山东不同云系降水微物理参数特征

研究不同云系降水的微物理参数特征,对研究降水机制、人工影响天气、雷达定量测量降水、数值预报模式中微物理参数化方案的选择等都有一定意义。本文针对2015年济南地区的液态降水过程,基于微降水雷达(Micro Rain Radar,简称MRR)资料,研究不同云系降水的微物理参数。在400 m高度上,层状云降水0.02~0.2 mm h-1雨强样本数很大,但对累计降水量的贡献很小。混合云和对流云降水在大粒子端数浓度较高。在垂直方向上,层状云降水中的粒子的尺度较集中,中值体积直径D0平均在1 mm左右,随高度的变化不大。对流云降水在雨强大于20 mm h-1时,强垂直气流(包括上升气流和下沉气流)对粒子直径的影响较大,进而影响空中微降水雷达反演降水参数的数据质量。而垂直气流的影响对层状云降水影响较小,在层状云降水时,微降水雷达可以用来分析零度层亮带以下雨滴谱在垂直方向上的演变。     

大气物理过程方案对PM2.5预报的敏感性试验

利用WRF-Chem大气化学模式,选择2015年12月中旬发生在我国的大范围空气污染过程,在采用同样化学方案条件下,针对模式中不同物理过程及其参数化方案开展了地面PM_(2.5)预报的敏感性试验。结果表明:该模式能较好展示此次PM_(2.5)污染的演变过程,与实况也较接近,但对青海经宁夏至内蒙的PM_(2.5)高值区出现了漏报现象,这可能是模式外边界未对污染物做更新所致。对地面PM_(2.5)的预报,各物理过程的敏感度不同,边界层(含近地面层)过程的影响要明显大于积云对流及微物理过程的影响,不同的参数化方案会造成不同的预报误差。边界层过程QNSE和与其配套的近地面层方案的组合是预报的较佳组合;而TEMF和与其配套的组合以及ACM2和Pleim-X的组合则不佳。合理的物理过程参数化方案有助于提高PM_(2.5)预报质量。模式预报对排放源也有适应过程,其Spin-up时间较气象要素长。     

2017年秋季河北一次飑线引发的雷暴大风过程分析

利用常规地面高空观测资料、地面自动站资料、NCEP 1°×1°再分析资料、卫星云图、多普勒天气雷达资料等,对2017年秋季发生在河北省中部的一次由飑线引发的雷暴大风天气进行分析。结果表明：本次雷暴大风过程发生在高空冷涡底部,槽后冷空气与低层暖平流叠加配合地面冷锋的有利天气背景下,由飑线回波直接造成。环境条件中水汽和热力达到了中国华北地区产生强雷暴大风的平均值,大气温度直减率和垂直风切变比夏季更适宜,但能量不如夏季充足。飑线的强度、形态与夏季产生雷暴大风的雷达回波特征无异,但依据低层径向速度大值区预警秋季飑线大风需提高阈值。秋季飑线过程中地面同样伴随风场辐合、雷暴高压等中尺度系统,冷池密度流作用有利于地面大风产生。     

不同参数化方案对风预报效果影响个例研究

中尺度气象模式对风场的预报效果与参数化方案的适应性紧密相关.以内蒙古高原丘陵地形、江苏平缓的海陆交界地形2种典型下垫面试验风电场为模拟区域,分别用WRFV3.2 （Weather Research and Forecast Model）模式自带的6种物理过程参数化组合方案预报了2010年1月和7月两个风电场区域的风速和风向,对比分析了参数化组合方案差异对风场预报的影响.结果表明：①内蒙古试验区,边界层MRF方案描述的边界层结构较MYJ方案合理;微物理方案WSM3在夏季的风速预报能力优于WSM6,而冬季相反;复杂地形区域的风场预报需考虑陆面过程参数化方案,尤其是夏季降水发生后,陆面过程对于边界层结构的影响增大,选用Noah优于无陆面过程.②江苏试验区,边界层MRF方案描述的边界层结构较MYJ方案合理;1月陆面过程RUC方案优于陆面热量扩散和Noah;7月陆面热量扩散方案优于RUC和Noah.③风向预报6个方案的预报风向统计与实际记录风向统计有较好的一致性,风向概率分布相似,盛行风向一致且稳定.     

Accuracy and precision of different sampling strategies and flux integration methods for runoff water: comparisons based on measurements of the electrical conductivity

Because of their fast response to hydrological events, small catchments show strong quantitative and qualitative variations in their water runoff. Fluxes of solutes or suspended material can be estimated from water samples only if an appropriate sampling scheme is used. We used continuous in‐stream measurements of the electrical conductivity of the runoff in a small subalpine catchment (64 ha) in central Switzerland and in a very small (0·16 ha) subcatchment. Different sampling and flux integration methods were simulated for weekly water analyses. Fluxes calculated directly from grab samples are strongly biased towards high conductivities observed at low discharges. Several regressions and weighted averages have been proposed to correct for this bias. Their accuracy and precision are better, but none of these integration methods gives a consistently low bias and a low residual error. Different methods of peak sampling were also tested. Like regressions, they produce important residual errors and their bias is variable. This variability (both between methods and between catchments) does not allow one to tell a priori which sampling scheme and integration method would be more accurate. Only discharge‐proportional sampling methods were found to give essentially unbiased flux estimates. Programmed samplers with a fraction collector allow for a proportional pooling and are appropriate for short‐term studies. For long‐term monitoring or experiments, sampling at a frequency proportional to the discharge appears to be the best way to obtain accurate and precise flux estimates. Copyright © 2006 John Wiley & Sons, Ltd.     

‘A Case Study on a Polar Cold Air Outbreak over Fram Strait using a Mesoscale Weather Prediction Model’

The mesoscale weather prediction model ’Lokal-Modell’ (LM) of the Deutscher Wetterdienst is applied to the situation of an Arctic cold air outbreak in the Fram Strait region in April 1998. Observations are available from a flight along 5⁰E carried out during the ARTIST campaign. Initial and time-dependent boundary data for the simulation are taken from a larger scale operational model system. Using the standard configuration of LM, the simulation reproduced the propagation of cold air and the characteristic structure of the atmospheric boundary layer (ABL) in fair agreement with the observations. However, a detailed comparison revealed three basic problems. Firstly, there is evidence that the available data on sea-ice conditions were insufficient approximations to the true state for several reasons. A modification of the sea-ice data towards observations revealed that parts of the discrepancies were due to the original sea-ice data. Secondly, a control run with the model in its standard configuration shows an insufficient warming of the ABL downstream of the ice edge due to underestimation of surface heat fluxes. A simple modification of the approach for the scalar roughness length resulted in the strongest benefit, while comparative studies showed only a slight sensitivity to different types of parametrisation of turbulent mixing or the inclusion of an additional moist convection parametrisation. Thirdly, in all the simulations the deepening of the convective ABL downstream of the ice edge is weaker than observed. This may be partly due to the thermal stratification above the ABL in the analysis data, which is more stable than observed; but it may also be a hint to the fact that processes near the inversion are insufficiently parametrised in mesoscale models with resolutions as used in LM. The simulated cloud layer in the convective ABL is similar to that observed with respect to condensate content, a sharply defined cloud top, a diffuse lower bound, and continuous light precipitation.     

A Parameterization of Third-order Moments for the Dry Convective Boundary Layer

We describe one-dimensional (1D) simulations of the countergradient zone of mean potential temperature observed in the convective boundary layer (CBL). The method takes into account the third-order moments (TOMs) in a turbulent scheme of relatively low order, using the turbulent kinetic energy equation but without prognostic equations for other second-order moments. The countergradient term is formally linked to the third-order moments and , and a simple parameterization of these TOMs is proposed. It is validated for several cases of a dry CBL, using large-eddy simulations that have been realized from the MESO-NH model. The analysis of the simulations shows that TOMs are responsible for the inversion of the sign of in the higher part of the CBL, and budget analysis shows that the main terms responsible for turbulent fluxes and variances are now well reproduced.     

一次阻塞型华北对流性暴雨过程的诊断分析和数值模拟

利用常规和地面加密气象观测资料、卫星云图、NCEP再分析资料及中尺度模式MM5,对2006年7月23日一次阻塞型华北暴雨过程进行了诊断分析和数值模拟。结果表明,此次暴雨过程发生在以东西伯利亚阻塞高压为典型特征的大尺度鞍型场的背景下,宽广的低压区及高、低空急流次级环流为中尺度对流系统(MCS)的发生、发展提供了适宜的环境和动力强迫;700hPa和850hPa天气尺度的偏西水汽输送和低涡北侧的偏东水汽输送改善了环北京地区的水汽条件,暴雨发生前850hPa中尺度西南水汽输送对北京暴雨的发生有直接影响;MM5模式再现了导致MCS的形成及演变过程;中-β尺度MCS是这次暴雨的直接触发系统,其水平尺度约为0.5个经距,垂直方向由地面伸展到300hPa左右,具有典型的暖心结构,辐合、辐散中心分别位于900hPa和400hPa;MCS北侧强垂直次级环流为强对流的产生创造了有利条件。同时,MCS南侧相当位温的强梯度高能区及该区域的不稳定能量输送也是暴雨发生的重要条件。     

山东一次区域性暴雨中尺度特征分析

利用常规资料、地面自动站资料、FY2C卫星云图TBB和多普勒天气雷达资料,对2009年5月9—10日发生在山东的春季区域性暴雨进行分析和研究。结果表明:①强降水是在低层冷空气和深厚西南暖湿气流交汇的过程中产生的,副高异常偏强,制约850～700hPa切变线和地面辐合线停滞少动,产生较长时间的降水。②地面辐合线的形成和维持激发了边界层的辐合上升运动,为暴雨区提供了充足的水汽,冷空气从边界层楔入,与暖湿气流汇合并抬升暖湿气流辐合上升,使上升运动加强,降水增幅。③中尺度对流系统是造成暴雨的主要中尺度系统,多个单体更迭并移经同一区域,形成"列车效应"而产生区域性暴雨。④雷达径向速度图中逆风区和不同高度(超)低空急流的大小对短时强降水预报有一定的指示意义。