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NASA NTRS1988

The GISS global climate-middle atmosphere model. I - Model structure and climatology

A coupled global climate/middle atmosphere model (GCMAM) is developed by extending the Hansen et al. (1983) GISS global climate model to include the middle atmosphere up to 85 km. The model includes numerical solutions of the primitive equations, calculations of the radiative and surface fluxes, and a complete hydrologic cycle with convective and cloud-cover parameterizations. In addition, a parameterized gravity wave drag cycle is incorporated, in which gravity-wave momentum fluxes due to flow over topography, wind shear, and convection are calculated at each grid box, using theoretical relationships between the grid-scale variables and expected source strengths. The results of the GCMAM demonstrate that the model produces a reasonable simulation of the stratosphere. It is shown that the improvements over the previous version were achieved largely through the incorporation of the parameterized gravity wave drag.

Rind, D.↗

NASA NTRS1988

Doppler imaging of AR Lacertae at three epochs

Observations from IUE were used to study the structure of the lower chromosphere of AR Lacertae in the light of Mg II k. Sequences of LWR/P-HI images distributed around the binary period at three epochs were obtained. Discrete plage-like regions of enhanced Mg II surface flux in this system are identified. There are temporal variations in the Mg II flux on timescales of hours as well as substantial changes in chromospheric morphology on timescales of years. Even with the limited S/N attainable with the IUE, one can map the gross structures of active stellar atmospheres. With such information, one can begin to study the true 3-D structure of the atmospheres of late-type stars.

Walter, Frederick M.↗

NASA NTRS1989

The active chromosphere binary HD 17433 (VY Arietis)

The sixth-magnitude K star HD 17433 (VY Ari) is shown to be an active-chromosphere binary with an orbital period of 13.198 days. A photometric (rotational) period of 17.4 days is indicated by the existing photometry, implying that HD 17433 is not in synchronous rotation. In the optical, H-alpha is seen in emission, with variable profile and intensity; He I lambda-5876 is present in absorption. IUE observations show chromospheric and transition-region emission lines with surface fluxes up to 200 times greater than those observed in the quiet sun. The luminosity and radius are appropriate for a subgiant, and the kinematics suggest Pleiades group membership. The presence of a lithium absorption feature indicates it may either be a young object, possibly a pre-main sequence star or an evolved spotted RS CVn system. Interpretations from the optical measurement of the magnetic field strength of HD 17433 are given.

Bopp, Bernard W.↗

NASA NTRS1988

Lidar observations of the planetary boundary layer during FASINEX

Data are presented on the planetary boundary layer (PBL) over the ocean acquired with an airborne downward-looking lidar during the Frontal Air-Sea Interaction Experiment (FASINEX) with the purpose of studying the impact of an ocean front on air-sea interactions. No changes in the PBL structure were detected by lidar. Lidar data were then used along with other readily available remotely-sensed data and a one-dimensional boundary-layer-growth model to infer the mean PBL moisture and temperature structure and to estimate the surface fluxes of heat and moisture.

Melfi, S. H.↗

NASA NTRS1989

A PBL-radiation model for application to regional numerical weather prediction

Often in the short-range limited-area numerical weather prediction (NWP) of extratropical weather systems the effects of planetary boundary layer (PBL) processes are considered secondarily important. However, it may not be the case for the regional NWP of mesoscale convective systems over the arid and semi-arid highlands of the southwestern and south-central United States in late spring and summer. Over these dry regions, the PBL can grow quite high up into the lower middle troposphere (600 mb) due to very effective solar heating and hence a vigorous air-land thermal interaction can occur. The interaction representing a major heat source for regional dynamical systems can not be ignored. A one-dimensional PBL-radiation model was developed. The model PBL consists of a constant-flux surface layer superposed with a well-mixed (Ekman) layer. The vertical eddy mixing coefficients for heat and momentum in the surface layer are determined according to the surface similarity theory, while their vertical profiles in the Ekman layer are specified with a cubic polynomial. Prognostic equations are used for predicting the height of the nonneutral PBL. The atmospheric radiation is parameterized to define the surface heat source/sink for the growth and decay of the PBL. A series of real-data numerical experiments has been carried out to obtain a physical understanding how the model performs under various atmospheric and surface conditions. This one-dimensional model will eventually be incorporated into a mesoscale prediction system. The ultimate goal of this research is to improve the NWP of mesoscale convective storms over land.

Chang, Chia-Bo↗

NASA NTRS1990

Chromospheres of metal-deficient field giants

Observations of the 2800-A Mg II line have been obtained with IUE for a sample of 10 metal-deficient field giant stars to search for chromospheric emission and signatures of mass loss, as well as to establish the level of chromospheric radiative energy losses from these stars. Mg II emission is probably present in all stars. High-resolution spectra of three of the brightest giants show asymmetric Mg II profiles which indicate a differentially expanding atmosphere, signaling the presence of outward mass motions. Surprisingly, the stellar surface fluxes in the Mg II lines are commensurate with the values found for disk giant stars (population I) of similar color. In spite of substantially depleted Mg abundances in the target stars (by factors of 10-100 relative to the solar abundance), the radiative losses implied by the Mg II fluxes, and possibly the chromospheric heating mechanism, appear to be reasonably independent of metallicity and age.

Dupree, A. K.↗

NASA NTRS1990

The distortion of the magnetotail equilibrium structure by a net cross-tail magnetic field

Explicit solutions are presented for quiet, quasi-static magnetotail configurations in the presence of a net cross-tail magnetic field component in addition to field components associated with tail flaring. The solutions are derived using the general three-dimensional equilibrium theory of Birn (1987). It is found that the net By(N) field varies only weakly along field lines and across the tail. The presence of the net By(N) in the neutral sheet implies that a part of the cross-tail current becomes field-aligned, flowing from the southern to the northern hemisphere for By(N) larger than zero and from north to south for By(n) less than zero. The deformations of magnetic flux surfaces suggest that there may be a class of field lines extending from the earth into the magnetosheath region through the low-latitude flanks of the tail. A topological connection is found of the low-latitude region near the magnetopause with the plasma sheet boundary layer, separating closed plasma sheet and open lobe field lines.

Birn, J.↗

NASA NTRS1990

A cloudiness transition in a marine boundary layer

In situ aircraft data and lidar data are used to analyze a transition in the boundary layer thermodynamic structure from a clear boundary layer through small cumulus and broken stratocumulus to a deck of solid stratocumulus. The data was collected in conjunction with a Landsat overpass on July 7, 1987 off the coast of southern California. A steady progression in mixing line stability is seen associated with the change in cloudiness. The (empirically based) stability threshold for the breakup of this stratocumulus is that the slope of the mixing line is 0.66 + or - 0.04 of the slope of the wet virtual adiabat. A simple linear parameterization is proposed for cloud fraction in terms of mixing line stability. Surface flux measurements are consistent with bulk aerodynamic estimates.

Boers, Reinout↗

NASA NTRS1990

Chromospheres of two red giants in NGC 6752

Two red giant stars, A31 and A59, in the globular cluster NGC 6752 exhibit Mg II (2800 A) emission with surface fluxes comparable to those observed among metal-deficient halo field giants, and among low-activity Population I giants. Optical echelle spectra of these cluster giants reveal emission in the core of the Ca II K (3933.7 A) line, and in the wing of the H-alpha (6562.8 A) profile. Asymmetries exist both in the emission profiles and the line cores. These observations demonstrate unequivocally the existence of chromospheres among old halo population giants, and the presence of mass outflow in their atmospheres. Maintenance of a relatively constant level of chromospheric activity on the red giant branch contrasts with the decay of magnetic dynamo activity exhibited by dwarf stars and younger giants. A purely hydrodynamic phenomenon may be responsible for heating the outer atmospheres of these stars, enhancing chromospheric emission, thus extending the atmospheres and facilitating mass loss.

Dupree, A. K.↗

NASA NTRS1990

Rotational modulation and flares on RS Canum Venaticorum and BY Draconis-type stars. XIV - Phase, eclipse and flare observations of YY Geminorum by Exosat and IUE

The eclipsing spectroscopic binary YY Geminorum has been observed at optical, ultraviolet, and X-ray wavelengths for rotational modulation, eclipse variability, and flaring. The epoch T(phi = 0) = JD 2425698.3561, and the phase P = 0.81428224 d. The quiescent level of Mg II emission is remarkably steady during the three-year observing interval, with F(Mg II) roughly 3.6 x 10 to the -12th erg/sq cm/s. Both stars appear to have identical Mg II surface fluxes, with F roughly 1.8 x 10 to the 6th erg/sq cm/s. Both stars appear to be covered with evenly distributed Mg II emitting regions consistent with the proposition of Doyle (1987) that saturation of the Mg II lines occurs for stars having P less than 4 d, implying that such stars are entirely covered by plage. The transition region lines show significantly more rotational modulation and/or secular variability than Mg II. Both Mg II and the transition region lines show preflare and postflare enhancement.

Haisch, B. M.↗

NASA NTRS1990

Radiative control on tropical convective boundary layer equilibrium

How the near-balance between the radiative cooling and the surface fluxes in the tropics controls boundary layer, the height of cloud-base and the top of the convective boundary layer over the tropical oceans is discussed. The impact of boundary layer cloudiness and upper level moisture on the equilibrium solutions is explored.

Betts, Alan K.↗

NASA NTRS1991

The impact of greenhouse climate change on the energetics and hydrologic processes of mid-latitude transient eddies

Atmospheric transient eddies contribute significantly to mid-latitude energy and water vapor transports. Changes in the global climate, as induced by greenhouse enhancement, will likely alter transient eddy behavior. Unraveling all the feedbacks that occur in general circulation models (GCMs) can be difficult. The transient eddies are isolated from the feedbacks and are focused on the response of the eddies to zonal-mean climate changes that result from CO2-doubling. Using a primitive-equation spectral model, the impact of climate change on the life cycles of transient eddies is examined. Transient eddy behavior in experiments is compared with initial conditions that are given by the zonal-mean climates of the GCMs with current and doubled amounts of CO2. The smaller meridional temperature gradient in a doubled CO2 climate leads to a reduction in eddy kinetic energy, especially in the subtropics. The decrease in subtropical eddy energy is related to a substantial reduction in equatorward flux of eddy activity during the latter part of the life cycle. The reduction in equatorward energy flux alters the moisture cycle. Eddy meridional transport of water vapor is shifted slightly poleward and subtropical precipitation is reduced. The water vapor transport exhibits a relatively small change in magnitude, compared to changes in eddy energy, due to the compensating effect of higher specific humidity in the doubled-CO2 climate. An increase in high-latitude precipitation is related to the poleward shift in eddy water vapor flux. Surface evaporation amplifies climatic changes in water vapor transport and precipitation in the experiments.

Branscome, Lee E.↗

NASA NTRS1990

Physical parameters for three chromospherically active binaries

High-resolution spectroscopy, photoelectric radial-velocity observations, and uvby photometry are reported for three southern late-type binaries. Data obtained at ESO during 1988 and 1989 are combined with previously published results in extensive tables and graphs and analyzed in detail. HD 57853 is found to be at least a triple system with period 122.2 d and components of strongly differing luminosity; the primary component rotates rapidly (v sin i = 22 km/sec) and has an age of about 80 Myr. HD 114630 comprises two components of equal mass (at least 1.07 solar mass) and luminosity, with orbital inclination about 90 deg, period 4.23 d, rotation v sin i = about 17 km/sec, and age about 2 Gyr. HD 119285 has rotational period 12.031 d, with a K2IVe primary rotating at v sin i = 6.5 km/sec and a very low-mass secondary; its X-ray surface flux is estimated as 5.5 x 10 to the 6th erg/sq cm sec.

Saar, S. H.↗

NASA NTRS1991

Does ion tearing exist?

Theoretical models of reconnection in the geomagnetic tail and its relationship to the collisionless tearing mode are examined analytically. The focus is on attempts to remove the stabilizing effects of electron compressibility in the quasi-neutral sheet. It is shown that this goal can be achieved by introducing spatial diffusion of electrons across the magnetic-flux surfaces, but not by wave-turbulent pitch-angle scattering (Coroniti, 1980) or nonadiabatic stochastic first-invariant diffusion (Buechner and Zelenyi, 1988). Hence the ion tearing mode does not exist, but the electron tearing mode does.

Pellat, R.↗

NASA NTRS1991

An overview of the results of the FIFE-87 and FIFE-89 campaigns

Results of a major land surface-atmosphere exchange study, the First ISLSCP Field Experiment (FIFE) are summarized. Findings result from nearly 80 days of field measurements over a 15 kilometer square area involving 30 science teams, seven remote sensing aircraft, and 5 satellites which acquired over 1200 low resolution and 35 high spatial resolution satellite images, maps of remotely sensed temperature, vegetation index and soil moisture, surface flux measurements from more than 20 ground stations and airborne sensors, and transects of vegetation, soil moisture and soil chemistry. Relationships between in situ, airborne, and spaceborne remote measurements and the problem of scaling will be discussed. Energy balance comparisons among sites were made. Diurnal measurements of latent heat fluxes indicated a strong correlation between the evaporative fraction at midday and the daytime average value.

Murphy, R. E.↗

NASA NTRS1991

Model atmospheres and X-ray spectra of bursting neutron stars

Model atmosphere equations are derived which correspond to plane-parallel nongray atmospheres of very hot neutron stars in hydrostatic and radiative equilibrium. The nonlinear transfer equation used implements an exact photon redistribution function which precisely traces even those scattering events having large photon-electron energy exchange. The numerical results thus obtained encompass tables of the surface fluxes for 20 model atmospheres whose T(eff) ranges from 6 to 30 million K. The spectra of high gravity models are virtually identical with the blackbody spectrum shifted toward higher energies, while models approaching the Eddington limit develop a large low-energy hump.

Madej, Jerzy↗

NASA NTRS1991

Digging in the coronal graveyard - A Rosat observation of the red giant Arcturus

A deep exposure of the bright star Arcturus (Alpha Bootis: K1 III) with the Roentgensatellit (Rosat) failed to detect soft X-ray emission from the archetype 'noncoronal' red giant. The 3-sigma upper limit in the energy band 0.1-2.4 keV corresponds to an X-ray luminosity of less than 3 x 10 to the 25th erg/s, equivalent to a coronal surface flux density of less than 0.0001 solar. The nondetection safely eliminates coronal irradiation as a possible mechanism to produce the highly variable He I 10830 feature and emphasizes the sharp decline in solarlike coronal activity that accompanies the evolution of low-mass single stars away from the main sequence. While the most conspicuous object in the Rosat field of view was not visible in X-rays, at least one fainter star is among the about 60 sources recorded: the Sigma Sct variable CN Boo, an A8 giant in the UMa Stream.

Ayres, Thomas R.↗

NASA NTRS1990

The inner region of the unstable boundary layer over hilly prairie for temperature and humidity

The analytical treatment of wind-speed, temperature, and humidity data above nonuniform surfaces is utilized to test the Monin-Obukhov similarity at the inner region of the atmospheric boundary layer. The data profiles are analyzed, and independent measurements of the surface fluxes of sensible heat and water vapor are incorporated into the analysis. It is shown that the mean profiles of the potential temperature and specific humidity can be described using the Monin-Obukhov similarity functions in an area with unstable conditions.

Brutsaert, Wilfried↗