Center for Atmospheric Research's (NCAR) 10-cm Doppler radar during the Program for Regional Observing and Forecasting Services (PROFS) 1985 Real-Time Forecast Exercise (RT-85) have been used to evaluate the Next Generation Weather Radar (NEXRAD) hail algorithm, a modified version of the Radar Data Processor (RADAP) II-based Severe Weather Probability (SWP) algorithm, tuned for Colorado High Plains thunderstorms, and a Precipitation Type/Intensity (PTI) hail detection algorithm that uses dual-polarization of echo return. Comparative analyses of algorithm performance are presented using two statistical scoring techniques. The results are consistent for the two methods. The SWP algorithm exhibited the best performance when a 0.10 probability threshold was used for the hail/no hail decision criterion. Probability of detection, false alarm rate and critical success index scores for the SWP algorithm were 0.58, 0.71 and 0.23, respectively. Appli...

Spatial Distribution of Lightning Strikes to Ground During Small Thunderstorms in Florida

Abstract: The spatial patterns of the strike points produced by cloud-to-ground lightning under three small thunderstorms have been analyzed to determine the area flash density as a function of radius from the storm center, the distribution of the nearest-neighbor distances, and the distribution of the horizontal distances between successive flashes. The storm average flash densities range from about 0.8 to 1.6 Fl/km squared, and the average lightning fluxes range from 0.03 to 0.05 Fl/km squared/min. The mean nearest-neighbor distances are about 0.7 km and smaller, but are still in good agreement with a theory that assumes an infinite and uniform flash density. The mean distance between successive flashes ranges from 3.2 to 4.2 km, and a sizable fraction of this variation could be due to channel geometry.

Fluctuations in millimeter-wave signals propagated through inclement weather

Abstract: Results are presented from measurements of the effects of inclement weather on the fluctuations in amplitude and phase of millimeter-wave (MMW) signals propagated through the atmosphere. These measurements were made at frequencies near 116, 140, 173, and 230 GHz at a site near Champaign-Urbana, Illinois, in a community chosen for its exceptional flatness and lack of terrain features that might perturb the atmosphere. It was found that this inclement weather fluctuations are generally smaller than those observed in clear air under sunny conditions, and are also smaller than the corresponding effects observed at visible and near-infrared wavelengths. Rain has the greatest effect on MMW transmission, causing large, slow changes in received signal strength as a function of rain rate. It should also be noted that rain caused the only observed loss of the propagation link, during a thunderstorm in which the rain rate was as high as 60 mm/h. It is concluded that, in general, millimeter-wave radiation propagates well in adverse weather, with rain causing the major problems. >

Lightning initiation on aircraft in thunderstorms

Abstract: The physical model of the initiation of lightning flashes by aircraft in thunderstorms is presented. The model is based on the 'bi-directional uncharged leader' concept by Kasemir, and is verified with airborne data from lightning strikes to an instrumented airplane (NASA F-106B and FAA CV-580). The characteristics of electromagnetic processes during lightning attachment are identified by comparison with those studied in natural flashes, those triggered by wire trailing rockets, and laboratory discharges. A triggered flash starts with either a negative corona or a positive leader that depends on the ambient electric field vector and the airplane form factor. The positive leader with continuous current that increases with time is followed in a few milliseconds by the negative stepped leader with current pulses of a few kA. The two leaders develop in space simultaneously and bi-directionally from the oppositely charged extremities of the airplane.

Low-Altitude Wind Shear Detection with Airport Surveillance Radars: Evaluation of 1987 Field Measurements

Abstract: : A field measurement program is being conducted to investigate the capabilities of airport surveillance radars (ASR) to detect low altitude wind shear (LAWS). This capability would require minor RF signal path modifications in existing ASRs and the addition of a signal processing channel to measure the radial velocity of precipitation wind tracers and automatically detect regions of hazardous velocity shear. A modified ASR-8 has been deployed in Huntsville, Alabama and is operated during periods of nearby thunderstorm activity. Data from approximately 30 'wet' (i.e., high radar reflectivity) microbursts during 1987 have been evaluated through comparison with simultaneous measurements from a collocated pencil beam weather radar. This report describes the 1987 approaches for LAWS detection with ASRs. Techniques are described for estimation of low altitude wind fields in the presence of interference such as ground clutter or weather aloft and for automatic detection of microburst wind shear from the resulting radial velocity fields. Evaluation of these techniques using case studies and statistical scoring of the automatic detection algorithm indicates that a suitably modified ASR could detect wet microbursts within 16 km of the radar with a detection probability in excess of 0.90 and a corresponding false alarm probability of less than 0.10. These favorable results indicate the need for careful consideration of implementation issues and the potential operational role of wind measurements from an ASR. Keywords: Meteorological radar, Radial velocity estimation, Hazard detection algorithm.

Using Lightning for Rainfall Estimation in Florida

Abstract: THE objective of this study was to evaluate the feasibility of using remote sensing methods, such as lightning technique, to quantify the area rainfall. Lightning and rainfall data collected during 1977 and 1978 by the Thunderstorm Research International Program located at the Kennedy Space Center were used in this study. The results showed that an increase of 1 lightning flash/min appears to be equivalent to an increase in rainfall of about 1.6 mm. Also, each increment in maximum flashing rate (FM) per minute is likely to be equivalent to increase of hourly rainfall by 1 mm. Using the FM appears to be a better tool for the area-average rainfall (RA) estimation than using average flashing rate. The RA can be predicted based on the FM with an equation of RA = 0.467 + 0.562 FM (r^ =: 0.74).

In-Flight Lightning Characterization Program on a CV-580 Aircraft

Abstract: : A CV-580 aircraft was instrumented with external displacement current density sensors, surface current density sensors, current shunts, and static electric field mill sensors and was flown in active thunderstorms in central Florida during the summer of 1984 and 1985. Electromagnetic data were collected and analyzed for 52 direct lightning strikes to the aircraft flying at altitudes between 2.000 and 18,000 ft. The data consisted of analog records with dc to 2- MHz bandwidth and 10-us windows of digital samples taken at 5-ns intervals. The data show the physical mechanism of lightning attachment to aircraft and suggest comparison of aircraft responses to lightning and simulated NEMP is also provided. Lightning, Lightning attachment, Direct strike, Triggered strike, Intercepted strike, Electromagnetic pulse.

Meteorological aspects of cloud-to-ground lightning in the Kennedy Space Center region

Abstract: This paper examines the meteorological environment in which cloud-to-ground lightning occurs at the Kennedy Space Center (KSC) region during summer. Using the lightning patterns of central Florida, flashes from individual thunderstorms were analyzed for five low-level wind regimes during three summers. The results showed large differences between the number, timing, and location of flashes for varying flow regimes, with southwest flow being the most prolific producer of flashes in the KSC area. In the second part of the study, short-term forecasts of lightning were investigated using the divergence measured by the KSC surface wind network. An interval of about 1 hour was found from the beginning of a convergence event to first lightning. Monitoring 5-min surface convergence, in combination with understanding the climatology of lightning related to the daily flow regime, made it possible to isolate the lightning. Results were used successfully in operations at KSC.

Interpretation of bipole pattern in a mesoscale storm

Abstract: Recent observations of bipole patterns of CG lightning strokes at ground level are interpreted in terms of the production of charge centers in a model cell of a Florida thunderstorm, and in the subsequent histories of these centers following collapse of the updraft that generated the centers.

Effects of stratospheric lapse rate on thunderstorm cloud-top structure in a three-dimensional numerical simulation. I - Some basic results of comparative experiments

Abstract: The effects of stratospheric temperature lapse rate on cloud top height/temperature structure for strongly sheared, mature, isolated midlatitude thunderstorms are investigated by performing three different experiments with an anelastic, three-dimensional model: (1) with an assumed stratospheric lapse rate of 0 K/km (i.e., the isothermal case), (2) with 3 K/km, and (3) with -3 K/km (i.e., the case of inversion). Kinematic storm structure is very similar in all three cases, especially in the troposphere; a strong quasi-steady updraft evolves and splits into a dominant cyclonic overshooting right-mover and a weaker, anticyclonic left-mover that does not reach the tropopause.

Thunderstorms and Aviation Safety — A Dialogue

Abstract: Barnett [Barnett, A. 1987. Lightning strikes twice. Interfaces 17(2) 21–26.] described his attempt to save lives in aviation by closing all airports during thunderstorms. He discussed the phenomenon of “windshear,” which is often associated with thunderstorms and pointed out that in the last 14 years four fatal accidents to commercial airliners have been caused by this phenomenon. As I have tried to suggest, the case for suspending airport operations in thunderstorms is rooted not in emotionalism or ignorance, but in hard-headed management science. That is why I am confident that, whether immediately or shortly, whether formally or informally, some variant of the policy will come into wide-spread use.