对引发密云泥石流的局地暴雨的分析和诊断

2002年8月1日晚北京东北部密云县发生了降雨量达到280.2 mm的局地短历时特大暴雨，并引发了洪水和泥石流。卫星云图分析表明，它是由一个中-β尺度对流系统在北京北部山区停留造成的。TBB图的等值线密集区和上冲云顶对暴雨落区有指示意义。大尺度环流诊断表明，暴雨前一天存在较深厚的下沉运动，其伴随的逆温层抑制对流的发生，使不稳定能量得以积累。各种稳定参数的计算结果证实了这一能量的积累过程。暴雨当天，下沉运动转为微弱的上升运动，使对流的发生成为可能。大尺度水汽场的诊断表明，北京处在西太平洋副热带高压边缘的高温高湿气流的控制下，有很强的水汽输送。地面中尺度分析表明，中尺度低压和辐合线是对流的触发系统，北京地区北部处于中尺度低压东部暖湿气流的辐合区中，在有利的地形条件下使密云县西部山区产生了局地特大暴雨。     

区域气候模拟中Betts-Miller对流参数化方案的敏感性试验

利用NCAR RegCM2气候模式,对Betts-Miller对流参数化方案中影响参考态的几个关键性参数:稳定度参数α、云底和冻结层参考态的饱和气压Δp*和对流调整时间τ进行了敏感性试验,分析这些参数的不同取值对区域气候模拟的影响及意义,并由此提出适合我国东部地区汛期区域气候模拟的Betts-Miller对流参数化方案中较优的关键性参数取值.     

NUMERICAL SIMULATION OF A MESOSCALE CONVECTIVE SYSTEM (MCS) DURING THE FIRST RAINY SEASON OVER SOUTH CHINA*

Based on the NCEP/NCAR reanalysis data and observations collected during the SCSMEX,a mesoscale convective system (MCS) occurring over South China during 23-24 May 1998 has been studied with a numerical simulation using the Fifth Generation Penn-State/NCAR Mesoseale Modeling System (MMS).The successful simulations present us some interesting findings.The simulated MCS was a kind of meso-β scale system with a life cycle of about 11 hours.It generated within a small vortex along a cold front shear line.The MCS was characterized by severe convection.The simulated maximum vertical velocity was greater than 90 cm s^-1,and the maximum divergence at about 400 hPa.The rainfall rate of MCS exceeded 20 mm h^-1.To the right of the simulated MCS,a mesoscale low-level jet (mLLJ) was found.A strong southwesterly current could also be seen to the right of MCS above the mLLJ.This strong southwesterly current might extend up to 400 hPa.A column of cyclonic vorticity extended through most part of the MCS in the vertical direction.Additionally,the simulated MCS was compared favorably with the observational data in terms of location,precipitation intensity and evolution.     

Initiation Mechanism of Meso-β Scale Convective Systems

With the aid of the Penn State-NCAR MM5 model, the initiation mechanism of meso-β scaleconvective systems (MCS) is investigated on the basis of simulation of the temporal and spatialthermodynamic structure of the MCS that occurred in Wuhan, Hubei, China and its surrounding area on 21July 1998. Using the PV inversion method, comparisons among the upper-, middle-, and low-leveltropospheric potential vorticity (PV) perturbations, as well as their effects on the initiation of MCS, indicatethat the low-level tropospheric PV perturbations play an important role in the triggering of MCS. Furtheranalysis reveals that the interaction between the southwest low-level jet and the gravity-inertia wave indeedinitiates MCS in the conditionally unstable ambient atmosphere.     

Cloud cover classification through simultaneous ground-based measurements of solar and infrared radiation

Simultaneous measurements of downwelling short-wave solar irradiance and incoming total radiation flux were performed at the Reeves Nevè glacier station (1200 m MSL) in Antarctica on 41 days from late November 1994 to early January 1995, employing the upward sensors of an albedometer and a pyrradiometer. The downwelling short-wave radiation measurements were analysed following the Duchon and O'Malley [J. Appl. Meteorol. 38 (1999) 132] procedure for classifying clouds, using the 50-min running mean values of standard deviation and the ratio of scaled observed to scaled clear-sky irradiance. Comparing these measurements with the Duchon and O'Malley rectangular boundaries and the local human observations of clouds collected on 17 days of the campaign, we found that the Duchon and O'Malley classification method obtained a success rate of 93% for cirrus and only 25% for cumulus. New decision criteria were established for some polar cloud classes providing success rates of 94% for cirrus, 67% for cirrostratus and altostratus, and 33% for cumulus and altocumulus.The ratios of the downwelling short-wave irradiance measured for cloudy-sky conditions to that calculated for clear-sky conditions were analysed in terms of the Kasten and Czeplak [Sol. Energy 24 (1980) 177] formula together with simultaneous human observations of cloudiness, to determine the empirical relationship curves providing reliable estimates of cloudiness for each of the three above-mentioned cloud classes. Using these cloudiness estimates, the downwelling long-wave radiation measurements (obtained as differences between the downward fluxes of total and short-wave radiation) were examined to evaluate the downwelling long-wave radiation flux normalised to totally overcast sky conditions. Calculations of the long-wave radiation flux were performed with the MODTRAN 3.7 code [Kneizys, F.X., Abreu, L.W., Anderson, G.P., Chetwynd, J.H., Shettle, E.P., Berk, A., Bernstein, L.S., Robertson, D.C., Acharya, P., Rothman, L.S., Selby, J.E.A., Gallery, W.O., Clough, S.A., 1996. In: Abreu, L.W., Anderson, G.P. (Eds.), The MODTRAN 2/3 Report and LOWTRAN 7 MODEL. Contract F19628-91-C.0132, Phillips Laboratory, Geophysics Directorate, PL/GPOS, Hanscom AFB, MA, 261 pp.] for both clear-sky and cloudy-sky conditions, considering various cloud types characterised by different cloud base altitudes and vertical thicknesses. From these evaluations, best-fit curves of the downwelling long-wave radiation flux were defined as a function of the cloud base height for the three polar cloud classes. Using these relationship curves, average estimates of the cloud base height were obtained from the three corresponding sub-sets of long-wave radiation measurements. The relative frequency histograms of the cloud base height defined by examining these three sub-sets were found to present median values of 4.7, 1.7 and 3.6 km for cirrus, cirrostratus/altostratus and cumulus/altocumulus, respectively, while median values of 6.5, 1.8 and 2.9 km were correspondingly determined by analysing only the measurements taken together with simultaneous cloud observations.     

DETECTION OF MULTILAYER CIRRUS CLOUD USING FY-1C DATA*

There are ten channels in the FY-1C polar-orbiting meteorological satellite of China.These channels cover visible,near-infrared and infrared spectral bands.Based on simulating analysis of single layer and multilayer clouds that cirrus clouds possibly overlap low water clouds,the case study using FY-1C data is performed.Results show that FY-1C data can be used to analyze multilayer clouds,especially for the ease of low water cloud overlaid by cirrus.     

A finite element-spherical harmonics model for radiative transfer in inhomogeneous clouds.: Part II. Some applications

EVENT has been used to examine the effects of 3D cloud structure, distribution, and inhomogeneity on the scattering of visible solar radiation and the resulting 3D radiation field. Large eddy simulation and aircraft measurements are used to create realistic cloud fields which are continuous or broken with smooth or uneven tops. The values, patterns and variance in the resulting downwelling and upwelling radiation from incident visible solar radiation at different angles are then examined and compared to measurements. The results from EVENT confirm that 3D cloud structure is important in determining the visible radiation field, and that these results are strongly influenced by the solar zenith angle. The results match those from other models using visible solar radiation, and are supported by aircraft measurements of visible radiation, providing confidence in the new model.     

MESOSCALE CONVECTIVE SYSTEMS DURING 20-23 JUNE 2002 REVEALED BY SATELLITE OBSERVATION

The 20-23 June 2002 mesoscale convective systems,which produced heavy rainfalls over the middle and lower reaches of the Changjiang River Basin,are studied using satellite imagery, satellite products and conventional sounding data. Results demonstrate that the torrential rain was caused by three MαCSs (Meso-a scale Convective System) and some MβCS (Meso-β-scale Convective System) activities in succession. The TBB (black-body temperature) analysis depicts that several meso-β-scale cloud clusters and convective cells were embedded and alternately developed within an MαCS.As the strongest convection gradually decreased,the cold TBB area expanded quickly in hours before dissipation. However,the heavy rainfall occurred in MαCS developing and maturing stages.And the minimum TBB fluctuation matched well to the precipitation trend with the lower TBB for the heavier rainfall. A kind of favorable synoptic environment for MαCS genesis and activities could be described as follows.The West Pacific subtropical high is stable with its western ridge reaching to the west region of South China.To its south and west sides,there is the robust ITCZ (Intertropical Convergence Zone),the active Indian-Bengal monsoon cloud surges carry warm and moist air by low-level jet (LLJ) to mid-latitudes where a frontal zone existed for days,meanwhile a 500 hPa short-wave trough moved eastward.At the upper troposphere,the southeasterly divergent flow dominates the environment due to the South-Asian high enhancing and moving eastward,and a monsoon water vapor plume (WVP) is stretched from the Bay of Bengal to the Changjiang River Basin.With a certain favorable configuration including a monsoon WVP,a frontal cloud system,a shortwave trough cloud system,and monsoon cloud clusters,MαCSs could initiate and develop successively in the same region.     

一次强飑线的成因及维持和加强机制分析

利用常规观测资料、多普勒天气雷达、自动气象站等资料对2004年7月12日影响上海的一次较长生命史的强飑线过程进行了综合分析，对这次强对流天气发生、发展、强度以及移动和传播的分析结果表明:副热带高压从华南沿海稳定地加强西伸，西风槽缓慢东移，导致华东地区850~500 hPa形成深厚西南急流，急流的加强促使低层锋生，配合K指数高能锋区的不稳定层结，大大增强了强对流天气发生的可能性；地面锋生作用和低层辐合、高层辐散造成的强抬升作用是主要的触发机制；较强的环境风垂直切变和雷暴内部上升气流与下沉气流的正反馈作用是飑线系统维持较长时间的原因，中尺度对流系统(MCS)多个雷暴单体间的相互作用使得南侧的雷暴单体加强、移动方向发生偏转。     

有利于火箭人工增雨作业的天气条件分析

对有利于火箭人工增雨的天气条件进行了分析,给出了石家庄容易出现对流天气的天气形势特征及关键指标,通过编程实现关键指标的客观判断,达到提醒预报员灾害天气发生的可能。常规资料和每小时一次的自动站资料可以较高时效来判断对流天气的威胁区,尤其通过自动站资料计算得到的假相当位温高值区配合风场辐合带可以更高精度预测强降水落区;每小时一次的卫星云图和亮温资料可以跟踪云系、云团的演变,判断云系的发生、发展、维持情况,文中给出了判断云系发展、维持、移向和移速的云系特征,为提前寻找适合于增雨作业的时间和部位打下基础,为山区火箭增雨作业争取了时间。