微物理过程分档处理的三维对流云模式研究

在三维完全弹性冰雹云参数化模式动力框架和二维面对称分档模式(微物理过程分档处理)的基础上,建立粒子全分档的三维对流云分档模式.利用建立的分档模式对2000年6月29日美国堪萨斯州的一次超级单体风暴进行了模拟,并将模拟结果与三维冰雹云参数化模式的模拟结果作了对比.结果表明:分档模式可以较好地模拟出强对流云中两支上升气流,模拟得到的最大上升气流速度大于参数化模式;分档模式模拟得到的雷达回波强度、顶高和回波宽度更接近实测.     

A radar-based verification of precipitation forecast for local convective storms

Local flash flood storms with a rapid hydrological response are a real challenge for quantitative precipitation forecasting (QPF). It is relevant to assess space domains, to which the QPF approaches are applicable. In this paper an attempt is made to evaluate the forecasting capability of a high-resolution numerical weather prediction (NWP) model by means of area-related QPF verification. The results presented concern two local convective events, which occurred in the Czech Republic (CR) on 13 and 15 July 2002 and caused local flash floods. We used the LM COSMO model (Lokall Model of the COSMO consortium) adapted to the horizontal resolution of 2.8 km over a model domain covering the CR. The 18 h forecast of convective precipitation was verified by using radar rainfall totals adjusted to the measured rain gauge data. The grid point-related root mean square error (RMSE) value was calculated over a square around the grid point under the assumption that rainfall values were randomly distributed within the square. The forecast accuracy was characterized by the mean RMSE over the whole verification domain. We attempt to show a dependence of both the RMSE field and the mean RMSE on the square size. The importance of a suitable merger between the radar and rain gauge datasets is demonstrated by a comparison between the verification results obtained with and without the gauge adjustment. The application of verification procedure demonstrates uncertainties in the precipitation forecasts. The model was integrated with initial conditions shifted by 0.5° distances. The four verifications, corresponding to the shifts in the four directions, show differences in the resulting QPF, which depend on the size of verification area and on the direction of the shift.     

浅谈台风雨性质的辨别

利用地面观测资料,以及观测经验,对台风产生的降水过程进行分析,从中鉴别出该降水是阵性还是非阵性。     

Parametrization of the precipitation in the Northern Hemisphere and its verification in Mexico

To improve results in monthly rainfall prediction, a parametrization of precipitation has been developed. The thermodynamic energy equation used in the Adem thermodynamic model (ATM) and the Clausius and Clapeyron equation, were used to obtain a linear parametrization of the precipitation anomalies as a function of the surface temperature and the 700 mb temperature anomalies. The observed rainfall in Mexico over 36 months, from January 1981 to December 1983, was compared with the results obtained of the heat released by condensation, which is proportional to precipitation, using our theoretical formula, and those obtained using a statistical formula, which was derived for the ATM using 12 years of hemispheric real data. The verification using our formula in Mexico, showed better results than the one using the statistical formula.     

Lattice dynamical system modelling of molecular clouds

Because a comprehensive microscopic treatment of interstellar molecular clouds is out of reach, an alternative approach is proposed in which most of the crucial ingredients of the problem are considered, but at some 'minimal' level of modelling. This leads to the elaboration of a lattice dynamical system , i.e. a time-dependent, spatially extended, deterministic system of macroscopic cells coupled through radiative transfer. Each cell is characterized by a small set of variables and supports a caricatural chemistry possessing the essential dynamical features of more realistic reaction schemes.   This approach naturally precludes quantitative results, but allows heretofore unavailable insights into some of the basic mechanisms at play. We focus on the response of the transfer process and the chemistry to a frozen 'turbulent' velocity field. It is shown that the system settles generically into a state where the effective coupling between cells is neither local nor global, and for which no single length-scale exists.   The spectral lines reconstructed from the spatiotemporal evolution of our model may, depending on the velocity field, exhibit profiles ranging from Gaussian to bimodal with strong realization effects. In the bimodal case, the model intrinsically displays an energy cascade transport mechanism to the cells that cool most efficiently: the feedback of chemistry on radiative transfer cannot be neglected.   Finally, extensions of this work are discussed and future developments are outlined.     

Diffuse interstellar bands in low E (B – V) objects

We analyse spectra of slightly reddened, early-type stars in which the major diffuse interstellar bands at 5870 and 5797 Å are either strong or weak in relation to E(B – V) . It is demonstrated that among the low E(B – V) objects one can find stars obscured by 'sigma' as well as 'zeta' type clouds. The profiles of the diffuse bands seem not to be dependent on the total opacities of the interstellar clouds. We also discuss the physical conditions leading to the formation of the diffuse band carriers in low-opacity interstellar clouds; the carriers are apparently formed in media in which one can observe complex velocity fields.     

Collapse and fragmentation of rotating, prolate clouds

We present the results of collapse calculations for uniformly rotating, prolate clouds performed using the numerical method: smoothed particle hydrodynamics (SPH). The clouds considered are isothermal, prolate spheroids with different axial ratios ( a/b ), and with different values of β, the ratio of the rotational to gravitational energy. Small density perturbations are added to the clouds, and different initial perturbation spectra are studied. All of the clouds considered are strongly unstable to gravitational contraction, and so collapse to form a spindle configuration. Such a linear structure is unstable to fragmentation, so that the clouds break up into a number of subcondensations. The long-term evolution of the system is then determined by the angular momentum possessed by these fragments.
It is found that a number of the calculations performed result in the formation of orbitally stable binary systems, composed of two rotationally supported discs in orbit about their common centre of mass. Tidal interactions during closest approach, close three-body interactions and the continued accretion of material with high specific angular momentum are all found to increase the orbital separation during these calculations, ensuring that the systems do not merge at later times. The calculations are therefore relevant to the problem of binary star formation, though the systems produced tend to have large orbital separations and periods. One of the strong points of the models presented, however, is their ability to produce systems with a range of mass ratios and orbital eccentricities, without the explicit inclusion of biases in the initial conditions.     

Gas cooling by dust during dynamical fragmentation

We suggest that the abrupt switch, from hierarchical clustering on scales ≳ 0.04 pc, to binary (and occasionally higher multiple) systems on smaller scales, which Larson has deduced from his analysis of the grouping of pre-main-sequence stars in Taurus, arises because pre-protostellar gas becomes thermally coupled to dust at sufficiently high densities. The resulting change — from gas cooling by molecular lines at low densities to gas cooling by dust at high densities — enables the matter to radiate much more efficiently, and hence to undergo dynamical fragmentation. We derive the domain in which gas cooling by dust facilitates dynamical fragmentation. Low-mass (∼ M⊙) clumps — those supported mainly by thermal pressure — can probably access this domain spontaneously, albeit rather quasi-statically, provided that they exist in a region in which external perturbations are few and far between. More massive clumps probably require an impulsive external perturbation, for instance a supersonic collision with another clump, in order for the gas to reach sufficiently high density to couple thermally to the dust. Impulsive external perturbations should promote fragmentation, by generating highly non-linear substructures which can then be amplified by gravity during the subsequent collapse.     

On mixing processes in cumulus clouds

An examination is made of the hypothesis that internal cloud properties are determined by the mixing of dry air from above the cloud top and cloud base air in such a way that the mixture is neutrally buoyant with respect to the clear air environment at each level. It is concluded that the resulting mixture is much drier than is actually observed. Comments are made about observed cloud properties which need to be taken into account in any model of the mixing process.     

赤道太平洋对流活动异常对我国夏季降水和气温的影响

利用月平均ＯＬＲ、降水和气温资料,研究了ＥＮＳＯ期间赤道太平洋对流活动与我国夏季降水和气温的关系。结果表明：春季、夏季中、西太平洋对流活动异常与我国夏季江淮地区的降水有密切关系;春季和前一年冬季西太平洋对流活动异常与东北地区夏季降水有显著相关。前一年冬季中、西太平洋对流活动异常与我国降水的显著相关区是不同的,前者为华南、西南地区,后者为东北至内蒙古一带。气温与同期中、西太平洋对流活动的相关不显著。