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NASA NTRS1991

Cloud albedo control by cloud-top entrainment

Marine stratus and stratocumulus clouds exert a considerable influence on the earth's heat budget, mainly due to their high albedos relative to the ocean surface. It is therefore important to understand the processes that control the radiative properties of these extensive cloud systems, particularly during daylight hours. Aircraft measurements of a stratocumulus cloud deck taken around local noon during the 1987 field phase of the First International Satellite Cloud Climatology Project Regional Experiment are the topic of this paper. A mixing line analysis of data from a series of flight tracks across a strong gradient in cloud albedo provides evidence that variations in the water vapor content of the air above the marine inversion can be responsible for the albedo change. The implications of this unexpected result for climate modeling are discussed.

Hanson, Howard P.↗

NASA NTRS1991

Theoretical studies of volatile processes in the outer solar system

Four studies of volatile processes in the outer solar system are discussed. Researchers suggest that the convective and conductive regions of Triton's atmosphere join at the tropopause near 10 km. A model of volatile transport on Triton's surface was constructed that predicts that Triton's surface north of 15 degrees north latitude is experiencing deposition of nitrogen frosts, as are the bright portions of the south polar cap near the equator. Also discussed are numerical models of the evolution of Titan's surface and atmosphere. Results of a study of the rheology of ammonia-water liquids were applied to the icy satellites of the outer solar system. Finally, the researchers examined the frictional heating, sublimation, and re-condensation of grains free-falling into the solar nebula from a surrounding interstellar cloud. The sublimation model includes the effect of various volatile species and accounts for the poor radiating properties of small grains using Mie theory.

Lunine, Jonathan I.↗

NASA NTRS1990

Polar ozone

The observation and interpretation of a large, unexpected ozone depletion over Antarctica has changed the international scientific view of stratospheric chemistry. The observations which show the veracity, seasonal nature, and vertical structure of the Antarctic ozone hole are presented. Evidence for Arctic and midlatitude ozone loss is also discussed. The chemical theory for Antarctic ozone depletion centers around the occurrence of polar stratospheric clouds (PSCs) in Antarctic winter and spring; the climatology and radiative properties of these clouds are presented. Lab studies of the physical properties of PSCs and the chemical processes that subsequently influence ozone depletion are discussed. Observations and interpretation of the chemical composition of the Antarctic stratosphere are described. It is shown that the observed, greatly enhanced abundances of chlorine monoxide in the lower stratosphere are sufficient to explain much if not all of the ozone decrease. The dynamic meteorology of both polar regions is given, interannual and interhemispheric variations in dynamical processes are outlined, and their likely roles in ozone loss are discussed.

Solomon, S.↗

NASA NTRS1992

Scattering and radiation analysis of three-dimensional cavity arrays via a hybrid finite element method

A hybrid numerical technique is presented for a characterization of the scattering and radiation properties of three-dimensional cavity arrays recessed in a ground plane. The technique combines the finite element and boundary integral methods and invokes Floquet's representation to formulate a system of equations for the fields at the apertures and those inside the cavities. The system is solved via the conjugate gradient method in conjunction with the Fast Fourier Transform (FFT) thus achieving an O(N) storage requirement. By virtue of the finite element method, the proposed technique is applicable to periodic arrays comprised of cavities having arbitrary shape and filled with inhomogeneous dielectrics. Several numerical results are presented, along with new measured data, which demonstrate the validity, efficiency, and capability of the technique.

Jin, Jian-Ming↗

NASA NTRS1990

Global patterns of cloud optical thickness variation with temperature

A global cloud climatology dataset is used to study patterns of cloud optical thickness variation with temperature. The data, which cover the period from July 1983 through June 1995, contain detailed information on the distribution of cloud radiative properties and their diurnal and seasonal variations, as well as information on the vertical distribution of temperature and humidity in the troposphere. For cold low clouds over land, the temperature coefficient of change in optical thickness has a value of about 0.04, which is similar to that deduced from Soviet aircraft observations and derived from thermodynamic considerations for the change of cloud liquid water with temperature. It is suggested that, in this cold-temperature range, cloud optical thickness variations are dominated by changes in the liquid water content of the cloud and that the liquid water content changes in accordance with the thermodynamic theory.

Tselioudis, George↗

NASA NTRS1990

Cirrus cloud spectra and layers observed during the FIRE and GASP projects

A general characterization is developed for cirrus clouds in terms of their spectra, shapes, optical thicknesses, and radiative properties for use in numerical models. Data sets from the Global Atmospheric Sampling Project (GASP) of the upper troposphere and the First ISCCP Regional Experiment (FIRE) are combined and analyzed to study general traits of cirrus clouds. A definition is given for 2D turbulence, and the GASP and FIRE data sets are examined with respect to cirrus layers and entrainment and to dominant turbulent scales. The approach employs conditional sampling in cloudy and clear air, power-spectral analysis, and mixing-line-type diagrams. Evidence is given for a well mixed cloud deck and for the tendency of cirrus to be formed in multilayer structures. The results are of use in mesoscale and global circulation models which predict cirrus, in small-scale cirrus modeling, and in studying the role of gravity waves in the horizontal structure of upper tropospheric clouds.

Flatau, Piotr J.↗

NASA NTRS1992

Radiation transport around axisymmetric blunt body vehicles using a modified differential approximation

A moment method for computing 3D radiative transport in axisymmetric thermochemical nonequilibrium flows is developed. The method uses the P-1 approximation to reduce the governing system of integro-differential equations to a coupled set of partial differential equations. The numerical solution of these equations for realistic variations of the radiation properties is discussed. Representative results from the method are shown and compared to tangent slab calculations. The agreement between the transport methods is found to be about 10 percent in the stagnation region, with the difference increasing along the flank of the vehicle.

Hartung, Lin C.↗

NASA NTRS1992

A novel antenna for ultra-wide-band applications

An ultrawideband antenna based on a slotline feed structure, a bowtie horn, and a rolled edge termination was developed, analyzed, and measured. Empirical data showed that its beamwidths and bandwidth are dependent on its physical dimensions which are easily controllable by an antenna designer. Measured patterns of models with various radiation properties are shown to substantiate these design rules. A flat plateau-like main beam, low voltage standing-wave ratio (VSWR), the ability to produce both wide (60 deg) and narrow (30 deg) half-power beamwidths, low sidelobes and backlobe (40-50 dB down), low cross-polarized levels (20-25 dB down), and independent control of E- and H-plane beamwidths over an ultrawide bandwidth, say 2-18 GHz, are some of the strong points of this antenna type.

Lai, Albert K. Y.↗

NASA NTRS1993

Spectral radiation analyses of the GOES solar illuminated hexagonal cell scan mirror back

A ray tracing analytical tool has been developed for the simulation of spectral radiation exchange in complex systems. Algorithms are used to account for heat source spectral energy, surface directional radiation properties, and surface spectral absorptivity properties. This tool has been used to calculate the effective solar absorptivity of the geostationary operational environmental satellites (GOES) scan mirror in the calibration position. The development and design of Sounder and Imager instruments on board GOES is reviewed and the problem of calculating the effective solar absorptivity associated with the GOES hexagonal cell configuration is presented. The analytical methodology based on the Monte Carlo ray tracing technique is described and results are presented and verified by experimental measurements for selected solar incidence angles.

Fantano, Louis G.↗

NASA NTRS1993

University of Wisconsin Cirrus Remote Sensing Pilot Experiment

During the period of 26 October 1989 through 6 December 1989 a unique complement of measurements was made at the University of Wisconsin-Madison to study the radiative properties of cirrus clouds. Simultaneous observations were obtained from a scanning lidar, two interferometers, a high spectral resolution lidar, geostationary and polar orbiting satellites, radiosonde launches, and a whole-sky imager. This paper describes the experiment, the instruments deployed, and, as an example, the data collected during one day of the experiment.

Ackerman, Steven A.↗

NASA NTRS1991

Numerical simulations of turbulent thermal convection with differential rotation

The solar nebula, from which the planets in our solar system formed, featured a disk of gas and dust grains in rapid, differential rotation, and at some stage was likely to have been unstable to thermal convection. This situation is suspected by many to lead to significant turbulent Reynolds stress production and angular momentum transport in such systems, and estimates of transport rates have been attempted from unsubstantiated phenomenological models. In order to determine the circumstances and physical conditions under which our own planetary system formed and to explain recent observations of young stellar systems, it is necessary to develop realistic models of heat and angular momentum transport for such flows. Developing an understanding of complicated flows featuring thermal convection, rotation, and shear is also of wide interest in stellar astrophysics and in planetary and terrestrial atmospheric studies. The ultimate objective is to develop workable models based on the numerical simulations for constructing global solar nebula models; viz., relatively simple prescriptions for heat and angular momentum fluxes from given system parameters (e.g., ratios of rotation, shear, and convective lapse rates) are characterized, quantified, and developed. Toward this end, our program has been to attempt to understand the behavior of the direct numerical simulations of Boussinesq convection, which, despite the complexity of the results, is still an overly simplified approximation to the real system and should be more amenable to analysis. These results are also intended to be tested against turbulence models, especially those designed for atmospheric boundary layers, and may provide a basis for subgrid-scale models. In order to make the numerical simulations more realistic with regard to the solar nebula problem, a fully compressible code that will allow incorporation of large density stratifications and realistic thermodynamic and radiative properties is developed. In order to explore the properties of these flows at the very high values of Re found in natural systems and the very low values of Pr found in most astrophysical contexts, we will need to employ large-eddy simulations for which we want to determine the most appropriate subgrid-scale model to incorporate.

Cabot, William H.↗

NASA NTRS1993

Comparative study of INPIStron and spark gap

An inverse pinch plasma switch, INPIStron, was studied in comparison to a conventional spark gap. The INPIStron is under development for high power switching applications. The INPIStron has an inverse pinch dynamics, opposed to Z-pinch dynamics in the spark gap. The electrical, plasma dynamics and radiative properties of the closing plasmas have been studied. Recently the high-voltage pulse transfer capabilities or both the INPIStron and the spark gap were also compared. The INPIStron with a low impedance Z = 9 ohms transfers 87 percent of an input pulse with a halfwidth of 2 mu s. For the same input pulse the spark gap of Z = 100 ohms transfers 68 percent. Fast framing and streak photography, taken with an TRW image converter camera, was used to observe the discharge uniformity and closing plasma speed in both switches. In order to assess the effects of closing plasmas on erosion of electrode material, emission spectra of two switches were studied with a spectrometer-optical multi channel analyzer (OMA) system. The typical emission spectra of the closing plasmas in the INPIStron and the spark gap showed that there were comparatively weak carbon line emission in 658.7 nm and copper (electrode material) line emissions in the INPIStron, indicating low erosion of materials in the INPIStron.

Han, Kwang S.↗

NASA NTRS1993

Vertical velocity in cirrus case obtained from wind profiler

Cirrus clouds play an important role in the climate and general circulation because they significantly modulate the radiation properties of the atmosphere. However understanding the processes that govern their presence is made difficult by their high altitude, variable thickness, complex microphysical structure, and relatively little knowledge of the vertical motion field. In the FIRE 2 (First International Satellite Cloud Climatology Regional Experiment) experiment, a 404 MHz wind profiler was set up to provide continuous measurements of clear air wind field at Parsons, Kansas. Simultaneously, the NOAA wind profiler network supplied a wider spacial scale observation. On 26 Nov. 1991, the most significant cirrus cloud phenomena during the experiment with a jet streak at 250 Mb occurred. Analyses of the vertical wind velocity are made by utilizing different methods based on wind profiler data, among them the direct measurements from CSU wind profiler and NOAA network wind profilers, VAD (Velocity Azimuth Display) technique and the kinematic method.

Song, Ran↗

NASA NTRS1993

Estimation of cirrus cloud particle fallspeeds from vertically pointing Doppler radar

The First ISCCP (International Satellite Cloud Climatology Project) Regional Experiment 2 (FIRE 2) was conducted in Coffeyville, Kansas in late 1991 to study the microphysical and radiative properties of cirrus clouds. A variety of active and passive remote sensors were employed, including an 8-mm-wavelength cloud-sensing Doppler radar developed at the Wave Propagation Laboratory (WPL). The radar, having excellent sensitivity to cloud particles (-30 dBZ at 10 km), good spatial resolution (37 m), and velocity precision (.05 ms -1), is an excellent tool for observing cirrus clouds. Having this radar directed toward the zenith for long periods of time during FIRE 2 permitted the reflectivity-weighted particle fallspeed to be related to reflectivity which allowed a separation of ice particle fallspeeds from vertical air motions. Additionally, such relationships proved useful in other multi-sensor techniques for determining vertical profiles of ice particle characteristic size and ice water content in cirrus clouds. The analysis method and the results of applying it to cirrus cloud reflectivity and velocity data collected during FIRE 2 are discussed.

Orr, Brad W.↗

NASA NTRS1993

Are tropical cirrus brighter than mid-latitude cirrus?

Recent measurements during FIRE II, in the tropics and elsewhere, support an emerging hypothesis about the role of stratospheric mixing in determining the microphysical and radiative properties of cirrus clouds. This is only a working hypothesis, and may change as new measurements become available. This report reviews the conditions under which certain types of ice crystals form.

Mitchell, David L.↗

NASA NTRS1993

An overview of the radiation component of the NASA TOGA-COARE experiment

During January and February, 1993, the Physical Climate Branch of NASA sponsored an aircraft program in support of the Tropical Ocean - Global Atmosphere (TOGA) Coupled Ocean-Atmosphere Response Experiment (COARE). The NASA program was integrated with and contributed directly to the COARE objectives, but had as its primary foci the measurement of convection and precipitation related to the Tropical Rainfall Measurement Mission (TRMM) and the measurement of the physical and radiative properties of tropical cirrus related to the International Satellite Cloud Climatology Project (ISCCP) and the First ISCCP Regional Experiment (FIRE). This brief overview will concentrate on the measurements associated with FIRE and ISCCP.

Ackerman, Thomas↗

NASA NTRS1993

Session on modeling of radiative transfer processes

The session on modeling of radiative transfer processes is reviewed. Six critical issues surfaced in the discussion concerning scale-interactive radiative processes relevent to the mesoscale convective systems (MCS's). These issues are the need to expand basic knowledge of how MCS's influence climate through extensive cloud shields and increased humidity in the upper troposphere; to improve radiation parameterizations used in mesoscale and General Circulation Model (GCM) models; to improve our basic understanding of the influence of radiation on MCS dynamics due to diabatic heating, production of condensate, and vertical and horizontal heat fluxes; to quantify our understanding of radiative impacts of MCS's on the surface and free atmosphere energy budgets; to quantify and identify radiative and microphysical processes important in the evolution of MCS's; and to improve the capability to remotely sense MCS radiative properties from space and ground-based systems.

Flatau, Piotr↗

NASA NTRS1994

Radiation and scattering from cylindrically conformal printed antennas

Microstrip patch antennas offer considerable advantages in terms of weight, aerodynamic drag, cost, flexibility, and observables over more conventional protruding antennas. These flat patch antennas were first proposed over thirty years ago by Deschamps in the United States and Gutton and Baisinot in France. Such antennas have been analyzed and developed for planar as well as curved platforms. However, the methods used in these designs employ gross approximations, suffer from extreme computational burden, or require expensive physical experiments. The goal of this thesis is to develop accurate and efficient numerical modeling techniques which represent actual antenna structures mounted on curved surfaces with a high degree of fidelity. In this thesis, the finite element method is extended to cavity-backed conformal antenna arrays embedded in a circular, metallic, infinite cylinder. Both the boundary integral and absorbing boundary mesh closure conditions will be used for terminating the mesh. These two approaches will be contrasted and used to study the scattering and radiation behavior of several useful antenna configurations. An important feature of this study will be to examine the effect of curvature and cavity size on the scattering and radiation properties of wraparound conformal antenna arrays.

Kempel, Leo C.↗