693 research outputs found
Atmosphere-Ocean Ozone Exchange – A Global Modeling Study of Biogeochemical, Atmospheric and Water-Side Turbulence Dependencies
The significance of the removal of tropospheric ozone by the oceans, covering ~2/3 of the Earth's surface, has only been addressed in a few studies involving water tank, aircraft, and tower flux measurements. On the basis of results from these few observations of the ozone dry deposition velocity (VdO3), atmospheric chemistry models generally apply an empirical, constant ocean uptake rate of 0.05 cm s-1. This value is substantially smaller than the atmospheric turbulent transport velocity for ozone. On the other hand, the uptake is higher than expected from the solubility of ozone in clean water alone, suggesting that there is an enhancement in oceanic ozone uptake, e.g., through a chemical destruction mechanism. We present an evaluation of a global-scale analysis with a new mechanistic representation of atmosphere-ocean ozone exchange. The applied atmosphere chemistry-climate model includes not only atmospheric but also waterside turbulence and the role of waterside chemical loss processes as a function of oceanic biogeochemistry. The simulations suggest a larger role of biogeochemistry in tropical and subtropical ozone oceanic uptake with a relative small temporal variability, whereas in midlatitude and high-latitude regions, highly variable ozone uptake rates are expected because of the stronger influence of waterside turbulence. Despite a relatively large range in the explicitly calculated ocean uptake rate, there is a surprisingly small sensitivity of simulated Marine Boundary Layer ozone concentrations compared to the sensitivity for the commonly applied constant ocean uptake approach. This small sensitivity points at compensating effects through inclusion of the process-based ocean uptake mechanisms to consider variability in oceanic O3 deposition consistent with that in atmospheric and oceanic physical, chemical, and biological processe
Mesoscale variability of free tropospheric humidity near San Nicolas Island during FIRE
Humidity variability at the top of the marine boundary layer (MBL) and in the free troposphere was examined using a variety of measurements taken on and around San Nicolas Island (SNI) during the FIRE IFO in July, 1987. Doppler wind profiler reflectivity recorded at two minute time resolution has provided the most continuous record and detail of small scale humidity fluctuations. Rawinsonde data were available from both an island site and the research vessel Point Sur. The information extractable from these sources is somewhat limited due to the frequency of launches (3 to 4/day at SNI and 6/day on the Point Sur). Some additional data were available from instrumented aircraft although scheduling flights in the neighborhood of the island was difficult due to restrictions on the air space. Other relevant data were collected at SNI near the radar and rawinsonde launch sites. A continuous record of cloud base altitude was logged by a ceilometer. Doppler acoustic sounder (sodar) reflectivity data provided a good record of inversion height. The sodar also monitored turbulent temperature fluctuations in the MBL. A small ground station recorded hourly averages of solar irradiance and downward longwave irradiance. The analysis in progress of the various data sets for two adjacent two day periods from 11 July to 14 July is described. The earlier period was chosen because the marine inversion was unusually high and there was increased frequency of rawinsonde launches at SNI. The later period was chosen because of the significant descent with time of an elevated inversion indicated by the radar data. Throughout the four day period, but especially in the first half, the turbulent humidity structure calculated from Doppler radar reflectivity shows excellent agreement with humidity profiles evaluated from rawinsonde data
Multicolour Optical Imaging of IR-Warm Seyfert Galaxies. I. Introduction and Sample Selection
The standard AGN unification models attempt to explain the diversity of
observed AGN types by a few fundamental parameters, where orientation effects
play a paramount role. Whether other factors, such as the evolutionary stage
and the host galaxy properties are equally important parameters for the AGN
diversity, is a key issue that we are addressing with the present data. Our
sample of IR-selected Seyfert galaxies is based on the important discovery that
their integrated IR spectrum contains an AGN signature. This being an almost
isotropic property, our sample is much less affected by orientation/obscuration
effects compared to most Seyfert samples. It therefore provides a test-bed for
the orientation-dependent models of Seyferts, involving dusty tori. We have
obtained multi-colour broad and narrow band imaging for a sample of mid-IR
``warm'' Seyferts and for a control sample of mid-IR ``cold'' galaxies. In the
present paper we describe the sample selection and briefly discuss their IR
properties. We then give an overview of the data collected and present
broad-band images for all our objects. Finally, we summarize the main issues
that will be addressed with these data in a series of forthcoming papers.Comment: 18 pages including 3 figures and 5 tables (tables 1,4,5 are included
as independent files
COLA II - Radio and Spectroscopic Diagnostics of Nuclear Activity in Galaxies
We present optical spectroscopic observations of 93 galaxies taken from the
infra-red selected COLA (Compact Objects in Low Power AGN) sample. The sample
spans the range of far-IR luminosities from normal galaxies to LIRGs. Of the
galaxies observed, 78 (84%) exhibit emission lines. Using a theoretically-based
optical emission-line scheme we classify 15% of the emission-line galaxies as
Seyferts, 77% as starbursts, and the rest are either borderline AGN/starburst
or show ambiguous characteristics. We find little evidence for an increase in
the fraction of AGN in the sample as a function of far-IR luminosity but our
sample covers only a small range in infrared luminosity and thus a weak trend
may be masked. As a whole the Seyfert galaxies exhibit a small, but
significant, radio excess on the radio-FIR correlation compared to the galaxies
classified as starbursts. Compact (<0.05'') radio cores are detected in 55% of
the Seyfert galaxies, and these galaxies exhibit a significantly larger radio
excess than the Seyfert galaxies in which cores were not detected. Our results
indicate that there may be two distinct populations of Seyferts,
``radio-excess'' Seyferts, which exhibit extended radio structures and compact
radio cores, and ``radio-quiet'' Seyferts, in which the majority of the radio
emission can be attributed to star-formation in the host galaxy. No significant
difference is seen between the IR and optical spectroscopic properties of
Seyferts with and without radio cores. (Abridged)Comment: 24 pages, 4 figures, 6 tables. Accepted for publication in ApJ,
February 200
Structure of Turbulence in Katabatic Flows below and above the Wind-Speed Maximum
Measurements of small-scale turbulence made over the complex-terrain
atmospheric boundary layer during the MATERHORN Program are used to describe
the structure of turbulence in katabatic flows. Turbulent and mean
meteorological data were continuously measured at multiple levels at four
towers deployed along the East lower slope (2-4 deg) of Granite Mountain. The
multi-level observations made during a 30-day long MATERHORN-Fall field
campaign in September-October 2012 allowed studying of temporal and spatial
structure of katabatic flows in detail, and herein we report turbulence and
their variations in katabatic winds. Observed vertical profiles show steep
gradients near the surface, but in the layer above the slope jet the vertical
variability is smaller. It is found that the vertical (normal to the slope)
momentum flux and horizontal (along the slope) heat flux in a slope-following
coordinate system change their sign below and above the wind maximum of a
katabatic flow. The vertical momentum flux is directed downward (upward)
whereas the horizontal heat flux is downslope (upslope) below (above) the wind
maximum. Our study therefore suggests that the position of the jet-speed
maximum can be obtained by linear interpolation between positive and negative
values of the momentum flux (or the horizontal heat flux) to derive the height
where flux becomes zero. It is shown that the standard deviations of all wind
speed components (therefore the turbulent kinetic energy) and the dissipation
rate of turbulent kinetic energy have a local minimum, whereas the standard
deviation of air temperature has an absolute maximum at the height of
wind-speed maximum. We report several cases where the vertical and horizontal
heat fluxes are compensated. Turbulence above the wind-speed maximum is
decoupled from the surface, and follows the classical local z-less predictions
for stably stratified boundary layer.Comment: Manuscript submitted to Boundary-Layer Meteorology (05 December 2014
Extragalactic Large-Scale Structures behind the Southern Milky Way. -- II. Redshifts Obtained at the SAAO in the Crux Region
In our systematic optical galaxy search behind the southern Milky Way, 3760
(mostly unknown) galaxies with diameters D > 0.2 arcminutes were identified in
the Crux region (287 < l < 318 degrees, |b| < 10 degrees, Woudt &
Kraan-Korteweg 1997). Prior to this investigation, only 65 of these galaxies
had known redshifts. In order to map the galaxy distribution in redshift space
we obtained spectra for 226 bright (B_J < 18.0 mag) objects with the 1.9m
telescope of the South African Astronomical Observatory (SAAO). Two main
structures crossing the Galactic Plane in the Crux region have now become
clear. A narrow, nearby filament from (l,b) = (340 deg, -25 deg) to the
Centaurus cluster can be traced. This filament runs almost parallel to the
extension of the Hydra-Antlia clusters found earlier and is part of what we
have earlier termed the ``Centaurus Wall'' extending in redshift-space between
0 < v < 6000 km/s (Fairall & Paverd 1995). The main outcome of this survey
however, is the recognition of another massive extended structure between 4000
< v < 8000 km/s. This broad structure, dubbed the Norma Supercluster (Woudt et
al. 1997), runs nearly parallel to the Galactic Plane from Vela to ACO 3627
(its centre) from where it continues to the Pavo cluster. This massive
structure is believed to be associated with the Great Attractor. The survey has
furthermore revealed a set of cellular structures, similar to those seen in
redshift space at higher galactic latitudes, but never before seen so clearly
behind the Milky Way.Comment: Accepted for publication in Astronomy & Astrophysics. 13 pages,
LaTex, 6 encapsulated postscript figures, requires l-aa.sty and epsf.sty.
Original figures and tables are available upon request (E-mail:
[email protected]
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