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At least 1,477 records · Page 82
All sourcesNASA NTRSDOE OSTI
NASA NTRS1974

Rain droplet erosion mechanisms in transparent plastic materials

Tests were conducted to determine the damaging effects of rain erosion on optically transparent materials. The rotating arm test equipment used for the tests is described. Typical transparent materials such as those found in windshields, infrared windows, lasers, and television systems were tested. Nominal velocities of 400, 500, and 600 miles per hour and rainfall conditions of one inch per hour simulated rainfall were used in the tests. It was determined that an 80 percent reduction in laser transmittance can occur in plastics submitted to rain erosion. Significant results of the environmental tests are explained.

Schmitt, G. F., Jr.↗
NASA NTRS1974

Dynamics of suspended sediment plumes in Lake Ontario

The absence of turbidity plumes during the winter was well documented in an ususually successful sequence of images obtained February 10 - 12, 1974. Useful imagery of the south shore of Lake Ontario was obtained on 3 successive days at a time when sky cover over the area normally approaches complete coverage. Imagery of the Oswego, Genesee, and Niagara rivers failed to detect any plumes, however faint. Despite strong northwest winds on February 11 there was no indication of shoreline erosion generated by wave action. Frozen ground, snow cover, shoreline icing and minimal construction and farm activity without doubt reduces the probability of sediment movement in winter. Thunderstorm activity over the study area is very rare during the cold season so that the erosive energy of rainfall is greatly reduced. Moreover, a fairly high percentage of the winter precipitation is in the form of snow or sleet further reducing the impact of rainfall energy on sediment transport.

Pluhowski, E. J.↗
NASA NTRS1975

A multisensor analysis of Nimbus-5 data recorded on 22 January 1973

The Nimbus 5 meteorological satellite has a full complement of radiation sensors. Data from these sensors were analyzed and intercompared for orbits 569 and 570. The electrically-scanning microwave radiometer (19.35-GHz region) delineated rain areas over the ocean off the U.S. east coast, in good agreement with radar imagery, and permitted the estimation of rainfall rates in this region. Residual ground water, from abnormal rainfall in the lower Mississippi Valley, was indicated under clear sky conditions by soil brightness temperature values in the Nimbus 5 electrically scanning microwave radiometer and U.S. Air Force Data Acquisition and Processing Program infrared data. The temperature-humidity infrared radiometer (6.7 micron and 11 micron) showed the height and spatial configuration of frontal clouds along the east coast and outlined the confluence of a polar jet stream with a broad subtropical jet stream along the U.S. Gulf Coast. Temperature profiles from three vertical temperature sounders are found to be in good agreement with related radiosonde ascents along orbit 569 from the subtropics to the Arctic Circle.

Allison, L. J.↗
NASA NTRS1975

Modern control concepts in hydrology

Two approaches to an identification problem in hydrology are presented, based upon concepts from modern control and estimation theory. The first approach treats the identification of unknown parameters in a hydrologic system subject to noisy inputs as an adaptive linear stochastic control problem; the second approach alters the model equation to account for the random part in the inputs, and then uses a nonlinear estimation scheme to estimate the unknown parameters. Both approaches use state-space concepts. The identification schemes are sequential and adaptive and can handle either time-invariant or time-dependent parameters. They are used to identify parameters in the Prasad model of rainfall-runoff. The results obtained are encouraging and confirm the results from two previous studies; the first using numerical integration of the model equation along with a trial-and-error procedure, and the second using a quasi-linearization technique. The proposed approaches offer a systematic way of analyzing the rainfall-runoff process when the input data are imbedded in noise.

Duong, N.↗
NASA NTRS1975

Drought in the Sahara - A biogeophysical feedback mechanism

Two integrations of a global general circulation model, differing only in the prescribed surface albedo in the Sahara, show that an increase in albedo resulting from a decrease in plant cover causes a decrease in rainfall. Thus any tendency for plant cover to decrease would be reinforced by a decrease in rainfall, and could initiate or perpetuate a drought.

Charney, J.↗
NASA NTRS1975

Satellite microwave observations of soil moisture variations

The electrically scanning microwave radiometer (ESMR) on the Nimbus 5 satellite was used to observe microwave emissions from vegetated and soil surfaces over an Illinois-Indiana study area, the Mississippi Valley, and the Great Salt Lake Desert in Utah. Analysis of microwave brightness temperatures (T sub B) and antecedent rainfall over these areas provided a way to monitor variations of near-surface soil moisture. Because vegetation absorbs microwave emission from the soil at the 1.55 cm wavelength of ESMR, relative soil moisture measurements can only be obtained over bare or sparsely vegetated soil. In general T sub B increased during rainfree periods as evaporation of water and drying of the surface soil occurs, and drops in T sub B are experienced after significant rainfall events wet the soil. Microwave observations from space are limited to coarse resolutions (10-25 km), but it may be possible in regions with sparse vegetation cover to estimate soil moisture conditions on a watershed or agricultural district basis, particularly since daily observations can be obtained.

Schmugge, T. J.↗
NASA NTRS1976

Tropical oceanic precipitation frequency from Nimbus 5 microwave data

Microwave brightness temperature data from the Nimbus 5 satellite are analyzed, using threshold brightness temperatures, to yield tropical oceanic precipitation frequencies for several classes of rainfall rates during the season December 1972 through February 1973. Data taken near local noon and near local midnight are analyzed. The overall results are consistent with both climatological precipitation frequency and with concurrent satellite-derived frequency of highly reflective clouds. The difference between the local noon and local midnight frequencies is small, but the heavier rainfall rates tend to occur more frequently near local noon. The ratios of the frequencies of light, moderate, and heavy rain were observed to be relatively constant over the tropical oceans. Passive microwave measurements from space seem to be an important step toward accurate measurement of oceanic precipitation.

Kidder, S. Q.↗
NASA NTRS1976

Data analysis report on ATS-F COMSAT millimeter wave propagation experiment, part 1

The 13/18 GHz COMSAT Propagation Experiment (CPE) was performed to measure attenuation caused by hydrometeors along slant paths from transmitting terminals on the ground to the ATS-6 satellite. The effectiveness of site diversity in overcoming this impairment was also studied. Problems encountered in assembling a valid data base of rain induced attenuation data for statistical analysis are considered. The procedures used to obtain the various statistics are then outlined. The graphs and tables of statistical data for the 15 dual frequency (13 and 18 GHz) site diversity locations are discussed. Cumulative rain rate statistics for the Fayetteville and Boston sites based on point rainfall data collected are presented along with extrapolations of the attenuation and point rainfall data.

Hyde, G.↗
NASA NTRS1976

Large Area Crop Inventory Experiment (LACIE). Results of LACIE integrated drought analysis (Southern U.S. Great Plains drought 1975-76)

The development and intensification of the drought in the United States southern Great Plains was monitored during the initial growing period of the 1975-76 winter wheat crop. Because of the severity of the drought conditions, a drought analysis plan was developed and implemented beginning on March 8, 1976. Sample segments and full-frame imagery were used at 9-day intervals to identify the drought area and quantify the effects on the wheat acreage. Yield model simulations were run to extrapolate the effects of the drought on yield estimates at harvest, assuming 10 and 90 percent of normal rainfall for subsequent months and 30-day forecast. A survey of LANDSAT data for improvement of distribution of rainfall patterns in the drought area was done for April and yield models run for drought affected crop reporting districts. Special aggregations were performed by the Crop Assessment Subsystem on the drought area to evaluate the utility of remote sensing to monitor the effect of the drought on wheat area, yield, and production.

Thompson, D. R.↗
NASA NTRS1977

Seasonal oceanic precipitation frequencies from Nimbus 5 microwave data

Microwave brightness temperature data from the Nimbus 5 satellite have been analyzed by using threshold brightness temperatures to yield tropical oceanic precipitation frequencies for several classes of rainfall rates during the season December 1972 through February 1973. Data taken near local noon and near local midnight were analyzed. The overall results are consistent with both climatological precipitation frequency and with concurrent satellite-derived frequency of highly reflective clouds. The difference between the local noon and the local midnight frequency is small, but the heavier rainfall rates tend to occur more frequently near local noon. The ratios of the frequencies of light, moderate, and heavy rain were observed to be relatively constant over the tropical oceans. Passive microwave measurements from space seem to be an important step toward accurate measurement of oceanic precipitation.

Kidder, S. Q.↗
NASA NTRS1978

Meteorological radar calibration

A meteorological radar calibration technique is developed. It is found that the integrated, range corrected, received power saturates under intense rain conditions in a manner analogous to that encountered for the radiometric path temperature. Furthermore, it is found that this saturation condition establishes a bound which may be used to determine an absolution radar calibration for the case of radars operating at attenuating wavelengths. In the case of less intense rainfall or for radars at nonattenuating wavelengths, the relationship for direct calibration in terms of an independent measurement of radiometric path temperature is developed. This approach offers the advantage that the calibration is in terms of an independent measurement of the rainfall through the same elevated region as that viewed by the radar.

Hodge, D. B.↗
NASA NTRS1976

Large Area Crop Inventory Experiment (LACIE). Detection of episodic phenomena on LANDSAT imagery

The author has identified the following significant results. Episodic phenomena such as rainfall shortly before data pass, thin translucent clouds, cloud shadows, and aircraft condensation trails and their shadows are responsible for changes in the spectral reflectivities of some surfaces. These changes are readily detected on LANDSAT full-frame imagery. Histograms of selected areas in Kansas show a distinct decrease in mean radiance values, but also, an increase in scene contrast, in areas where recent rains had occurred. Histograms from a few individual fields indicate that the mean radiance values for winter wheat followed a different trend after a rainfall than alfalfa or grasses.

Chesnutwood, C. M.↗
NASA NTRS1979

Time-dependent gravity in southern California, May 1974 - Apr 1979

Gravity measurements were coordinated with the long baseline three dimensional geodetic measurements of the Astronomical Radio Interferometric Earth Surveying project which used radio interferometry with extra-galactic radio sources. Gravity data from 28 of the stations had a single reading standard deviation of 11 microgal which gives a relative single determination between stations a standard deviation of 16 microgal. The largest gravity variation observed, 80 microgal, correlated with nearby waterwell variations and with smoothed rainfall. Smoothed rainfall data appeared to be a good indicator of the qualitative response of gravity to changing groundwater levels at other suprasediment stations, but frequent measurement of gravity at a station was essential until the quantitative calibration of the station's response to groundwater variations was accomplished.

Whitcomb, J. H.↗
NASA NTRS1980

A study of the dynamics of droughts in Northern Brazil: Observations, theory, and numerical experiments with a global atmospheric circulation model

The monthly mean sea surface temperature anomalies over tropical Altantic and rainfall anomalies over two selected stations for 25 years (1948-1972) were examined. It is found that the most severe drought events are associated with the simultaneous occurrence of warm sea surface temperature anomalies over north and cold sea surface temperature anomalies over south tropical Atlantic. Simultaneous occurrences of warm sea surface temperature anomaly at 15 deg N, 45 deg W and cold sea surface temperature anomaly at 15 deg S, 5 deg W were always associated with negative anomalies of rainfall, and vice versa. A simple primitive equation model is used to calculate the frictionally controlled and thermally driven circulation due to a prescribed heating function in a resting atmosphere.

Moura, A. D.↗
NASA NTRS1958

A Flight Evaluation and Analysis of the Effect of Icing Conditions on the ZPG-2 Airship

A series of test flights was conducted by the U. S. Navy over a 3- year period to evaluate the effects of icing on the operation of the ZPG-2 airship. In supercooled. clouds, ice formed only on the forward edges of small protuberances and wires and presented no serious hazard to operation. Ice accretions of the glaze type which occurred in conditions described as freezing drizzle adversely affected various components to a somewhat greater extent. The results indicated, a need for protection of certain components such as antennas, propellers, and certain parts of the control system. The tests showed that icing of the large surface of the envelope occurred only in freezing rain or drizzle. Because of the infrequent occurrence of these conditions, the potential maximum severity could not be estimated from the test results. The increases in heaviness caused by icing in freezing rain and drizzle were substantial, but well within the operational capabilities of the airship. In order to estimate the potential operational significance of icing in freezing rain, theoretical calculations were used to estimate: (1) the rate of icing as a function of temperature and rainfall intensity, (2) the climatological probability of occurrence of various combinations of these variables, and (3) the significance of the warming influence of the ocean in alleviating freezing-rain conditions. The results of these calculations suggest that, although very heavy icing rates are possible in combinations of low temperature and high rainfall rate, the occurrence of such conditions is very infrequent in coastal areas and virtually impossible 200 or 300 miles offshore.

Lewis, Willilam↗
NASA NTRS1981

An evaluation of the spatial resolution of soil moisture information

Rainfall-amount patterns in the central regions of the U.S. were assessed. The spatial scales of surface features and their corresponding microwave responses in the mid western U.S. were investigated. The usefulness for U.S. government agencies of soil moisture information at scales of 10 km and 1 km. was ascertained. From an investigation of 494 storms, it was found that the rainfall resulting from the passage of most types of storms produces patterns which can be resolved on a 10 km scale. The land features causing the greatest problem in the sensing of soil moisture over large agricultural areas with a radiometer are bodies of water. Over the mid-western portions of the U.S., water occupies less than 2% of the total area, the consequently, the water bodies will not have a significant impact on the mapping of soil moisture. Over most of the areas, measurements at a 10-km resolution would adequately define the distribution of soil moisture. Crop yield models and hydrological models would give improved results if soil moisture information at scales of 10 km was available.

Hardy, K. R.↗
NASA NTRS1981

The initialization of a mesoscale weather prediction model using satellite and precipitation data

An analysis and initialization procedure was developed capable of utilizing mesoscale satellite and radar data to resolve features in moisture and momentum fields on a finer scale than is discernible from radiosonde observation networks. The radiosonde moisture field is enhanced, using infrared satellite imagery and precipitation data from the surface observation network (also obtainable from the radar network) while the observed mesoscale rainfall rates are also used to define the initial divergent wind component. Results show that the predicted rainfall amounts are significantly improved when these additional data are taken into account.

Tarbell, T. C.↗
NASA NTRS1982

Adaptively pointing spaceborne radar for precipitation measurements

An auxiliary concept of adaptive pointing is presented as an aid to space-based measurements of precipitation using microwave radar. It is recommended that the instantaneous field of view (IFOV) be limited to cloudy regions of possible precipitation rather than scanning clear-sky areas of no interest. A microwave imager is proposed for scanning ahead of the satellite in order to identify areas of precipitation, and then an on-board antenna controller can reposition the scanning antenna to obtain reflectivities for use in rainfall-reflectivity relationships. The precursor beam could delineate the storm boundaries and determine points of peak intensity which can then be selectively sampled to find the rainfall rate distribution for the storm. The adaptive-beam system is suggested to be implementable using an IR sensor on-board geosynchronous satellites.

Atlas, D.↗