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At least 235 records · Page 13
All sourcesNASA NTRSDOE OSTI
NASA NTRS1987

Composite climatology of Florida summer thunderstorms

An attempt is made to produce a more comprehensive peninsular Florida summer thunderstorm climatology using composites based upon the longest available time series of best-resolution manually digitized radar (MDR) return. The MDR composites are compared to a more limited set of satellite composites, and temporal variation in the radar composites is related to synoptic-scale variability. The results objectively demonstrate the dominance of recurrent convective patterns over south Florida, even at scales as small as local coastline variation.

Michaels, Patrick J.↗
NASA NTRS1988

Lightning initiation on aircraft in thunderstorms

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.

Mazur, Vladislav↗
NASA NTRS1987

The electric field alignment of ice particles in thunderstorms

Electrical and aerodynamic torques on atmospheric ice particles are calculaed in order to assess the degree of alignment of these particles with the electric fields in thunderstorms. In such clouds fields of many tens of kilovolts per meter are commonly measured, and values of 100 to 200 kV/m are not rare. For E = 100 kV/m the calculations indicate that electric field alignment occurs for crystals with major dimensions up to maximum values in the range from 200 microns to 1 mm, depending upon crystal type. Columns are aligned more easily than platelike crystals, except for dendrites which, by virtue of their smaller assumed density, have smaller fall velocities thereby experiencing weaker aerodynamic torques. Thus a substantial degree of alignment is expected for E = 100 kV/m. For E = 10 kV/m only much smaller crystals will be aligned, probably only ones with major dimensions of less than 50 microns or so.

Weinheimer, Andrew J.↗
NASA NTRS1988

Comparison of high-altitude remote aircraft measurements with the radar structure of an Oklahoma thunderstorm - Implications for precipitation estimation from space

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.

Heymsfield, Gerald M.↗
NASA NTRS1988

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

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.

Schlesinger, Robert E.↗
NASA NTRS1988

Lightning initiation on aircraft in thunderstorms

A physical model of the initiation of lightning flashes in thunderstorms is presented. The model is based on the bi-directional charged leader concept of Kasemir, and is verified with airborne data from lightning strikes to instrumented airplanes (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.

Mazur, Vladislav↗
NASA NTRS1988

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

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.

Krider, E. Philip↗
NASA NTRS1988

Passive microwave observations of thunderstorms from high-altitude aircraft

A high-altitude (20 km) aircraft made overflights of severe and nonsevere Midwest thunderstorms in the central and southeast U.S. during 2 separate experiments. Down-looking instruments on the aircraft are the imaging Multi-Channel Cloud Radiometer with channels in the visible, IR, and near IR, and two passive microwave instruments, the imaging Advanced Microwave Moisture Sounder at 92 (atmospheric window) and 183 GHz (centered on a water vapor line) and the 45 deg foward-of-nadir Multi-Channel Precipitation Radiometer at the 18 and 37 GHz window channels. Over land, the 92 GHz frequency distinguishes quite well the precipitating region from the nonprecipitating anvil region. The interpretation of the microwave measurements is complicated by differences in the cloud microphysics between different climatic regions.

Heymsfield, Gerald M.↗
NASA NTRS1987

Better understanding of intense and tornadic thunderstorms through research using geostationary satellite data

The use of satellite data to study convective storm behavior and better understand intense and tornadic storm development is discussed. The influence of terrain and cloud cover on thunderstorm development, the life cycle and dynamics of the arc cloud line, and the difference in the thermodynamic characteristics of the air into which it advances are examined. The properties of convective-scale interaction are outlined, and the way in which severe convective storms can be isolated using satellite imagery is described. Examples of studies of severe convective and tornadic storms using GOES imagery are presented.

Purdom, James F. W.↗
NASA NTRS1988

Thunderstorm ice induced brightness temperature depressions at 18, 37, and 92 GHz during Cohmex and their implications for satellite precipitation retrievals

Measurements of thunderstorm ice scattering at 18, 37, and 92 GHz, and with high spatial and temporal resolution from the Cooperative Huntsville Meterorlogical Experiment (Cohmex) are discussed. The Cohmex experiment consisted of ground-based, airborne, and satellite experiments in June and July, 1986. The rainfall rates from the experiment are compared with other measurements and the implications of the results for satellite precipitation retrievals are examined.

Hood, Robbie E.↗
NASA NTRS1988

Mesoscale modeling of the severe thunderstorm environment

The abilities of limited-area mesoscale models to provide accurate predictions of the environment of midlatitude severe thunderstorms and the possible feedback effects of the storms upon their environment are reviewed. Mesoaplha-scale models, mesobeta models, and terrain-induced mesoscale systems are discussed. The importance of the initial state and model numerics and physics is examined. It is found that mesoscale models must be run locally if they are to be used for short-range forecasting.

Koch, Steven E.↗
NASA NTRS1988

Satellite-observed characteristics of midwest severe thunderstorm anvils

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.

Heymsfield, Gerald M.↗
NASA NTRS1988

Lightning initiation on aircraft in thunderstorms

A physical model of the initiation of lightning flashes by aircraft in thunderstorms is presented. The model is based on the 'bidirectional uncharged leader' concept of Kasemir, and is verified with airborne data from lightning strikes to instrumented airplanes (NASA F-106B and FAA CV-580). 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 several milliseconds by the negative stepped leader with current pulses of several kA. The two leaders develop in space simultaneously and bidirectionally from the oppositely charged extremities of the airplane.

Mazur, Vladislav↗
NASA NTRS1988

Electrical measurements over active thunderstorms

During the summer of 1986, the air conductivity and the vertical electric field were measured over active thunderstorms from a high altitude U-2 airplane. The conductivity at 20 km remained relatively steady above storms with variations less than + or - 15 percent while vertical electric fields in excess of 5 kV/m were regularly observed. Estimates of the storm generator current and the C.T.R. Wilson (i.e., conduction) current can be obtained using the current densities derived from the conductivity and field measurements and integrated over area. Upward flowing currents of 0.05 to 7.7 A were observed with an average of 2.2 A. More importantly, the U-2 data show that the storm current varies linearly with flash rate, therefore, the average charge transfer per discharge should be independent of storm development or activity.

Blakeslee, Richard J.↗
NASA NTRS1989

Analysis of lightning field changes during active Florida thunderstorms

A computer algorithm for deriving accurate values of lightning-caused changes in cloud electric fields under active storm conditions was developed and applied to data obtained during two thunderstorms from a network of ground-based electric field mills at the NASA Kennedy Space Center and the Cape Canaveral Air Force Station. The resulting field changes were analyzed using a least-squares optimization procedure and point-charge (Q) and point-dipole (P) models. The values and the time variations of the Q-model parameters under active storm conditions were found to be similar to those reported by Maier and Krider (1986) for small storms, when the computations were carried out with the same analysis criteria and comparable biases. The parameters of the P solutions were found to vary with time within the storm interval and from storm to storm.

Koshak, William J.↗
NASA NTRS1989

Cockpit display of ground-based weather data during thunderstorm research flights

This paper describes an integrated system for providing ground-based cockpit display, transmitting to an aircraft, upon request via VHF radio, important ground-based thunderstorm data such as radar precipitation reflectivity contours, aircraft ground track, and cloud-to-ground lightning locations. Examples of the airborne X-band weather radar display and the ground-based display are presented for two different missions during the NASA Storm Hazards Program. In spite of some limitation, the system was found to be helpful in the selection of the route of flight, the general ground track to be used, and, occasionally, in clarifying the location of a specific cell of interest.

Fisher, Bruce D.↗
NASA NTRS1989

The evolution and discharge of electric fields within a thunderstorm

An analysis of the present three-dimensional thunderstorm electrical model and its finite-difference approximations indicates unconditional stability for the discretization that results from the approximation of the spatial derivatives by a box-schemelike method and of the temporal derivative by either a backward-difference or Crank-Nicholson scheme. Lightning propagation is treated through numerical techniques based on the inverse-matrix modification formula and Cholesky updates. The model is applied to a storm observed at the Kennedy Space Center in 1978, and numerical comparisons are conducted between the model and the theoretical results obtained by Wilson (1920) and Holzer and Saxon (1952).

Hager, William W.↗
NASA NTRS1989

Calculations on the electrical development of a small thunderstorm

Data obtained in airborne studies of a small thundercloud have been examined in terms of the noninductive mechanism. Crude calculations are consistent with the observation that only a few percent of ice hydrometeors carry detectable charges. Observed particle charges are reproduced reasonably well. The results suggest that this mechanism can explain the observed electrical development, and predict that most of the charge in thunderstorms is carried on particles greater than about 0.2 mm in size.

Latham, J.↗