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At least 1,387 records · Page 77
All sourcesNASA NTRSDOE OSTI
NASA NTRS1989

A mesoscale analysis of the pre-storm environment on the 17 June 1986 COHMEX day

The study presented utilized special mesoscale Cooperative Huntsville Meteorological Experiment (COHMEX) data to understand the evolution of the preconvective environment on June 17, 1986. Using the special mesoscale COHMEX data, several mechanisms for triggering the convection are investigated. Afternoon heating probably was a major factor as observed noontime temperatures were near the sounding-derived convection temperatures. The special surface network revealed a quasi-stationary area of convergence not aligned with the front that may be associated with the orography of the area. This study demonstrates that rapid, small scale atmospheric variations preceded convective development on June 17, 1986.

Fuelberg, Henry E.↗
NASA NTRS1991

Gyro-phase effects near the storm-time boundary of energetic plasma

The nonadiabatic acceleration of plasma sheet ions during the expansion phase of substorms is examined by means of single-particle codes. It is shown that, in the near-earth plasma sheet, the gyration phase at substorm onset controls the net ion energization, as the gyro-period is locally comparable to the field variation time scale. This can yield a particularly significant decrease of the particle magnetic moment. It is accordingly argued that, via adiabatic invariant violation, the dipolarization of magnetospheric field lines can induce a 'de-trapping' of inner plasma sheet populations and, hence, give rise to short-lived precipitation enhancements over the auroral zone. As this effect depends upon cyclotron frequency, it is expected to occur at distinct latitudes for different ion species. This contrasts with nonadiabatic ion behavior further out into the geotail, which is characterized by intense perpendicular heating and a likely collective trapping.

Delcourt, D. C.↗
NASA NTRS1992

The contribution of ion-cyclotron waves to electron heating and SAR-arc excitation near the storm-time plasmapause

The potential role of ion-cyclotron waves in the electron heating process has been studied, using the HOTRAY code. It is demonstrated that ion-cyclotron waves can play an important role in both the energy transfer to plasmaspheric electrons and the subsequent downward heat conduction to SAR arc altitudes. In particular, such waves can experience enhanced path integrated amplification along the steep plasmapause density gradient. The latter tends to keep the wave normal angle small on several successive bounces across the equator, thus allowing cyclotron-resonant amplification leading to a total gain of up to 20 e-foldings. When the wave propagation vector becomes highly oblique, absorption occurs during Landau resonance with thermal plasmaspheric electrons, increasing the electron temperature in the direction parallel to the ambient field and leading directly to heat conduction into ionosphere.

Thorne, Richard M.↗
NASA NTRS1992

Formation of the Leonid meteor stream and storm

It is well known that some meteor showers display a very high level of activity at certain times, the most famous being the Leonid shower with very spectacular displays at roughly 33 year intervals. This period being also the period of the parent comet of the stream, Comet Tempel-Tuttle. An investigation of the geometry of the comet and the Earth at the time of each high activity occurrence by Yeomans suggests that most of the meteoroids are found outside the cometary orbit and lagging the comet. The formation process of such a stream by numerically integrating the orbits of dust particles ejected from the comet and moving under the influence of gravity and radiation pressure are simulated. The intersection of these dust particles with the Earth is also considered and it is concluded that about 12 percent of the ejected particles may be observed and that of those observable, 63 percent will be outside the cometary orbit and behind the comet.

Wu, Zidian↗
NASA NTRS1993

Climatic variation of storms

Long-term variation of U.S. tornadoes were studied by obtaining the best possible data during the 75 years, 1916-90. The most difficult task was to estimate the number of early tornadoes in 1916-50 when the reporting efficiency was very bad, resulting in undercounting of the incidents. In generating the best possible data, Fujita's Tornado Tape produced at the University of Chicago, the book Significant Tornadoes, 1880-1989, by Thomas P. Grazulis, and the NSSFC Tornado Tape were combined. First, the annual number of tornadoes were smoothed to obtain the smoothed number of annual tornadoes which was normalized to the standard number of tornadoes defined as the mean number in the recent 30 years 1960-89. The normalization factor was obtained by computing the ration of smoothed and standard number. In the early years, the factor was in excess of 7. Thereafter, the normalized number of annual tornadoes was computed and plotted, finding that the results are very satisfactory. In order to visualize the variation of tornado activities, the tornado activity number (TAN) was initiated and computed. The TAN including all annual tornadoes show certain periodicity. For climatological evaluation, the peak-activity day of each year, such as Superoutbreak day, 3 April 1974, Palm Sunday, 11 April 1965 were eliminated. The TAN, excluding peak-day tornadoes, is of extreme interest.

Fujita, Ted↗
NASA NTRS1992

Climatology of Station Storm Rainfall in the Continental United States: Parameters of the Bartlett-Lewis and Poisson Rectangular Pulses Models

The parameters of two stochastic models of point rainfall, the Bartlett-Lewis model and the Poisson rectangular pulses model, are estimated for each month of the year from the historical records of hourly precipitation at more than seventy first-order stations in the continental United States. The parameters are presented both in tabular form and as isopleths on maps. The Poisson rectangular pulses parameters are useful in implementing models of the land surface water balance. The Bartlett-Lewis parameters are useful in disaggregating precipitation to a time period shorter than that of existing observations. Information is also included on a floppy disk.

Hawk, Kelly Lynn↗
NASA NTRS1993

Low-frequency synoptic-eddy activity in the Pacific storm track

The GCM presently used to examine the nature of LF variations in synoptic-eddy activity over the North Pacific, when compared with observations, yields realistic time-mean and LF synoptic-eddy forcing of the 200-mb zonal wind. In the time mean, this forcing exhibits an east-west dipole structure that is consistent with the general view of the life cycle of baroclinic waves; these show marked upward and eastward propagation in the western and central Pacific, as well as meridional propagation to the east. Composites of the hemispheric distribution of synoptic-eddy forcing in the GCM, based on the extremes of the dominant Pacific empirical orthogonal function, show a seesaw behavior with enhanced eddy forcing in the North Pacific basin which is associated with suppressed forcing in the North Atlantic basin (and vice versa).

Higgins, R. W.↗
NASA NTRS1993

Case study of a low-reflectivity pulsating microburst - Numerical simulation of the Denver, 8 July 1989, storm

An intense, low-reflectivity, and pulsating microburst event, detected on July 8, 1989 by the Low-Level Windshear Alert System north of Denver Stapleton Airport is described using a Terminal Area Simulation System (TASS) model (Proctor, 1987). This 3d nonhydrostatic cloud model includes parametrizations for both liquid- and ice-phase microphysics. Simulation results show that the dissipating cloud induces a type-1 pulsating microburst event, with at least three distinct microburst pulses over a period of 20 min. Virga from the dissipating cloud continued to maintain hazardous windshear for a period much longer than the typical lifetime of a single microburst.

Proctor, Fred H.↗
NASA NTRS1993

Techniques and resources for storm-scale numerical weather prediction

The topics discussed include the following: multiscale application of the 5th-generation PSU/NCAR mesoscale model, the coupling of nonhydrostatic atmospheric and hydrostatic ocean models for air-sea interaction studies; a numerical simulation of cloud formation over complex topography; adaptive grid simulations of convection; an unstructured grid, nonhydrostatic meso/cloud scale model; efficient mesoscale modeling for multiple scales using variable resolution; initialization of cloud-scale models with Doppler radar data; and making effective use of future computing architectures, networks, and visualization software.

Droegemeier, Kelvin↗
NASA NTRS1993

Case study of a low-reflectivity pulsating microburst - Numerical simulation of the Denver, 8 July 1989, storm

Results from a 3D simulation of a very strong microburst, detected by the Low-Level Windshear Alert System, are presented. The simulation was performed using the Terminal Area Simulation System (TASS) model (Proctor, 1987) which is a 3D nonhydrostatic cloud model that includes parameterizations for both liquid and ice-phase microphysics. Emphasis is placed om the pulsating characteristic and the very-low radar reflectivity of the event. The simulation revealed that the dissipating cloud induces a type-1 pulsating microburst event, with at least three distinct microburst pulses over a period of 20 min.

Proctor, Fred H.↗
NASA NTRS1994

Case study of a low-reflectivity pulsating microburst: Numerical simulation of the Denver, 8 July 1989, storm

On 8 July 1989, a very strong microburst was detected by the Low-Level Windshear Alert system (LLWAS), within the approach corridor just north of Denver Stapleton Airport. The microburst was encountered by a Boeing 737-200 in a 'go-around' configuration which was reported to have lost considerable air speed and altitude during penetration. Data from LLWAS revealed a pulsating microburst with an estimated peak velocity change of 48 m/s. Wilson et al. reported that the microburst was accompanied by no apparent visible clues such as rain or virga, although blowing dust was present. Weather service hourly reports indicated virga in all quadrants near the time of the event. A National Center for Atmospheric Research (NCAR) research Doppler radar was operating; but according to Wilson et al., meaningful velocity could not be measured within the microburst due to low radar-reflectivity factor and poor siting for windshear detection at Stapleton. This paper presents results from the three-dimensional numerical simulation of this event, using the Terminal Area Simulation System (TASS) model. The TASS model is a three-dimensional nonhydrostatic cloud model that includes parameterizations for both liquid and ice phase microphysics, and has been used in investigations of both wet and dry microburst case studies. The focus of this paper is the pulsating characteristic and the very-low radar reflectivity of this event. Most of the surface outflow contained no precipitation. Such an event may be difficult to detect by radar.

Proctor, Fred H.↗
NASA NTRS1997

The Impact of Aerosols Generated from Biomass Burning, Dust Storms, and Volcanoes Upon the Earth's Radiative Energy Budget

A new technique for detecting aerosols from biomass burning and dust is developed. The radiative forcing of aerosols is estimated over four major ecosystems in South America. A new smoke and fire detection scheme is developed for biomass burning aerosols over South America. Surface shortware irradiance calculations are developed in the presence of biomass burning aerosols during the SCAR-B experiment. This new approach utilizes ground based, aircraft, and satellite measurements.

Christopher, Sundar A.↗
NASA NTRS1998

Assessing the Geomorphic Evolution and Hydrographic Changes Induced by Winter Storms along the Louisiana Coast

The influence that cold front passages have on Louisiana coastal environments, including land loss and land building processes, has been the primary topic of this multidisciplinary research. This research has combined meteorological, remote sensing, and coastal expertise from the University of Wisconsin (UW) and Louisiana State University (LSU). Analyzed data sets include remotely sensed radiometric data (AVHRR on NOAA-12,13,14, Multispectral Atmospheric Mapping Sensor (MAMS) and MODIS Airborne Simulator (MAS) on NASA ER-2), U.S. Army Corps of Engineers (USACE) water level data, water quality data from the Coastal Studies Institute (CSI) at LSU, USACE river discharge data, National Weather Service (NWS) and CSI wind in sitzi measurements, geomorphic measurements from aerial photography (NASA ER-2 and Learjet), and CSI ground based sediment burial pipes (for monitoring topographic change along the Louisiana coast) and sediment cores. The work reported here-in is a continuation of an initial investigation into coastal Louisiana landform modification by cold front systems. That initial effort demonstrated the importance of cold front winds in the Atchafalaya Bay sediment plume distribution (Moeller et al.), documented the sediment transport and deposition process of the western Louisiana coast (Huh et al.) and developed tools (e.g. water types identification, suspended solids estimation) from multispectral radiometric data for application to the current study. This study has extended that work, developing a Geomorphic Impact Index (GI(sup 2)) for relating atmospheric forcing to coastal response and new tools to measure water motion and sediment transport.

Menzel, W. Paul↗
NASA NTRS1999

Localized Upper Tropospheric Warming During Tropical Depression and Storm Formation Revealed by the NOAA-15 AMSU

The warm core of hurricanes as measured by microwave temperature sounders has been related to various azimuthally averaged measures of hurricane strength by several researchers Unfortunately, the use of these instruments (e.g. the Microwave Sounding Units, MSU) for the routine monitoring of tropical cyclone genesis and intensity has been hampered by poor resolution. The recent launch of the NOAA-15 AMSU represents a significant advance in our ability to monitor subtle atmospheric temperature variations (0.1-0.2 C) at relatively high spatial resolution (50 km) in the presence of clouds. Of particular interest is the possible capability of the AMSU to observe the slight warming associated with depression formation, and the relationship of the spatial characteristics of the warming to the surface pressure and wind field, without azimuthal averaging. In order to present the AMSU data as imagery, we have developed a method for precise limb-correction of all 15 AMSU channels. Through a linear combination of several neighboring channels, we can very closely match the nadir weighting functions of a given AMSU sounding channel with the non-nadir data. It is found that there is discernible, localized upper tropospheric warming associated with depression formation in the Atlantic basin during the 1998 hurricane season. Also, it is found that uncertainty in positioning of tropical cyclone circulation centers can be reduced, as in the example of Hurricane Georges as it approached Cuba. Finally, to explore the potential utility of a future high resolution microwave temperature sounder, we present an analysis of the relationship between the modeled surface wind field and simulated high -resolution AMSU-type measurements, based upon cloud resolving model simulations of hurricane Andrew in 1992.

Spencer, Roy W.↗