The analysis of indexed astronomical time-series – VIII. Cross-correlating noisy autoregressive series     

Chris Koen 《Monthly notices of the Royal Astronomical Society》2003,344(3):798-808

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A search for short time-scale I-band variability in ultracool dwarfs     

Chris Koen 《Monthly notices of the Royal Astronomical Society》2003,346(2):473-482

76 hours of CCD photometry of 11 L dwarfs and one T dwarf is described. Three objects, including the T dwarf ε Indi B, showed convincing evidence for variability. A periodicity of 8 h is clearly visible in the light curve of 2MASS J1155395−372735.  相似文献   

Another look at theR-modes     

Chris Koen 《Astrophysics and Space Science》1987,135(2):393-398

Possible oscillation modes for a rotating star are listed. The only assumption made is that oscillations are adiabatic and that rotation is uniform. It is found that two modes not present for non-rotating stars are possible. Oscillation frequencies of these modes are rather different from those given in the literature for so-calledr-modes.  相似文献   

The Nyquist frequency for irregularly spaced time-series: a calculation formula     

Chris Koen 《Monthly notices of the Royal Astronomical Society》2006,371(3):1390-1394

Eyer & Bartholdi showed that the Nyquist frequency of irregularly sampled time-series can be very high. In this paper, a calculation formula for the Nyquist frequency is presented. In practice there is an upper limit of 0.5/Δ on this frequency, where Δ is the best accuracy with which time is recorded.  相似文献   

The analysis of indexed astronomical time series – X. Significance testing of O−C data     

Chris Koen 《Monthly notices of the Royal Astronomical Society》2006,365(2):489-495

It is assumed that   O − C   ('observed minus calculated') values of periodic variable stars are determined by three processes, namely measurement errors, random cycle-to-cycle jitter in the period, and possibly long-term changes in the mean period. By modelling the latter as a random walk, the covariances of all   O − C   values can be calculated. The covariances can then be used to estimate unknown model parameters, and to choose between alternative models. Pseudo-residuals which could be used in model fit assessment are also defined. The theory is illustrated by four applications to spotted stars in eclipsing binaries.  相似文献   

The DA+dMe eclipsing binary EC13471–1258: its cup runneth over … just     

D. O'Donoghue  C. Koen  D. Kilkenny  R. S. Stobie  D. Koester  M. S. Bessell  N. Hambly  H. MacGillivray 《Monthly notices of the Royal Astronomical Society》2003,345(2):506-528

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A search for short time-scale JHK variability in ultracool dwarfs     

C. Koen  N. Matsunaga  J. Menzies 《Monthly notices of the Royal Astronomical Society》2004,354(2):466-476

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Quantitative estimation of skin soil moisture with the Special Sensor Microwave/Imager   总被引：1，自引：0，他引：1  

Koen De Ridder 《Boundary-Layer Meteorology》2000,96(3):421-432

A physically based retrieval scheme is described toinfer near surface moisture content of bare soilpatches from time series of the Special SensorMicrowave/Imager (SSM/I) Polarisation DifferenceTemperature (PDT) at 19.35 GHz. The PDT is modelled asa function of soil moisture using Fresnel's relationsfor specular reflection at the air-soil boundary,together with a dielectric mixing model for themoisture-dependent soil dielectric constant. Theproblem of the specification of parameters describingatmospheric and soil roughness effects is avoided byconsidering temporal variations of the PDT. Theretrieval method is applied to the FIFE site for atwo-week period in summer, during which heavy rainfalloccurred on a dry upper soil. Although differences insensing depths made a rigorous validation impossible,remotely sensed soil moisture showed a high degree ofconsistency with ground measurements. An erroranalysis shows that the method is capable of providingan accuracy of approximately 20% of moisture contentat field capacity. Finally, it is demonstrated thatthe method is relatively insensitive to soil texturalcharacteristics when moisture content is scaled withfield capacity.  相似文献   

The Impact of Vegetation Cover on Sahelian Drought Persistence     

Koen De Ridder 《Boundary-Layer Meteorology》1998,88(2):307-321

The sensitivity of the potential for convective precipitation to land surface characteristics in the West African Sudano-Sahelian zone is studied. Simulations are performed with a one-dimensional version of a mesoscale atmospheric model coupled to a detailed land surface model to investigate the influence of the land surface on rain infiltration, surface evapotranspiration, and their impact on the convective available potential energy. The simulations show that the presence of a densely vegetated surface acts as a catalyst in the hydrological cycle, creating a positive feedback and enhancing precipitation recycling. It is argued that this is due to the relation between the characteristic drying-out time of the soil and the return frequency of the rain-triggering African easterly waves.  相似文献   

The Urban Heat Island Intensity of Paris: A Case Study Based on a Simple Urban Surface Parametrization   总被引：1，自引：0，他引：1  

Abhijit Sarkar  Koen De Ridder 《Boundary-Layer Meteorology》2011,138(3):511-520

We explore the ability of a simple urban surface parametrization, embedded in a mesoscale meteorological model, to correctly reproduce observed values of the urban heat island (UHI) intensity, which is defined as the urban-rural surface air temperature difference. To do so, a simple urban scheme was incorporated into the Advanced Regional Prediction System (ARPS). Subsequently, a simulation was performed with the coupled model over the wider area of Paris, for a 12-day period in June 2006 that was characterised by conditions prone to UHI development. Simulated 2-m air temperature was compared with observed values for urban and rural stations, yielding mean errors of 1.4 and 1.5 K, respectively. More importantly, it was found that the model also displayed an overall good capability of reproducing the observed temperature differences. In particular, the magnitude (up to 6 K) and timing of the diurnal cycle of the UHI intensity was simulated well, the model exhibiting a mean error of 1.15 K. As a result, our conclusion is that the ARPS model, extended with simple urban surface physics, is able to capture observed urban-rural air temperature differences well, at least for the domain and period studied.  相似文献