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831.
Jinho Kim Hee Il Kim Hyung Mok Lee 《Monthly notices of the Royal Astronomical Society》2009,399(1):229-238
We obtain equilibrium solutions for rotating compact stars, including special relativistic effects. The gravity is assumed to be Newtonian, but we use the active mass density, which takes into account all energies such as the motion of the fluid, internal energy and pressure energy in addition to the rest-mass energy, in computing the gravitational potential using Poisson's equation. Such a treatment could be applicable to neutron stars with relativistic motions or a relativistic equation of state. We applied Hachisu's self-consistent field (SCF) method to find spheroidal as well as toroidal sequences of equilibrium solutions. Our solutions show better agreement with general relativistic solutions than the Newtonian relativistic hydrodynamic approach, which does not take into account the active mass. Physical quantities such as the peak density and equatorial radii in our solutions agree with the general relativistic ones to within 5 per cent. Therefore our approach can be used as a simple alternative to the fully relativistic one when a large number of model calculations is necessary, as it requires much fewer computational resources. 相似文献
832.
Jun Ma Richard de Grijs Zhou Fan Soo-Chang Rey Zhen-Yu Wu Xu Zhou Jiang-Hua Wu Zhao-Ji Jiang Jian-Sheng Chen Kyungsook Lee Sangmo Tony Sohn 《中国天文和天体物理学报》2009,9(6)
Mayall Ⅱ = G1 is one of the most luminous globular clusters (GCs) in M31. Here, we determine its age and mass by comparing multicolor photometry with theo-retical stellar population synthesis models. Based on far- and near-ultraviolet GALEX photometry, broad-band UBV RI, and infrared JHK8 2MASS data, we construct the most extensive spectral energy distribution of G1 to date, spanning the wavelength range from 1538 to 20000A. A quantitative comparison with a variety of simple stellar pop-ulation (SSP) models yields a mean age which is consistent with G1 being among the oldest building blocks of M31 and having formed within ~1.7Gyr after the Big Bang. Irrespective of the SSP model or stellar initial mass function adopted, the resulting mass estimates (of order 107M⊙) indicate that G1 is one of the most massive GCs in the Local Group. However, we speculate that the cluster's exceptionally high mass suggests that it may not be a genuine GC. Our results also suggest that G1 may contain, on average, (1.65±0.63) × 102L⊙ far-ultraviolet-bright, hot, extreme horizontal-branch stars, depend-ing on the adopted SSP model. In addition, we demonstrate that extensive multi-passband photometry coupled with SSP analysis enables one to obtain age estimates for old SSPs that have similar accuracies as those from integrated spectroscopy or resolved stellar pho-tometry, provided that some of the free parameters can be constrained independently. 相似文献
833.
A Modelling Study of Flux Imbalance and the Influence of Entrainment in the Convective Boundary Layer 总被引:3,自引:2,他引:1
It is frequently observed in field experiments that the eddy covariance heat fluxes are systematically underestimated as compared
to the available energy. The flux imbalance problem is investigated using the NCAR’s large-eddy simulation (LES) model imbedded
with an online scheme to calculate Reynolds-averaged fluxes. A top–down and a bottom–up tracer are implemented into the LES
model to quantify the influence of entrainment and bottom–up diffusion processes on flux imbalance. The results show that
the flux imbalance follows a set of universal functions that capture the exponential decreasing dependence on u
*/w
*, where u
* and w
* are friction velocity and the convective velocity scale, respectively, and an elliptic relationship to z/z
i
, where z
i
is the mixing-layer height. The source location in the boundary layer is an important factor controlling the imbalance magnitude
and its horizontal and vertical distributions. The flux imbalance of heat and the bottom–up tracer is tightly related to turbulent
coherent structures, whereas for the top–down diffusion, such relations are weak to nonexistent. Our results are broadly consistent
with previous studies on the flux imbalance problem, suggesting that the published results are robust and are not artefacts
of numerical schemes. 相似文献
834.
Dong-Hyuk Kim Hwa-Woon Lee Soon-Hwan Lee 《Asia-Pacific Journal of Atmospheric Sciences》2010,46(4):393-403
The wind distribution over the Korean Peninsula was analyzed using numerically optimized wind data to reduce the uncertainties in estimating the wind resources. The simulated data were validated by a comparison with surface wind observations and three statistical indexes. According to the simulated surface winds, mesoscale circulation, such as land-sea breeze and mountain-valley winds affect the wind characteristics of the hub height at coastal and inland regions. However, the prevailing winds are strongly associated with the synoptic forcing at the island and mountainous regions, not the regional circulation. On the other hand, the atmospheric stability definitely affects the strength of the daytime and nocturnal wind speed at a hub height. Overall, there was a significant difference between the numerical and logarithmic method to estimate the wind energy at hub height. Moreover, the discrepancy in the wind density estimated using the two methods becomes clear over inland and mountainous areas. 相似文献
835.
Dense observations and numerical experiments were carried out to estimate the modification of mesoscale circulation, particularly cold drainage wind. It was confirmed that nocturnal drainage flow can develop on clear calm summer day and change due to orographical forcing and the heterogeneity of heat flux induced by the discontinuity of land-use. The temperature of nocturnal drainage flow at Sungji Valley, Busan Korea, tended to increase as it passed over the urban surface due to anthropogenic heat. The increase in temperature reached 2.9 K at night. The roughness associated with the exchange of momentum flux alone and the pass of nocturnal drainage flow is important for modifying the characteristics of flow Numerical simulations carried out under various surface conditions showed good agreement with the observations. Urban heat fluxes from the surface during the day are fundamental causes of the changes in the urban mesoscale circulation. In addition, the impact of a discontinuity of surface heat flux on mesoscale flow modification tends to be greater at night than during the day because the direction of urban surface heat fluxes at night is different from that in rural areas. In addition, the criterion to estimate the increase in temperature nocturnal drainage flow was also proposed, and provided results that generally agreed with the numerical results. 相似文献
836.
Dong-Kyou Lee Dae-Yong Eom Joo-Wan Kim Jae-Bok Lee 《Asia-Pacific Journal of Atmospheric Sciences》2010,46(3):341-353
The WRF-based real-time forecast system (http://jhwc.snu.ac.kr/weather) of the Joint Center for High-impact Weather and Climate Research (JHWC) has been in operation since November 2006; this system has three nested model domains using GFS (Global Forecast System) data for its initial and boundary conditions. In this study, we evaluate the improvement in daily and hourly weather prediction, particularly the prediction of summer rainfall over the Korean Peninsula, in the JHWC WRF (Weather Research and Forecasting) model system by 3DVAR (three-Dimensional Variational) data assimilation using the data obtained from KEOP (Korea Enhanced Observation Program). KEOP was conducted during the period June 15 to July 15, 2007, and the data obtained included GTS (Global Telecommunication System) upper-air sounding, AWS (Automatic Weather System), wind profiler, and radar observation data. Rainfall prediction and its characteristics should be verified by using the precipitation observation and the difference field of each experiment. High-resolution (3 km in domain 3) summer rainfall prediction over the Korean peninsula is substantially influenced by improved synoptic-scale prediction in domains 1 (27 km) and 2 (9 km), in particular by data assimilation using the sounding and wind profiler data. The rainfall prediction in domain 3 was further improved by radar and AWS data assimilation in domain 3. The equitable threat score and bias score of the rainfall predicted in domain 3 indicated improvement for the threshold values of 0.1, 1, and 2.5 mm with data assimilation. For cases of occurrence of heavy rainfall (7 days), the equitable threat score and bias score improved considerably at all threshold values as compared to the entire period of KEOP. Radar and AWS data assimilation improved the temporal and spatial distributions of diurnal rainfall over southern Korea, and AWS data assimilation increased the predicted rainfall amount by approximately 0.3 mm 3hr?1. 相似文献
837.
Soon-Hwan Lee Soo-Jin Hwang Chan-Su Ryu 《Asia-Pacific Journal of Atmospheric Sciences》2010,46(3):355-367
In order to clarify the characteristics of Mesoscale Convective System (MCS) development and understand the impact of the trigger effect of isolated islands, observational and numerical analysis of the heavy rainfall were carried out over the southwestern part of the Korean Peninsula on July 14, 2004. Satellite based remote sensing data and numerical model MM5 with observational data adjustment were used in this study. The MCS development, in this case, was accompanied not by directional wind shear, but by speed shear which was strongly associated with development of the updraft cloud. An inversion layer at a 750 hPa level is one of the fundamental factors in increasing instability. Effective separation of the upper and lower level atmospheric structure due to an inversion layer at a 750 hPa level creates a suitable condition to develop a MCS. According to numerical analysis it has been found that isolated islands located off the southwestern part of the Korean Peninsula are not a principal factor in causing the heavy rainfall due to the evolution of MCS in this case. Transferable topographic forcing of the downwind side can often induce the variation of MCS intensity, while associated with the precipitation amount over the lee side of the isolated islands at a mature stage of MCS development. 相似文献
838.
A systematic procedure for calibrating system gain bias (so-called “calibration error”) of radar reflectivity measurements from the Korea Meteorological Administration (KMA) operational radar network is presented. First, the RJNI radar located at Jindo Island is calibrated by comparing with radar reflectivities simulated theoretically by a scattering algorithm using drop spectra collected by a disdrometer from June 19 to 29, 2009. Once the RJNI radar is calibrated, the reflectivity measurements from nearby radars are compared with the RJNI radar reflectivities to determine existing gain biases of nearby radars. This radar-radar calibration procedure was repeated with the other radars within the network. For isolating a system gain bias, echoes affected by partial beam blockage due to ground clutter and by attenuation due to precipitation were removed. The system gain biases of the RJNI and RPSN radars were ?3 and ?4.2 dB, respectively, during the experimental period. The RBRI and RDNH radars revealed relatively large biases, above ?8 dB. The other radars (RKSN, RGSN, RSSP, RKWK, RGDK, RIIA, and RMYN) revealed biases from ?6 to ?7 dB. Thus, the reflectivity measurements from all of the KMA radars were severely biased. New R-Z relations of R = 3.350 × 10?2Z0.624 (Z = 231.1R 1.6) for stratiform and R=1.546 × 10?2 Z 0.714 (Z = 342.4R 1.4) for convective precipitations were derived using disdrometer data. Using these R-Z relations, the radar-derived total rainfall amounts from the reflectivity measurements without calibration produced significant underestimations, compared to gauge measurements at about 80 sites, with a normalized bias of about ?56%. On the other hand, after calibrating the above system biases, the radar-derived rainfall amounts corresponded well with the gauge measurements, with a normalized bias of about ?3%. In conclusions, the radar reflectivity measurements from the KMA radar network are severely biased and the procedure presented in this study can be used to resolve the system gain biases. 相似文献
839.
840.
T. J. Griffis S. D. Sargent X. Lee J. M. Baker J. Greene M. Erickson X. Zhang K. Billmark N. Schultz W. Xiao N. Hu 《Boundary-Layer Meteorology》2010,134(2):307-325
Impacts of different terrain configurations on the general behaviour of idealised katabatic flows are investigated in a numerical
model study. Various simplified terrain models are applied to unveil modifications of the dynamics of nocturnal cold drainage
of air as a result of predefined topographical structures. The generated idealised terrain models encompass all major topographical
elements of an area in the tropical eastern Andes of southern Ecuador and northern Peru, and the adjacent Amazon. The idealised
simulations corroborate that (i) katabatic flows develop over topographical elements (slopes and valleys), that (ii) confluence
of katabatic flows in a lowland basin with a concave terrainline occur, and (iii) a complex drainage flow system regime directed
into such a basin can sustain the confluence despite varying slope angles and slope distances. 相似文献