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At least 163 records · Page 9
All sourcesNASA NTRSDOE OSTI
NASA NTRS2015

Initial Results from Radiometer and Polarized Radar-Based Icing Algorithms Compared to In-Situ Data

In early 2015, a field campaign was conducted at the NASA Glenn Research Center in Cleveland, Ohio, USA. The purpose of the campaign is to test several prototype algorithms meant to detect the location and severity of in-flight icing (or icing aloft, as opposed to ground icing) within the terminal airspace. Terminal airspace for this project is currently defined as within 25 kilometers horizontal distance of the terminal, which in this instance is Hopkins International Airport in Cleveland. Two new and improved algorithms that utilize ground-based remote sensing instrumentation have been developed and were operated during the field campaign. The first is the 'NASA Icing Remote Sensing System', or NIRSS. The second algorithm is the 'Radar Icing Algorithm', or RadIA. In addition to these algorithms, which were derived from ground-based remote sensors, in-situ icing measurements of the profiles of super-cooled liquid water (SLW) collected with vibrating wire sondes attached to weather balloons produced a comprehensive database for comparison. Key fields from the SLW-sondes include air temperature, humidity and liquid water content, cataloged by time and 3-D location. This work gives an overview of the NIRSS and RadIA products and results are compared to in-situ SLW-sonde data from one icing case study. The location and quantity of super-cooled liquid as measured by the in-situ probes provide a measure of the utility of these prototype hazard-sensing algorithms.

flight test↗
NASA NTRS1990

A parameterization for the shortwave transmissivity of stratiform water clouds based on empirical data and radiative transfer theory

Surface measurements of solar flux and total integrated liquid-water content, radiosonde data, and infrared satellite images are analyzed in conjunction with radiative transfer calculations to derive an empirical parameterization for the shortwave transmissivity of continental stratiform water clouds. The data were collected near Denver, Colorado, over a period of six years. A multiple reflection radiative transfer model is employed to estimate stratus cloud transmissivity in terms of the measurable liquid-water path (LWP). A nonlinear fit of estimated transmissivities to the corresponding observations of LWP yields close agreement with a previous, more complicated parameterization. The derived expression for cloud transmissivity is used to predict mean daily surface fluxes for 61 days during which periods of stratiform clouds were observed over the Denver area. A comparison between predicted and measured fluxes shows agreement to within + or - 4 percent, with best agreement for clouds of moderate optical thickness. Potential sources of error are identified with sensitivity studies.

Derr, V. E.↗
NASA NTRS2008

Radiative Susceptibility of Cloudy Atmospheres to Droplet Number Perturbations: 1. Theoretical Analysis and Examples from MODIS

Theoretical and satellite-based assessments of the sensitivity of broadband shortwave radiative fluxes in cloudy atmospheres to small perturbations in the cloud droplet number concentration (N) of liquid water clouds under constant water conditions are performed. Two approaches to study this sensitivity are adopted: absolute increases in N, for which the radiative response is referred to as absolute cloud susceptibility, and relative increases in N or relative cloud susceptibility. Estimating the former is more challenging as it requires an assumed value for either cloud liquid water content or geometrical thickness; both susceptibilities require an assumed relationship between the droplet volume and effective radius. Expanding upon previous susceptibility studies, present radiative calculations include the effect of AN perturbations on droplet asymmetry parameter and single-scattering albedo, in addition to extinction. Absolute cloud susceptibility has a strong nonlinear dependence on the droplet effective radius as expected, while relative cloud susceptibility is primarily dependent on optical thickness. Molecular absorption and reflecting surfaces both reduce the relative contribution of the cloud to the top-of-atmosphere (TOA) flux and therefore also reduce the TOA albedo susceptibility. Transmittance susceptibilities are negative with absolute values similar to albedo susceptibility, while atmospheric absorptance susceptibilities are about an order of magnitude smaller than albedo susceptibilities and can be either positive or negative. Observation-based susceptibility calculations are derived from MODIS pixel-level retrievals of liquid water cloud optical thickness, effective radius, and cloud top temperature; two data granule examples are shown. Susceptibility quantifies the aerosol indirect effect sensitivity in a way that can be easily computed from model fields. As such, susceptibilities derived from MODIS observations provide a higher-order test of model cloud properties used for indirect effect studies. MODIS-derived global distributions of cloud susceptibility and radiative forcing calculations are presented in a companion paper.

Platnick, Steven↗
NASA NTRS2006

Instrument for Aircraft-Icing and Cloud-Physics Measurements

The figure shows a compact, rugged, simple sensor head that is part of an instrumentation system for making measurements to characterize the severity of aircraft-icing conditions and/or to perform research on cloud physics. The quantities that are calculated from measurement data acquired by this system and that are used to quantify the severity of icing conditions include sizes of cloud water drops, cloud liquid water content (LWC), cloud ice water content (IWC), and cloud total water content (TWC). The sensor head is mounted on the outside of an aircraft, positioned and oriented to intercept the ambient airflow. The sensor head consists of an open housing that is heated in a controlled manner to keep it free of ice and that contains four hot-wire elements. The hot-wire sensing elements have different shapes and sizes and, therefore, exhibit different measurement efficiencies with respect to droplet size and water phase (liquid, frozen, or mixed). Three of the hot-wire sensing elements are oriented across the airflow so as to intercept incoming cloud water. For each of these elements, the LWC or TWC affects the power required to maintain a constant temperature in the presence of cloud water.

Lilie, Lyle↗
NASA NTRS1978

Studies of snowpack properties by passive microwave radiometry

Research involving the microwave characteristics of snow was undertaken in order to expand the information content currently available from remote sensing, namely the measurement of snowcovered area. Microwave radiation emitted from beneath the snow surface can be sensed and thus permits information on internal snowpack properties to be inferred. The intensity of radiation received is a function of the average temperature and emissivity of the snow layers and is commonly referred to as the brightness temperature (T sub b). The T sub b varies with snow grain and crystal sizes, liquid water content and snowpack temperature. The T sub b of the 0.8 cm wavelength channel was found to decrease moreso with increasing snow depth than the 1.4 cm channel. More scattering of the shorter wavelength radiation occurs thus resulting in a lower T sub b for shorter wavelengths in a dry snowpack. The longer 21.0 cm wavelength was used to assess the condition of the underlying ground. Ultimately it may be possible to estimate snow volume over large areas using calibrated brightness temperatures and consequently improve snowmelt runoff predictions.

Chang, A. T. C.↗
NASA NTRS1979

Studies of snowpack properties by passive microwave radiometry

Research involving the microwave characteristics of snow was undertaken in order to expand the information content currently available from remote sensing, namely the measurement of snowcovered area. Microwave radiation emitted from beneath the snow surface can be sensed and thus permits information on internal snowpack properties to be inferred. The intensity of radiation received is a function of the average temperature and emissivity of the snow layers and is commonly referred to as the brightness temperature (T sub B). The T sub B varies with snow grain and crystal sizes, liquid water content, and snowpack temperature. The T sub B of the 0.8 cm wavelength channel was found to decrease more so with increasing snow depth than the 1.4 cm channel. More scattering of the shorter wavelength radiation occurs thus resulting in a lower T sub B for shorter wavelengths in a dry snowpack. The longer 21.0 cm wavelength was used to assess the condition of the underlying ground.

Chang, A. T. C.↗
NASA NTRS1972

Project Fog Drops. Part 1: Investigations of warm fog properties

A detailed study was made of the micrometeorological and microphysical characteristics of eleven valley fogs occurring near Elmira, New York. Observations were made of temperature, dew point, wind speed and direction, dew deposition, vertical wind velocity, and net radiative flux. In fog, visibility was continuously recorded and periodic measurements were made of liquid water content and drop-size distribution. The observations were initiated in late evening and continued until the time of fog dissipation. The vertical distribution of temperature in the lowest 300 meters and cloud nucleus concentration at several heights were measured from an aircraft before fog nucleus concentrations at several heights were measured from an aircraft before fog formation. A numerical model was developed to investigate the life cycle of radiation fogs. The model predicts the temporal evolution of the vertical distributions of temperature, water vapor, and liquid water as determined by the turbulent transfer of heat and moisture. The model includes the nocturnal cooling of the earth's surface, dew formation, fog drop sedimentation, and the absorption of infrared radiation by fog.

Pilie, R. J.↗
NASA NTRS2010

Total Water Content Measurements with an Isokinetic Sampling Probe

The NASA Glenn Research Center has developed a Total Water Content (TWC) Isokinetic Sampling Probe. Since it is not sensitive to cloud water particle phase nor size, it is particularly attractive to support super-cooled large droplet and high ice water content aircraft icing studies. The instrument is comprised of the Sampling Probe, Sample Flow Control, and Water Vapor Measurement subsystems. Analysis and testing have been conducted on the subsystems to ensure their proper function and accuracy. End-to-end bench testing has also been conducted to ensure the reliability of the entire instrument system. A Stokes Number based collection efficiency correction was developed to correct for probe thickness effects. The authors further discuss the need to ensure that no condensation occurs within the instrument plumbing. Instrument measurements compared to facility calibrations from testing in the NASA Glenn Icing Research Tunnel are presented and discussed. There appears to be liquid water content and droplet size effects in the differences between the two measurement techniques.

Reehorst, Andrew L.↗
NASA NTRS2006

Effects of Vegetation and of Heat and Vapor Fluxes from Soil on Snowpack Evolution and Radiobrightness

The radiobrightness of a snowpack is strongly linked to the snow moisture content profile, to the point that the only operational inversion algorithms require dry snow. Forward dynamic models do not include the effects of freezing and thawing of the soil beneath the snowpack and the effect of vegetation within the snow or above the snow. To get a more realistic description of the evolution of the snowpack, we reported an addition to the Snow-Soil-Vegetation-Atmosphere- Transfer (SSVAT) model, wherein we coupled soil processes of the Land Surface Process (LSP) model with the snow model SNTHERM. In the near future we will be adding a radiobrightness prediction based on the modeled moisture, temperature and snow grain size profiles. The initial investigations with this SSVAT for a late winter and early spring snow pack indicate that soil processes warm the snowpack and the soil. Vapor diffusion needs to be considered whenever the ground is thawed. In the early spring, heat flow from the ground into a snow and a strong temperature gradient across the snow lead to thermal convection. The buried vegetation can be ignored for a late winter snow pack. The warmer surface snow temperature will affect radiobrightness since it is most sensitive to snow surface characteristics. Comparison to data shows that SSVAT provides a more realistic representation of the temperature and moisture profiles in the snowpack and its underlying soil than SNTHERM. The radiobrightness module will be optimized for the prediction of brightness when the snow is moist. The liquid water content of snow causes considerable absorption compared to dry snow, and so longer wavelengths are likely to be most revealing as to the state of a moist snowpack. For volumetric moisture contents below about 7% (the pendular regime), the water forms rings around the contact points between snow grains. Electrostatic modeling of these pendular rings shows that the absorption of these rings is significantly higher than a sphere of the same volume. The first implementation of the radiobrightness module will therefore be a simple radiative transfer model without scattering.

soil↗
NASA NTRS1969

Ground-based microwave probing

Theoretical and experimental activities in remote sensing were conducted. The projects are: (1) the linear statistical inversion method, (2) vertical temperature profile determination from microwave emission measurements near 60 GHz, (3) radio path length correction determination from emission measurements at 20.6 GHz, and (4) liquid water content determination of thunderstorm cells by emission measurements at 10.7 GHz.

Westwater, E. R.↗
NASA NTRS1972

Further developments in cloud statistics for computer simulations

This study is a part of NASA's continued program to provide global statistics of cloud parameters for computer simulation. The primary emphasis was on the development of the data bank of the global statistical distributions of cloud types and cloud layers and their applications in the simulation of the vertical distributions of in-cloud parameters such as liquid water content. These statistics were compiled from actual surface observations as recorded in Standard WBAN forms. Data for a total of 19 stations were obtained and reduced. These stations were selected to be representative of the 19 primary cloud climatological regions defined in previous studies of cloud statistics. Using the data compiled in this study, a limited study was conducted of the hemogeneity of cloud regions, the latitudinal dependence of cloud-type distributions, the dependence of these statistics on sample size, and other factors in the statistics which are of significance to the problem of simulation. The application of the statistics in cloud simulation was investigated. In particular, the inclusion of the new statistics in an expanded multi-step Monte Carlo simulation scheme is suggested and briefly outlined.

Chang, D. T.↗
NASA NTRS1972

Recent developments in global cloud statistics.

The cloud-type and cloud-layer statistics were stratified by cloud climatological regime, month, and time of day at the three hourly intervals beginning with 01 LST. Use was made of the format previously adopted for the derivation of cloud cover statistics by Sherr et al. (1968) and Greaves et al. (1971). With the new data, it is now possible to provide realistic simulations of the four-dimensional aspects of the cloud field. Furthermore, by the addition of models of the microphysical properties of each of the major cloud types, such as those developed by Gaut and Reifenstein (1969), it will be possible to extend the cloud simulation to the specification of the vertical distributions of such intensive parameters as drop size distribution and liquid water content.

Chang, D. T.↗
NASA NTRS1972

Some microphysical features of coastal and inland fogs.

Radiation fogs sampled at Los Angeles were found to possess low visibilities, high liquid water content (LWC), relatively small drops, narrow peaked drop-size distributions, and high concentrations of micron sized haze droplets. Typical characteristics of valley fogs observed at Elmira, N.Y., were low visibilities, moderately high LWC, low concentrations of haze droplets, and similar drop size distributions to those measured in Los Angeles. On the other hand, advection fogs observed in Vandenberg were characterized by less severe visibilities, low LWC, large drops in a broad drop-size distribution, and very low haze nucleus concentrations.

Mack, E. J.↗
NASA NTRS1972

Cloud microstructure studies

Over two thousand individual cloud droplet size distributions were measured with an optical cloud particle spectrometer flown on the NASA Convair 990 aircraft. Representative droplet spectra and liquid water content, L (gm/cu m) were obtained for oceanic stratiform and cumuliform clouds. For non-precipitating clouds, values of L range from 0.1 gm/cu m to 0.5 gm/cu m; with precipitation, L is often greater than 1 gm/cu m. Measurements were also made in a newly formed contrail and in cirrus clouds.

Blau, H. H., Jr.↗
NASA NTRS1973

Polar sea ice observations by means of microwave radiometry

Principles pertinent to the utilization of 1.55 cm wavelength radiation emanating from the surface of the earth for studying the changing characteristics of polar sea ice are briefly reviewed. Recent data obtained at that wavelength with an imaging radiometer on-board the Nimbus 5 satellite are used to illustrate how the seasonal changes in extent of sea ice in both polar regions may be monitored free of atmospheric interference. Within a season, changes in the compactness of the sea ice are also observed from the satellite. Some substantial areas of the Arctic sea ice canopy identified as first-year ice in the past winter were observed not to melt this summer, a graphic illustration of the eventual formation of multiyear ice in the Arctic. Finally, the microwave emissivity of some of the multiyear ice areas near the North Pole was found to increase significantly in the summer, probably due to liquid water content in the firm layer.

Gloersen, P.↗
NASA NTRS1974

Advances in water resources monitoring from space

Nimbus-5 observations indicate that over the oceans the total precipitable water in a column of atmosphere can be estimated to within + or - 10%, the liquid water content of clouds can be estimated to within + or - 25%, areas of precipitation can be delineated, and broad estimates of the precipitation rate obtained. ERTS-1 observations permit the measurement of snow covered area to within a few percent of drainage basin area and snowline altitudes can be estimated to within 60 meters. Surface water areas as small as 1 hectare can be inventoried over large regions such as playa lakes region of West Texas and Eastern New Mexico. In addition, changes in land use on water-sheds occurring as a result of forest fires, urban development, clear cutting, or strip mining can be rapidly obtained.

Salomonson, V. V.↗