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At least 847 records · Page 47

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NASA NTRS1990

DFW microburst model based on AA-539 data

Analysis of the August 2, 1985 crash for an L-1011 jumbo jet (DL-191) on approach to the Dallas-Ft. Worth International Airport (DFW) in a thunderstorm indicates that the severe windshear microburst that caused the crash was composed not only of a strong downflow and outflow but also included several large-scale vortex rings entrained in the flowfield. This paper presents a detailed two-dimensional model of the DFW microburst based on data from the MD-80 (AA-539) that followed behind DL-191 and flew through the microburst about two minutes after the crash of DL-191. The model was developed using wind-vector and flight-path data reconstructed by NASA Ames Research Center and a combination of interactive graphics and least-squares error best fit between the modeled and measured wind vectors along the AA-539 flight path. The model indicates that the flowfield contains some significant elements and vortices not previously reported. The alternating direction of rotation of the vortices in the model suggests a microburst structure based on a von Karman vortex street rather than on a Kelvin-Helmholtz instability. The model also indicates that the reconstructed wind-vector data contain a time lag of at least one second in the horizontal winds.

Grantham, Walter J.↗

NASA NTRS1991

Determination of precipitation profiles from airborne passive microwave radiometric measurements

This study presents the first quantitative retrievals of vertical profiles of precipitation derived from multispectral passive microwave radiometry. Measurements of microwave brightness temperature (Tb) obtained by a NASA high-altitude research aircraft are related to profiles of rainfall rate through a multichannel piecewise-linear statistical regression procedure. Statistics for Tb are obtained from a set of cloud radiative models representing a wide variety of convective, stratiform, and anvil structures. The retrieval scheme itself determines which cloud model best fits the observed meteorological conditions. Retrieved rainfall rate profiles are converted to equivalent radar reflectivity for comparison with observed reflectivities from a ground-based research radar. Results for two case studies, a stratiform rain situation and an intense convective thunderstorm, show that the radiometrically derived profiles capture the major features of the observed vertical structure of hydrometer density.

Kummerow, Christian↗

NASA NTRS1991

Surface wind convergence as a short-term predictor of cloud-to-ground lightning at Kennedy Space Center

Cloud-to-ground lightning is a significant forecast problem at the Kennedy Space Center (KSC) in Florida. In this study, cloud-to-ground lightning is related in time and space to surface convergence for 244 days during the convective seasons of 1985 and 1986 over a 790 sq-km network at KSC. The method uses surface convergence, particularly the average over the area, to identify the potential for new, local thunderstorm growth, and it can be used to specify the likely time and location of lightning during the life cycle of the convection. A threshold of 75 x 10 to the -6th/s change in divergence is the main criterion used to define a convergence event, and a set of flashes less than 30 min apart defines a lightning event. Time intervals are found from the study to be approximately 1 h from beginning convergence to first flash, and another hour to the end of lightning. The influences of low-level winds and midlevel moisture in determining the location and intensity of convection are discussed. This is the first known dynamically-based forecast method for lightning prediction. The technique, currently in use at KSC, has been shown to be a systematic, quantitative tool for predicting lightning onset in situations where conventional analysis tools such as radar and satellite are limited.

Watson, Andrew I.↗

NASA NTRS1991

Automatic analysis of stereoscopic satellite image pairs for determination of cloud-top height and structure

Results are presented on an automatic stereo analysis of cloud-top heights from nearly simultaneous satellite image pairs from the GOES and NOAA satellites, using a massively parallel processor computer. Comparisons of computer-derived height fields and manually analyzed fields show that the automatic analysis technique shows promise for performing routine stereo analysis in a real-time environment, providing a useful forecasting tool by augmenting observational data sets of severe thunderstorms and hurricanes. Simulations using synthetic stereo data show that it is possible to automatically resolve small-scale features such as 4000-m-diam clouds to about 1500 m in the vertical.

Hasler, A. F.↗

NASA NTRS1991

Structure of an internal bore and dissipating gravity current as revealed by Raman lidar

The Raman lidar observations of a weak gravity current and an internal bore associated with a thunderstorm gust front are presented. These observations have been complemented by conventional surface meteorologial analyses, special radiosonde data, spectral and bandpass filter analysis of barograph data, and infrared satellite imagery. Results obtained reveal the time-space continuity and dynamic nature of two boundary-layer disturbances seen in the lidar data. A comparison of the lidar display with the rawinsonde data makes it possible to determine the thermal fields associated with these disturbances at high temporal resolution (2 min) and an altitude of 6 km. The airflow associated with the disturbances was inferred by synthesizing the lidar and rawinsonde data. One of the two disturbances represents a dissipating outflow boundary (gust front) and can be characterized as a gravity current. The second disturbance represents an internal bore propagating ahead of the gravity current on a surface-based stable layer, which acted as a waveguide. The lidar revealed a mean bore depth of 1.9 km, observed and calculated speeds were in good agreement (about + or - 20 percent).

Koch, Steven E.↗

NASA NTRS1991

Atmospheric electrodynamics in the U.S. - 1987-1990

Atmospheric electrodynamics research is summarized, focusing on three general areas: the ionosphere as a source for middle atmospheric electrodynamics, regional and global scale electrodynamics, and thunderstorms and lightning. New or improved instrumentation techniques which have furthered atmospheric electrodynamics research are also discussed.

Holzworth, R. H.↗

NASA NTRS1991

Stratospheric soot - Cloud forming properties and transport processes

Soot enters the stratosphere either from the troposphere in deep thunderstorm convection, predominantly in the tropics, from midlatitude forest fires, and directly from aircraft flying above the tropopause. Particles emitted from jet aircraft are relatively ineffective as cloud droplet condensation nuclei (less than 1 percent) compared with those from forest fires (above 50 percent). Chemical and other modification during aging of the particles may influence these results in some circumstances; cloud nucleating properties can be used as a soot tracer. Such aerosols, as they grow to haze drops and ilute in increasing humidity, may form ice particles in the cold polar stratosphere or in tropical cumuli anvils at 18 to 20 km altitude at temperatures as low as -90 C. Ice may also form as particles are removed by descent through troposphere folds and enter midlatitude cirrus at levels below -40 C. In the stratosphere, ice particles may provide surfaces that catalyze heterogeneous chemical reactions; in the upper troposphere, the optical depths of cirrus clouds may be significantly increased.

Hallett, John↗

NASA NTRS1992

Lightning Imaging Sensor (LIS) for the Earth Observing System

Not only are scientific objectives and instrument characteristics given of a calibrated optical LIS for the EOS but also for the Tropical Rainfall Measuring Mission (TRMM) which was designed to acquire and study the distribution and variability of total lightning on a global basis. The LIS can be traced to a lightning mapper sensor planned for flight on the GOES meteorological satellites. The LIS consists of a staring imager optimized to detect and locate lightning. The LIS will detect and locate lightning with storm scale resolution (i.e., 5 to 10 km) over a large region of the Earth's surface along the orbital track of the satellite, mark the time of occurrence of the lightning, and measure the radiant energy. The LIS will have a nearly uniform 90 pct. detection efficiency within the area viewed by the sensor, and will detect intracloud and cloud-to-ground discharges during day and night conditions. Also, the LIS will monitor individual storms and storm systems long enough to obtain a measure of the lightning flashing rate when they are within the field of view of the LIS. The LIS attributes include low cost, low weight and power, low data rate, and important science. The LIS will study the hydrological cycle, general circulation and sea surface temperature variations, along with examinations of the electrical coupling of thunderstorms with the ionosphere and magnetosphere, and observations and modeling of the global electric circuit.

Christian, Hugh J.↗

NASA NTRS1990

A DFW microburst model based on DL-191 data

Analysis of the August 2, 1985 crash of an L-1011 jumbo jet (DL-191), on approach to the Dallas-Ft. Worth International Airport (DFW) in a thunderstorm indicates that the severe windshear microburst which caused the crash was composed not only of a strong downflow and outflow but also included several large-scale vortex rings entrained in the flowfield. A detailed two-dimensional model of the DFW microburst, based on data from DL-191, is presented. The model uses wind vector and flight path data reconstructed by NASA-Ames Research Center from the L-1011's digital flight data records and ground-based radar measurements. The model was developed using a combination of interactive graphics and a least-square-error best fit between the modeled and measured wind vectors along the DL-191 flight path. The model confirms a microburst structure based on a von Karman vortex street rather than on a Kelvin-Helmholtz instability and indicates that the L-1011 flew near or directly through several strong vortices. In addition, the results also confirm that the reconstructed wind vector data contain a time lag in the horizontal winds. For the DL-191 this lag is approximately three seconds.

Grantham, Walter J.↗

NASA NTRS1990

Microphysical and radiative characteristics of convective clouds during COHMEX

The microphysical structure and the evolution of two strong isolated thunderstorms that occurred on July 11, 1986 in northern Alabama were investigated using data obtained during the Cooperative Huntsville Meteorological Experiment by CP-2 multiparameter radar, together with passive microwave measurements from ER-2 aircraft. A correlation was found between the magnitude of the microwave T(B) depression and the storm intensity, although a clear relationship was not always obvious between radar-deduced microphysical characteristics and passive microwave radiative observations.

Fulton, Richard↗

NASA NTRS1990

A parameterization of radiation heating at the surface in a numerical cloud model

The development of the parameterized radiation heat flux incorporated in a 2D time-dependent model that was tested for its predictive capabilities during the COHMEX project in 1986 is described, together with two experiments for using the model in the predictive mode. Using the 12 Z sounding during the North Dakota Thunderstorm Project in 1989, it was found that the COHMEX model was quite successful in simulating the convection and predicting the cloud characteristics.

Kopp, Fred J.↗

NASA NTRS1991

Polarization of the impulsive signals observed in the nightside ionosphere of Venus

The impulsive plasma wave bursts detected by the Pioneer Venus Orbiter electric field detector in the nightside ionosphere of Venus have been attributed to atmospheric lightning. However, it has also been argued that the wave bursts are generated locally by plasma instabilities. The waves associated with local instabilities are most probably electrostatic in nature, while lightning-generated waves should be whistler mode waves, at least at the lowest frequencies. It is shown that for typical ionospheric parameters the whistler mode wave electric field should be polarized predominantly perpendicular to the ambient magnetic field. It is shown that the 100-Hz waves are polarized perpendicular to the ambient magnetic field provided the data are restricted to those intervals in which the magnetic field is sufficiently far from horizontal to allow vertical propagation within the whistler mode resonance cone. The 100-Hz waves detected outside of the resonance cone are polarized parallel to the magnetic field, as are the waves at higher frequency. The waves consequently fall into two classes: whistler mode waves which are most likely due to atmospheric lightning, and a mode that is polarized parallel to the ambient field. This latter mode may be analogous to the anomalous parallel polarized wave fields detected in the terrestrial ionosphere above thunderstorms.

Strangeway, R. J.↗

NASA NTRS1989

Nonlinear triggered lightning models for use in finite difference calculations

Two nonlinear triggered lightning models have been developed for use in finite difference calculations. Both are based on three species of air chemistry physics and couple nonlinearly calculated air conductivity to Maxwell's equations. The first model is suitable for use in three-dimensional modeling and has been applied to the analysis of triggered lightning on the NASA F106B Thunderstorm Research Aircraft. The model calculates number densities of positive ions, negative ions, and electrons as a function of time and space through continuity equations, including convective derivative terms. The set of equations is closed by using experimentally determined mobilities, and the mobilities are also used to determine the air conductivity. Results from the model's application to the F106B are shown. The second model is two-dimensional and incorporates an enhanced air chemistry formulation. Momentum conservation equations replace the mobility assumption of the first model. Energy conservation equations for neutrals, heavy ions, and electrons are also used. Energy transfer into molecular vibrational modes is accounted for. The purpose for the enhanced model is to include the effects of temperature into the air breakdown, a necessary step if the model is to simulate more than the very earliest stages of breakdown. Therefore, the model also incorporates a temperature-dependent electron avalanche rate. Results from the model's application to breakdown around a conducting ellipsoid placed in an electric field are shown.

Rudolph, Terence↗

NASA NTRS1992

Lightning induced brightening in the airglow layer

This report describes a transient luminosity observed at the altitude of the airglow layer (about 95 km) in coincidence with a lightning flash in a tropical oceanic thunderstorm directly beneath it. This event provides new evidence of direct coupling between lightning and ionospheric events. This luminous event in the ionosphere was the only one of its kind observed during an examination of several thousand images of lightning recorded under suitable viewing conditions with Space Shuttle cameras. Several possible mechanisms and interpretations are discussed briefly.

Boeck, W. L.↗

NASA NTRS1992

Measuring ionospheric electron density using the plasma frequency probe

During the past decade, the plasma frequency probe (PFP) has evolved into an accurate, proven method of measuring electron density in the ionosphere above about 90 km. The instrument uses an electrically short antenna mounted on a sounding rocket that is immersed in the plasma and notes the frequency where the antenna impedance is large and nonreactive. This frequency is closely related to the plasma frequency, which is a direct function of free electron concentration. The probe uses phase-locked loop technology to follow a changing electron density. Several sections of the plasma frequency probe circuitry are unique, especially the voltage-controlled oscillator that uses both an electronically tuned capacitor and inductor to give the wide tuning range needed for electron density measurements. The results from two recent sounding rocket flights (Thunderstorm II and CRIT II) under vastly different plasma conditions demonstrate the capabilities of the PFP and show the importance of in situ electron density measurements of understanding plasma processes.

Jensen, Mark D.↗

NASA NTRS1990

Tornadogenesis via squall line and supercell interaction revisited - The 15 November 1989 Huntsville tornado

A case study analyzing the environmental setting and storm system morphology that provides observational evidence of a mechanism involving the interaction of a gust front with a preexisting mesocyclone is presented. This case serves to reemphasize the existence of a high conditional probability of tornado occurrence, given the merger of a gust front (or storm outflow) with a moderate to strong thunderstorm ahead of it. The resultant data serve to illustrate some important unresolved issues relevant to the severe weather warning system that involve the present and planned local warning and network radars, and future algorithms that might intelligently integrate other data sources and models with the radar data.

Goodman, Steven J.↗

NASA NTRS1990

Expert knowledge techniques applied to the analysis of electric field mill data

NASA operates a network of ground-based electric field mills at Kennedy Space Center (KSC) to identify clouds which might be an electrical hazard to space vehicle prior to and during launch or to the various ground operations performed at the center. Artificial intelligence has been used to develop an expert system for analyzing electric field mill data. The application of the system to expert system to small thunderstorms at KSC is shown.

Nicholson, James R.↗

NASA NTRS1992

Dimension statistics of rain cell cores and associated rain rate isopleths derived from radar measurements in the mid-Atlantic coast of the United States

Employing a multi-year radar database of precipitation in a mid-Atlantic coast region, the authors have characterized rain cell size statistics. The measurements were made with a high resolution, high power radar in which equicircle diameter contours of resolutions of 1 km and greater were identified out to ranges of 100 km from the radar. The rain cell was constructed by its core intensity isopleth and a family of enveloping contours having defined rain rate intervals in the regions between them. The isopleth statistics were extracted from a culled database whose cores had intensities of RLN = 7 (18-24 mm/h) and greater. This implies a bias of the statistics toward various types of convective rains generally associated with thunderstorms in the mid-Atlantic coast. Since the radar measurements were made in the mid-Atlantic coast region, the isopleth statistics may not be directly applicable to other geographic regions (such as the tropics).

Goldhirsh, Julius↗