187 research outputs found
Precision Photometry for Q0957+561 Images A and B
Since the persuasive determination of the time-delay in Q0957+561, much
interest has centered around shifting and subtracting the A and B light-curves
to look for residuals due to microlensing. Solar mass objects in the lens
galaxy produce variations on timescales of decades, with amplitudes of a few
tenths of a magnitude, but MACHO's (with masses of order to
) produce variations at only the 5% level. To detect such small
variations, highly precise photometry is required.
To that end, we have used 200 observations over three nights to examine the
effects of seeing on the light-curves. We have determined that seeing itself
can be responsible for correlated 5% variations in the light-curves of A and B.
We have found, however, that these effects can be accurately removed, by
subtracting the light from the lens galaxy, and by correcting for cross
contamination of light between the closely juxtaposed A and B images. We find
that these corrections improve the variations due to seeing from 5% to a level
only marginally detectable over photon shot noise (0.5%).Comment: 21 Pages with 9 PostScript figures, AASTeX 4 (preprint style
Radio Wavelength Constraints on the Sources of the Far Infrared Background
The cosmic far infrared background detected recently by the COBE-DIRBE team
is presumably due, in large part, to the far infrared (FIR) emission from all
galaxies. We take the well-established correlation between FIR and radio
luminosity for individual galaxies and apply it to the FIR background. We find
that these sources make up about half of the extragalactic radio background,
the other half being due to AGN. This is in agreement with other radio
observations, which leads us to conclude that the FIR-radio correlation holds
well for the very faint sources making up the FIR background, and that the FIR
background is indeed due to star-formation activity (not AGN or other possible
sources). If these star-forming galaxies have a radio spectral index between
0.4 and 0.8, and make up 40 to 60% of the extragalactic radio background, we
find that they have redshifts between roughly 1 and 2, in agreement with recent
estimates by Madau et al. of the redshift of peak star-formation activity. We
compare the observed extragalactic radio background to the integral over the
logN-logS curve for star-forming radio sources, and find that the slope of the
curve must change significantly below about 1 microjansky. At 1 microjansky,
the faint radio source counts predict about 25 sources per square arcminute,
and these will cause SIRTF to be confusion limited at 160micron.Comment: 10 pages including 1 figure, AASTeX, accepted by Ap
Faint Radio Sources and Star Formation History
Faint extragalactic radio sources provide important information about the
global history of star formation. Sensitive radio observations of the Hubble
Deep Field and other fields have found that sub-mJy radio sources are
predominantly associated with star formation activity rather than AGN. Radio
observations of star forming galaxies have the advantage of being independent
of extinction by dust. We use the FIR-radio correlation to compare the radio
and FIR backgrounds, and make several conclusions about the star forming
galaxies producing the FIR background. We then use the redshift distribution of
faint radio sources to determine the evolution of the radio luminosity
function, and thus estimate the star formation density as a function of
redshift.Comment: 12 pages, 9 figures, latex using texas.sty, to appear in the CD-ROM
Proceedings of the 19th Texas Symposium on Relativistic Astrophysics and
Cosmology, held in Paris, France, Dec. 14-18, 1998. Eds.: J. Paul, T.
Montmerle, and E. Aubourg (CEA Saclay). No changes to paper, just updated
publication info in this commen
Dominant Modes of Variability in the South Atlantic: A Study with a Hierarchy of Ocean-Atmosphere Models.
Abstract
Using an atmosphere model of intermediate complexity and a hierarchy of ocean models, the dominant modes of interannual and decadal variability in the South Atlantic Ocean are studied. The atmosphere Simplified Parameterizations Primitive Equation Dynamics (SPEEDY) model has T30L7 resolution. The physical package consists of a set of simplified physical parameterization schemes, based on the same principles adopted in the schemes of state-of-the-art AGCMs. It is at least an order of magnitude faster, whereas the quality of the simulated climate compares well with those models. The hierarchy of ocean models consists of simple mixed layer models with an increasing number of physical processes involved such as Ekman transport, wind-induced mixing, and wind-driven barotropic transport. Finally, the atmosphere model is coupled to a regional version of the Miami Isopycnal Coordinate Ocean Model (MICOM) covering the South Atlantic with a horizontal resolution of 1° and 16 vertical layers.
The coupled modes of mean sea level pressure and sea surface temperature simulated by SPEEDYâMICOM strongly resemble the modes as analyzed from the NCEPâNCAR reanalysis, indicating that this model configuration possesses the required physical mechanisms for generating these modes of variability. Using the ocean model hierarchy the authors were able to show that turbulent heat fluxes, Ekman transport, and wind-induced mixing contribute to the generation of the dominant modes of coupled SST variability. The different roles of these terms in generating these modes are analyzed. Variations in the wind-driven barotropic transport mainly seem to affect the SST variability in the BrazilâMalvinas confluence zone.
The spectra of the mixed layer models appeared to be too red in comparison with the fully coupled SPEEDYâMICOM model due to the too strong coupling between SST and surface air temperatures (SATs), resulting from the inability to advect and subduct SST anomalies by the mixed layer models. In SPEEDYâMICOM anomalies in the southeastern corner of the South Atlantic are subducted and advected toward the north Brazilian coast on a time scale of about 6 yr
Further Investigation of the Time Delay, Magnification Ratios, and Variability in the Gravitational Lens 0218+357
High precision VLA flux density measurements for the lensed images of
0218+357 yield a time delay of 10.1(+1.5-1.6)days (95% confidence). This is
consistent with independent measurements carried out at the same epoch (Biggs
et al. 1999), lending confidence in the robustness of the time delay
measurement. However, since both measurements make use of the same features in
the light curves, it is possible that the effects of unmodelled processes, such
as scintillation or microlensing, are biasing both time delay measurements in
the same way. Our time delay estimates result in confidence intervals that are
somewhat larger than those of Biggs et al., probably because we adopt a more
general model of the source variability, allowing for constant and variable
components. When considered in relation to the lens mass model of Biggs et al.,
our best-fit time delay implies a Hubble constant of H_o = 71(+17-23) km/s-Mpc
for Omega_o=1 and lambda_o=0 (95% confidence; filled beam). This confidence
interval for H_o does not reflect systematic error, which may be substantial,
due to uncertainty in the position of the lens galaxy. We also measure the flux
ratio of the variable components of 0218+357, a measurement of a small region
that should more closely represent the true lens magnification ratio. We find
ratios of 3.2(+0.3-0.4) (95% confidence; 8 GHz) and 4.3(+0.5-0.8) (15 GHz).
Unlike the reported flux ratios on scales of 0.1", these ratios are not
strongly significantly different. We investigate the significance of apparent
differences in the variability properties of the two images of the background
active galactic nucleus. We conclude that the differences are not significant,
and that time series much longer than our 100-day time series will be required
to investigate propagation effects in this way.Comment: 33 pages, 9 figures. Accepted for publication in ApJ. Light curve
data may be found at http://space.mit.edu/RADIO/papers.htm
Assessing bias corrections of oceanic surface conditions for atmospheric models
Future sea surface temperature and sea-ice concentration from coupled
oceanâatmosphere general circulation models such as those from the CMIP5
experiment are often used as boundary forcings for the downscaling of future
climate experiments. Yet, these models show some considerable biases when
compared to the observations over present climate. In this paper, existing
methods such as an absolute anomaly method and a quantileâquantile method
for sea surface temperature (SST) as well as a look-up table and a relative
anomaly method for sea-ice concentration (SIC) are presented. For SIC, we
also propose a new analogue method. Each method is objectively evaluated with
a perfect model test using CMIP5 model experiments and some real-case
applications using observations. We find that with respect to other
previously existing methods, the analogue method is a substantial improvement
for the bias correction of future SIC. Consistency between the constructed
SST and SIC fields is an important constraint to consider, as is consistency
between the prescribed sea-ice concentration and thickness; we show that the
latter can be ensured by using a simple parameterisation of sea-ice thickness
as a function of instantaneous and annual minimum SIC.</p
Impact of climate model resolution on soil moisture projections in central-western Europe
Global climate models project widespread
decreases in soil moisture over large parts of Europe. This paper
investigates the impact of model resolution on the magnitude and seasonality
of future soil drying in central-western Europe. We use the general
circulation model EC-Earth to study two 30-year periods representative of the
start and end of the 21st century under low-to-moderate greenhouse gas
forcing (RCP4.5). In our study area, central-western Europe, at high spatial
resolution (âŒ25 km) soil drying is more severe and starts earlier in
the season than at standard resolution (âŒ112 km). Here, changes in the
large-scale atmospheric circulation and local soil moisture feedbacks lead to
enhanced evapotranspiration in spring and reduced precipitation in summer. A
more realistic position of the storm track at high model resolution leads to
reduced biases in precipitation and temperature in the present-day
climatology, which act to amplify future changes in evapotranspiration in
spring. Furthermore, in the high-resolution model a stronger anticyclonic
anomaly over the British Isles extends over central-western Europe and
supports soil drying. The resulting drier future land induces stronger soil
moisture feedbacks that amplify drying conditions in summer. In addition,
soil-moisture-limited evapotranspiration in summer promotes sensible heating
of the boundary layer, which leads to a lower relative humidity with less
cloudy conditions, an increase in dry summer days, and more incoming solar
radiation. As a result a series of consecutive hot and dry summers appears in
the future high-resolution climate. The enhanced drying at high spatial
resolution suggests that future projections of central-western European soil
drying by CMIP5 models have been potentially underestimated. Whether these
results are robust has to be tested with other global climate models with
similar high spatial resolutions.</p
Mitigating Climate Biases in the Midlatitude North Atlantic by Increasing Model Resolution: SST Gradients and Their Relation to Blocking and the Jet
Starting to resolve the oceanic mesoscale in climate models is a step change in model fidelity. This study examines how certain obstinate biases in the midlatitude North Atlantic respond to increasing resolution (from 18 to 0.258 in the ocean) and how such biases in sea surface temperature (SST) affect the atmosphere. Using a multimodel ensemble of historical climate simulations run at different horizontal resolutions, it is shown that a severe cold SST bias in the central North Atlantic, common to many ocean models, is significantly reduced with increasing resolution. The associated bias in the time-mean meridional SST gradient is shown to relate to a positive bias in low-level baroclinicity, while the cold SST bias causes biases also in static stability and diabatic heating in the interior of the atmosphere. The changes in baroclinicity and diabatic heating brought by increasing resolution lead to improvements in European blocking and eddy-driven jet variability. Across the multimodel ensemble a clear relationship is found between the climatological meridional SST gradients in the broader Gulf Stream Extension area and two aspects of the atmospheric circulation: the frequency of high-latitude blocking and the southern-jet regime. This relationship is thought to reflect the two-way interaction (with a positive feedback) between the respective oceanic and atmospheric anomalies. These North Atlantic SST anomalies are shown to be important in forcing significant responses in the midlatitude atmospheric circulation, including jet variability and the storm track. Further increases in oceanic and atmospheric resolution are expected to lead to additional improvements in the representation of Euro-Atlantic climate
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