Engineering topics

Keyser, D. A.

Publications and source records attributed to Keyser, D. A..

8 records

NASA NTRS1987

An evaluation of various forms of VAS retrievals in the analysis of a preconvective environment

VISSR Atmospheric Sounder (VAS) radiance data obtained over the continental United States on July 20, 1981 are used to evaluate a variety of VAS retrieval procedures and parameters in the qualitative analysis and forecasting of severe weather events. The particular case analyzed contains two significantly different mesoscale convective events in the central plains. Retrievals of temperature, dewpoint temperature, equivalent potential temperature, total column precipitable water, and lifted index are shown to be physically consistent in space and time and to compare well with available radiosonde data. The analysis of the VAS retrievals identified significant spatial gradients and temporal changes in the thermal and moisture fields, including times and locations between radiosonde observations.

Petersen, R. A.↗

NASA NTRS1984

Delineating mid- and low-level water vapor patterns in pre-convective environments using VAS moisture channels

Infrared and visible imagery from VAS are used to delineate mid- and lower-tropospheric moisture fields for a variety of severe storm cases in the southern and central United States. The ability of sequences of images to isolate areas of large negative vertical moisture gradients and apparent convective instability prior to the onset of convective storms is assessed. A variety of image combination procedures are used to deduce the stability fields which are then compared with the available radiosonde data. The results for several severe storm cases indicate that VAS can detect mid- and low-level mesoscale water vapor fields as distinct radiometric signals. The VAS imagery shows a strong tendency for thunderstorms to develop along the edges of bands of midlevel dryness as they overtake either preexisting or developing low-level moisture maxima. Image sequences depict the speed with which deep moist and dry layers can develop and move.

Petersen, R. A.↗

NASA NTRS1984

Effects of diabatic heating on the ageostrophic circulation of an upper tropospheric jet streak

Higher temporal resolution data from NASA's fourth Atmospheric Variability Experiment (April 24-25, 1975) are used to study the role of diabatic heating within the convective region in altering the ageostrophic mass circulation in the entrance region of the jet streak. The modifying effect this diabatic heating in the mesoscale storm exerts on the synoptic-scale environmental flow through dynamical mechanisms which link the mass circulation of these two scales is examined. The effect of diabatic heating on the ageostrophic motion within the thermally direct circulation of the jet streak and the mesoscale convective complexes is investigated. Diabatic modes of ageostrophic motion that intensify the upper tropospheric jet streak downstream are identified.

Keyser, D. A.↗

NASA NTRS1984

The accuracy of mesoscale temperature and dewpoint fields retrieved from VAS satellite and conventional surface data

The Visible Infrared Spin-Scan Radiometer (VISSR) Atmospheric Sounder (VAS) is a twelve channel imaging radiometer on the Geostationary Operational Environmental Satellites (GOES). It is pointed out that the instrument is designed to monitor the evolution of tropospheric temperature and moisture fields at mesoscale resolution. Since the numerical qualities of VAS soundings affect their usefulness, a quantitative verification of the absolute and relative accuracy of VAS soundings appears highly desirable. The present investigation is concerned with an objective verification for two VAS case studies. Absolute accuray is measured by comparing the VAS retrievals to the radiosonde values at the network sites. Absolute temperature accuracy is found to be + or - 2 C below 500 mb, while absolute dewpoint accuracy is + or - 2 to + or - 3 C below 850 mb.

Chesters, D.↗

NASA NTRS1984

Advective forecasts of VAS imagery for use in anticipating convective destabilization

Wind field forecasts, based on data from the Visible Atmospheric Sounder (VAS) instrument on board the most recent GOES satellite, are described. The forecasts were used to generate a series of water vapor images for the Central U.S. according to an isentropic prediction scheme. The ability of VAS imagery to detect regions of mesoscale convective instability was contrasted with data from a 9-12 hr advective forecast and the results are discussed. It is shown that the VAS imagery was free from masking by convective outflows and lends itself to repeated applications for updating throughout the day. The incorporation of surface data into the VAS forecast is recommended in order to identify areas of persistent moisture convergence. Some examples of the VAS imagery are provided.

Petersen, R. A.↗

NASA NTRS1983

Evaluation of the synoptic and mesoscale predictive capabilities of a mesoscale atmospheric simulation system

The overall performance characteristics of a limited area, hydrostatic, fine (52 km) mesh, primitive equation, numerical weather prediction model are determined in anticipation of satellite data assimilations with the model. The synoptic and mesoscale predictive capabilities of version 2.0 of this model, the Mesoscale Atmospheric Simulation System (MASS 2.0), were evaluated. The two part study is based on a sample of approximately thirty 12h and 24h forecasts of atmospheric flow patterns during spring and early summer. The synoptic scale evaluation results benchmark the performance of MASS 2.0 against that of an operational, synoptic scale weather prediction model, the Limited area Fine Mesh (LFM). The large sample allows for the calculation of statistically significant measures of forecast accuracy and the determination of systematic model errors. The synoptic scale benchmark is required before unsmoothed mesoscale forecast fields can be seriously considered.

Koch, S. E.↗

NASA NTRS1983

Techniques for diagnosing mesoscale phenomena affecting aviation using VAS satellite data

A major advance in geostationary satellite observations occurred recently with the inclusion of an experimental instrument aboard the latest Geostationary Operational Environmental Satellites (GOES). This instrument, called the Visible Infrared Spin-Scan Radiometer (VISSR) Atmospheric Sounder (VAS), permits simultaneous observations of the atmosphere in up to 12 infrared channels, in addition to the visible image. In the present investigation, particular emphasis is placed on the ability of VAS imagery and derived soundings to delineate isolated, meso-scale regions of convective instability. The results of the investigation point to the ability of the VAS instrument to delineate meteorologically meaningful patterns of low-level moisture and mid-level dryness and to the utility of the image overlay procedure for isolating, in real time, regions of convective potential, at time and space scales unresolvable using radiosonde data alone.

Petersen, R. A.↗

NASA NTRS1982

Effects of diabatic heating on the ageostrophic circulation of an upper tropospheric jet streak

Interaction between the mass circulation within a mesoscale convective complex (MCC) and a direct mass circulation in the entrance region of an upper tropospheric polar jet streak was examined within the isentropic structure to investigate mechanisms responsible for linking these two scales of motion. The results establish that latent heating in the MCC modifies the direct mass circulation in the jet streak entrance region through the diabatically induced components of ageostrophic motion analyzed within isentropic coordinates. Within the strong mesoscale mass circulation of each MCC, strong horizontal mass flux convergence into the MCC at low levels is balanced by strong horizontal mass flux divergence away from the convergence at upper levels. Locations of large diabatic heating rates correspond well to the MCC position for each case; diabatic heating forces the upward vertical branch for the mesoscale mass circulation.

Keyser, D. A.↗