15,821 research outputs found
Towards More Precise Survey Photometry for PanSTARRS and LSST: Measuring Directly the Optical Transmission Spectrum of the Atmosphere
Motivated by the recognition that variation in the optical transmission of
the atmosphere is probably the main limitation to the precision of ground-based
CCD measurements of celestial fluxes, we review the physical processes that
attenuate the passage of light through the Earth's atmosphere. The next
generation of astronomical surveys, such as PanSTARRS and LSST, will greatly
benefit from dedicated apparatus to obtain atmospheric transmission data that
can be associated with each survey image. We review and compare various
approaches to this measurement problem, including photometry, spectroscopy, and
LIDAR. In conjunction with careful measurements of instrumental throughput,
atmospheric transmission measurements should allow next-generation imaging
surveys to produce photometry of unprecedented precision. Our primary concerns
are the real-time determination of aerosol scattering and absorption by water
along the line of sight, both of which can vary over the course of a night's
observations.Comment: 41 pages, 14 figures. Accepted PAS
SeaWiFS Technical Report Series. Volume 7: Cloud screening for polar orbiting visible and infrared (IR) satellite sensors
Methods for detecting and screening cloud contamination from satellite derived visible and infrared data are reviewed in this document. The methods are applicable to past, present, and future polar orbiting satellite radiometers. Such instruments include the Coastal Zone Color Scanner (CZCS), operational from 1978 through 1986; the Advanced Very High Resolution Radiometer (AVHRR); the Sea-viewing Wide Field-of-view Sensor (SeaWiFS), scheduled for launch in August 1993; and the Moderate Resolution Imaging Spectrometer (IMODIS). Constant threshold methods are the least demanding computationally, and often provide adequate results. An improvement to these methods are the least demanding computationally, and often provide adequate results. An improvement to these methods is to determine the thresholds dynamically by adjusting them according to the areal and temporal distributions of the surrounding pixels. Spatial coherence methods set thresholds based on the expected spatial variability of the data. Other statistically derived methods and various combinations of basic methods are also reviewed. The complexity of the methods is ultimately limited by the computing resources. Finally, some criteria for evaluating cloud screening methods are discussed
Hypotheses for near-surface exchange of methane on Mars
The Curiosity rover recently detected a background of 0.7 ppb and spikes of 7
ppb of methane on Mars. This in situ measurement reorients our understanding of
the Martian environment and its potential for life, as the current theories do
not entail any geological source or sink of methane that varies sub-annually.
In particular, the 10-fold elevation during the southern winter indicates
episodic sources of methane that are yet to be discovered. Here we suggest a
near-surface reservoir could explain this variability. Using the temperature
and humidity measurements from the rover, we find that perchlorate salts in the
regolith deliquesce to form liquid solutions, and deliquescence progresses to
deeper subsurface in the season of the methane spikes. We therefore formulate
the following three testable hypotheses. The first scenario is that the
regolith in Gale Crater adsorbs methane when dry and releases this methane to
the atmosphere upon deliquescence. The adsorption energy needs to be 36 kJ/mol
to explain the magnitude of the methane spikes, higher than existing laboratory
measurements. The second scenario is that microorganisms convert organic matter
in the soil to methane when they are in liquid solutions. This scenario does
not require regolith adsorption, but entails extant life on Mars. The third
scenario is that deep subsurface aquifers produce the bursts of methane.
Continued in situ measurements of methane and water, as well as laboratory
studies of adsorption and deliquescence, will test these hypotheses and inform
the existence of the near-surface reservoir and its exchange with the
atmosphere.Comment: Accepted for publication on Astrobiolog
Doppler sodar observations of the winds and structure in the lower atmosphere over Fairbanks, Alaska
Thesis (M.S.) University of Alaska Fairbanks, 2007Fairbanks, Alaska (64°49ʹ N, 147°52ʹ W) experiences strong temperature inversions which when combined with the low wind speeds prevailing during the winter cause serious air pollution problems. The SODAR (Sound Detection And Ranging) or acoustic sounder is a very useful instrument for studying the lower atmosphere as it can continuously and reliably measure the vertical profiles of wind speed and direction,vertical motions, turbulence and the thermal structure in the lower part of the troposphere. A Doppler sodar was operated from December 2005 to April 2006 at the National Weather Service site in Fairbanks. The wind observations from the sodar indicate that the majority of the winds during the winter months were from the North, Northeast or the East, which is in good agreement with the radiosonde measurements and the long term trends in the wind patterns over Fairbanks area. Case studies were carried out using the sodar data depicting drainage winds, low-level jets, formation and breakup of inversions and estimation of the mixing layer height.1. Introduction -- 1.1. Climatic features in Fairbanks during winter -- 1.1.1. Temperature inversions -- 1.1.2. Valley winds and drainage winds -- 1.1.3. Urban heat island -- 1.1.4. Air pollution and ice fog -- 1.2. SODAR and its applications -- 1.2.1 Acoustic sounder observations at Fairbanks in the past -- 2. Theory and instrumentation 2.1. Estimation of Ct² -- 2.1.1. Scattering theory -- 2.1.2. Sodar equation -- 2.2. Wind speed and direction -- 2.3. Sodar installation and data acquisition -- 2..4. Sodar dataset and additional sources of data -- 2.5. Algorithm to detect strong layers of temperature inversion -- 3. Results and discussion -- 3.1. Results from the inversion detection algorithm -- 3.1.1. Diurnal variations in inversion characteristics -- 3.1.2. Effect of cloud cover on inversion characteristics -- 3.2. Wind observations from sodar data -- 3.3. Case studies from sodar observations -- 3.3.1. Drainage winds overflowing the stable layer of air beneath -- 3.3.2. Nocturnal jet associated with a temperature inversion -- 3.3.3. Destruction of an inversion due to forced mixing and increasing cloud cover -- 3.3.4. Estimation of the mixing layer height from the backscatter intensity -- 4. Conclusions and future work -- References
Astroclimatic Characterization of Vallecitos: A candidate site for the Cherenkov Telescope Array at San Pedro Martir
We conducted an 18 month long study of the weather conditions of the
Vallecitos, a proposed site in Mexico to harbor the northern array of the
Cherenkov Telescope Array (CTA). It is located in Sierra de San Pedro Martir
(SPM) a few kilometers away from Observatorio Astron\'omico Nacional. The study
is based on data collected by the ATMOSCOPE, a multi-sensor instrument
measuring the weather and sky conditions, which was commissioned and built by
the CTA Consortium. Additionally, we compare the weather conditions of the
optical observatory at SPM to the Vallecitos regarding temperature, humidity,
and wind distributions. It appears that the excellent conditions at the optical
observatory benefit from the presence of microclimate established in the
Vallecitos.Comment: 16 pages, 16 figures, Publication of the Astronomical Society of the
Pacific, accepte
Titan Science with the James Webb Space Telescope (JWST)
The James Webb Space Telescope (JWST), scheduled for launch in 2018, is the
successor to the Hubble Space Telescope (HST) but with a significantly larger
aperture (6.5 m) and advanced instrumentation focusing on infrared science
(0.6-28.0 m ). In this paper we examine the potential for scientific
investigation of Titan using JWST, primarily with three of the four
instruments: NIRSpec, NIRCam and MIRI, noting that science with NIRISS will be
complementary. Five core scientific themes are identified: (i) surface (ii)
tropospheric clouds (iii) tropospheric gases (iv) stratospheric composition and
(v) stratospheric hazes. We discuss each theme in depth, including the
scientific purpose, capabilities and limitations of the instrument suite, and
suggested observing schemes. We pay particular attention to saturation, which
is a problem for all three instruments, but may be alleviated for NIRCam
through use of selecting small sub-arrays of the detectors - sufficient to
encompass Titan, but with significantly faster read-out times. We find that
JWST has very significant potential for advancing Titan science, with a
spectral resolution exceeding the Cassini instrument suite at near-infrared
wavelengths, and a spatial resolution exceeding HST at the same wavelengths. In
particular, JWST will be valuable for time-domain monitoring of Titan, given a
five to ten year expected lifetime for the observatory, for example monitoring
the seasonal appearance of clouds. JWST observations in the post-Cassini period
will complement those of other large facilities such as HST, ALMA, SOFIA and
next-generation ground-based telescopes (TMT, GMT, EELT).Comment: 50 pages, including 22 figures and 2 table
Effective Cloud Detection and Segmentation using a Gradient-Based Algorithm for Satellite Imagery; Application to improve PERSIANN-CCS
Being able to effectively identify clouds and monitor their evolution is one
important step toward more accurate quantitative precipitation estimation and
forecast. In this study, a new gradient-based cloud-image segmentation
technique is developed using tools from image processing techniques. This
method integrates morphological image gradient magnitudes to separable cloud
systems and patches boundaries. A varying scale-kernel is implemented to reduce
the sensitivity of image segmentation to noise and capture objects with various
finenesses of the edges in remote-sensing images. The proposed method is
flexible and extendable from single- to multi-spectral imagery. Case studies
were carried out to validate the algorithm by applying the proposed
segmentation algorithm to synthetic radiances for channels of the Geostationary
Operational Environmental Satellites (GOES-R) simulated by a high-resolution
weather prediction model. The proposed method compares favorably with the
existing cloud-patch-based segmentation technique implemented in the
PERSIANN-CCS (Precipitation Estimation from Remotely Sensed Information using
Artificial Neural Network - Cloud Classification System) rainfall retrieval
algorithm. Evaluation of event-based images indicates that the proposed
algorithm has potential to improve rain detection and estimation skills with an
average of more than 45% gain comparing to the segmentation technique used in
PERSIANN-CCS and identifying cloud regions as objects with accuracy rates up to
98%
- …