SARAL/AltiKa Global Statistical Assessment and Cross-Calibration with Jason-2     

Pierre Prandi  Sabine Philipps  Vincent Pignot  Nicolas Picot 《Marine Geodesy》2015,38(3):297-312

The CNES/ISRO mission SARAL/AltiKa was successfully launched on 25 February 2013. It reached its nominal orbit on 13 March 2013. AltiKa is the first altimeter using the Ka-band frequency. This article presents the results of the calibration and validation activities perfromed on the first year of the SARAL/AltiKa mission. The main objective of the article is to assess the SARAL/AltiKa data quality and to estimate the altimeter system performance using GDR products. To achieve this goal, we present mono-mission metrics and compare them with Jason-2 over the same period. Even if these missions do not have the same ground track, precise comparisons are still possible. They allow assessing parameter discrepancies and SSH consistency between both missions in order to detect geographically correlated biases, jumps or drifts. These results show that SARAL/AltiKa data quality is excellent: ocean data coverage is greater than 99.5%, standard deviation at cross-overs is 5.4 cm. The mission therefore fulfills the requirements of high precision altimetry and can be used (in conjunction with Jason-2) to monitor the global mean sea level, ensuring the continuity of the record over ERS/Envisat historical ground track. Possible improvements and open issues are also identified, foreseeing an even better mission performance.  相似文献   

Comet McNaught (260P/2012 K2): spin axis orientation and rotation period     

Federico Manzini  Virginio Oldani  Roberto Crippa  José Borrero  Erik Bryssink  Martin Mobberley  Joel Nicolas 《Astrophysics and Space Science》2014,351(2):435-450

Extensive observations of comet 260P/McNaught were carried out between August 2012 and January 2013. The images obtained were used to analyze the comet’s inner coma morphology at resolutions ranging from 250 to about 1000 km/pixel. A deep investigation of the dust features in the inner coma allowed us to identify only a single main active source on the comet’s nucleus, at an estimated latitude of ?50^°±15^°. A thorough analysis of the appearance and of the motion of the morphological structures, supported by graphic simulations of the geometrical conditions of the observations, allowed us to determine a pole orientation located within a circular spot of a 15^°-radius centered at RA=60^°, Dec=0^°. The rotation of the nucleus seems to occur on a single axis and is not chaotic, furthermore no precession effects could be estimated from our measurements. The comet’s spin axis never reached the plane of the sky from October 2012 to January 2013; during this period it did not change its direction significantly (less than 30^°), thus giving us the opportunity to observe mainly structures such as bow-shaped jets departing from the single active source located on the comet’s nucleus. Only during the months of August 2012 and January 2013 the polar axis was directed towards the Earth at an angle of about 45^° from the plane of the sky; this made it possible to observe the development of faint structures like fragments of shells or spirals. A possible rotation period of 0.340±0.01 days was estimated by means of differential photometric analysis.  相似文献   

SPIRE point source photometry: within the Herschel interactive processing environment (HIPE)     

Chris Pearson  Tanya Lim  Chris North  George Bendo  Luca Conversi  Darren Dowell  Matt Griffin  Terry Jin  Nicolas Laporte  Andreas Papageorgiou  Bernhard Schulz  Dave Shupe  Anthony J. Smith  Kevin Xu 《Experimental Astronomy》2014,37(2):175-194

The different algorithms appropriate for point source photometry on data from the SPIRE instrument on-board the Herschel Space Observatory, within the Herschel Interactive Processing Environment (HIPE) are compared. Point source photometry of a large ensemble of standard calibration stars and dark sky observations is carried out using the 4 major methods within HIPE: SUSSEXtractor, DAOphot, the SPIRE Timeline Fitter and simple Aperture Photometry. Colour corrections and effective beam areas as a function of the assumed source spectral index are also included to produce a large number of photometric measurements per individual target, in each of the 3 SPIRE bands (250, 350, 500μm), to examine both the accuracy and repeatability of each of the 4 algorithms. It is concluded that for flux densities down to the level of 30mJy that the SPIRE Timeline Fitter is the method of choice. However, at least in the 250 and 350μm bands, all 4 methods provide photometric repeatability better than a few percent down to at approximately 100mJy. The DAOphot method appears in many cases to have a systematic offset of ～8 % in all SPIRE bands which may be indicative of a sub-optimal aperture correction. In general, aperture photometry is the least reliable method, i.e. largest scatter between observations, especially in the longest wavelength band. At the faintest fluxes, <30mJy, SUSSEXtractor or DAOphot provide a better alternative to the Timeline Fitter.  相似文献   

The Herschel-PACS photometer calibration     

Zoltan Balog  Thomas Müller  Markus Nielbock  Bruno Altieri  Ulrich Klaas  Joris Blommaert  Hendrik Linz  Dieter Lutz  Attila Moór  Nicolas Billot  Marc Sauvage  Koryo Okumura 《Experimental Astronomy》2014,37(2):129-160

This paper provides an overview of the PACS photometer flux calibration concept, in particular for the principal observation mode, the scan map. The absolute flux calibration is tied to the photospheric models of five fiducial stellar standards (α Boo, α Cet, α Tau, β And, γ Dra). The data processing steps to arrive at a consistent and homogeneous calibration are outlined. In the current state the relative photometric accuracy is ～2 % in all bands. Starting from the present calibration status, the characterization and correction for instrumental effects affecting the relative calibration accuracy is described and an outlook for the final achievable calibration numbers is given. After including all the correction for the instrumental effects, the relative photometric calibration accuracy (repeatability) will be as good as 0.5 % in the blue and green band and 2 % in the red band. This excellent calibration starts to reveal possible inconsistencies between the models of the K-type and the M-type stellar calibrators. The absolute calibration accuracy is therefore mainly limited by the 5 % uncertainty of the celestial standard models in all three bands. The PACS bolometer response was extremely stable over the entire Herschel mission and a single, time-independent response calibration file is sufficient for the processing and calibration of the science observations. The dedicated measurements of the internal calibration sources were needed only to characterize secondary effects. No aging effects of the bolometer or the filters have been found. Also, we found no signs of filter leaks. The PACS photometric system is very well characterized with a constant energy spectrum νF _ν = λF _λ = const as a reference. Colour corrections for a wide range of sources SEDs are determined and tabulated.  相似文献   

Cloud bands over southern Africa: seasonality, contribution to rainfall variability and modulation by the MJO     

Neil C. G. Hart  Chris J. C. Reason  Nicolas Fauchereau 《Climate Dynamics》2013,41(5-6):1199-1212

Tropical-extratropical cloud band systems over southern Africa, known as tropical temperate troughs (TTTs), are known to contribute substantially to South African summer rainfall. This study performs a comprehensive assessment of the seasonal cycle and rainfall contribution of TTTs by using a novel object-based strategy that explicitly tracks these systems for their full life cycle. The methodology incorporates a simple assignment of station rainfall data to each event, thereby creating a database containing detailed rainfall characteristics for each TTT. This is used to explore the importance of TTTs for rain days and climatological rainfall totals in October–March. Average contributions range from 30 to 60 % with substantial spatial heterogeneity observed. TTT rainfall contributions over the Highveld and eastern escarpment are lower than expected. A short analysis of TTT rainfall variability indicates TTTs provide substantial, but not dominant, intraseasonal and interannual variability in station rainfall totals. TTTs are however responsible for a high proportion of heavy rainfall days. Of 52 extreme rainfall events in the 1979–1999 period, 30 are associated with these tropical-extratropical interactions. Cut-off lows were included in the evolution of 6 of these TTTs. The study concludes with an analysis of the question: does the Madden-Julian Oscillation influence the intensity of TTT rainfall over South Africa? Results suggest a weak but significant suppression (enhancement) of intensity during phase 1(6).  相似文献   

Homogenized Daily Relative Humidity Series in China during 1960?2017   总被引：1，自引：0，他引：1  

Zhen LI  Zhongwei YAN  Yani ZHU  Nicolas FREYCHET  Simon TETT 《大气科学进展》2020,37(4):318-327

Surface relative humidity(RH)is a key element for weather and climate monitoring and research.However,RH is not as commonly applied in studying climate change,partly because the observation series of RH are prone to inhomogeneous biases due to non-climate changes in the observation system.A homogenized dataset of daily RH series from 746 stations in Chinese mainland for the period 1960–2017,ChinaRHv1.0,has been developed.Most(685 or 91.82%of the total)station time series were inhomogeneous with one or more break points.The major breakpoints occurred in the early 2000s for many stations,especially in the humid and semi-humid zones,due to the implementation of automated observation across the country.The inhomogeneous biases in the early manual records before this change are positive relative to the recent automatic records,for most of the biased station series.There are more break points detected by using the MASH(Multiple Analysis of Series for Homogenization)method,with biases mainly around?0.5%and 0.5%.These inhomogeneous biases are adjusted with reference to the most recent observations for each station.Based on the adjusted observations,the regional mean RH series of China shows little long-term trend during 1960–2017[0.006%(10 yr)^?1],contrasting with a false decreasing trend[?0.414%(10 yr)?1]in the raw data.It is notable that ERA5 reanalysis data match closely with the interannual variations of the raw RH series in China,including the jump in the early 2000s,raising a caveat for its application in studying climate change in the region.  相似文献   

Controls on upper Campanian–Maastrichtian chalk deposition in the eastern Danish Basin          下载免费PDF全文

Myriam Boussaha  Nicolas Thibault  Kresten Anderskouv  Julien Moreau  Lars Stemmerik 《Sedimentology》2017,64(7):1998-2030

Detailed facies analysis of a 350 m long core of upper Campanian–Maastrichtian chalk at Stevns Peninsula, eastern Denmark, shows that four mudstone and wackestone chalk facies account for close to 95% of the succession, and that bioturbated mudstone chalk alone accounts for nearly 55% of the sediment. Sedimentation took place in deep water, below the photic zone and storm‐wave base, and is characterized by decimetre to metre‐scale variations in facies and trace fossil assemblages indicating repeated shifts in depositional environment. Integration of facies with published data on sea‐surface temperature and accumulation rates suggests that sea‐surface temperature is the most important parameter in controlling stratification of the water column and thereby, indirectly, the observed variations in depositional facies. However, bioturbated mudstone chalk occurs in all stratigraphic levels independent of accumulation rates and sea temperatures and is interpreted to represent a very broad set of deep water environmental conditions with an ample supply of calcareous nannofossil debris and intense bioturbation. Longer term shifts in deposition are best expressed by distribution of clay, flint and bioturbated micro‐wackestone, bioturbated wackestone and laminated mudstone chalk facies, whereas the trace fossil assemblages appear less useful. The data set indicates overall shallowing over time with two distinctive events of clay influx to the basin during the late Campanian–earliest Maastrichtian and late Maastrichtian.  相似文献   

Predicting significant wave height with artificial neural networks in the South Atlantic Ocean: a hybrid approach     

Marangoni Gazineu Marinho Pinto  Paula  Martins Campos  Ricardo  Gallo  Marcos Nicolas  Parente Ribeiro  Carlos Eduardo 《Ocean Dynamics》2023,73(6):303-315

Ocean Dynamics - Accurate simulations of significant wave height (Hs) are extremely important for the safety of navigation, port operations, and oil and gas exploration. Thus, accurate forecasts of...  相似文献   

Evaluation of the Global Mean Sea Level Budget between 1993 and 2014     

Don P. Chambers  Anny Cazenave  Nicolas Champollion  Habib Dieng  William Llovel  Rene Forsberg  Karina von Schuckmann  Yoshihide Wada 《Surveys in Geophysics》2017,38(1):309-327

Evaluating global mean sea level (GMSL) in terms of its components—mass and steric—is useful for both quantifying the accuracy of the measurements and understanding the processes that contribute to GMSL rise. In this paper, we review the GMSL budget over two periods—1993 to 2014 and 2005 to 2014—using multiple data sets of both total GMSL and the components (mass and steric). In addition to comparing linear trends, we also compare the level of agreement of the time series. For the longer period (1993–2014), we find closure in terms of the long-term trend but not for year-to-year variations, consistent with other studies. This is due to the lack of sufficient estimates of the amount of natural water mass cycling between the oceans and hydrosphere. For the more recent period (2005–2014), we find closure in both the long-term trend and for month-to-month variations. This is also consistent with previous studies.  相似文献