Airborne microphysical measurements and radar reflectivity observations near a cold frontal rainband observed during the FRONTS 87 experiment

Airborne microphysical measurements and ground-based radar-reflectivity observations, collected at the near of a narrow cold frontal rainband during the FRONTS 87 experiment, are used to infer some characteristics of the precipitation: size-distribution, liquid or ice-water content, mass-weighted fallspeed, radar-reflectivity and relationships between pairs of these quantities. In the ice phase region, two different methods are proposed to calculate the radar reflectivity from the aircraft data. The influence of the choice of the ice particle type on the calculated quantities is discussed from the results of the first method. The second method is new: it is based upon the use of a roughness parameter estimated from the analysis of the two-dimensional particle images of ice precipitation. Although this method is tentative, it gives conclusive results for the two different flight levels studied here.     

陆地气旋爆发性发展的动力学分析和数值试验

利用中国地面加密自动站观测资料、北京地区雷达探测资料、NCEP（1°×1°）FNL资料、ECMWF ERA Interim（0.125°×0.125°）逐日再分析资料等,对造成2016年7月19-20日华北极端暴雨中的低涡系统发展演变的结构特征和加强机制进行了研究。华北地区这次特大暴雨过程出现了3个阶段降水,其中与低涡系统强烈发展对应的第2阶段降水是本次华北暴雨过程的主要降水阶段。针对该低涡的分析表明：（1）850 hPa以西南低涡为中心的低压带中,在河南西北部新生低涡系统,并且其在向华北地区移动过程中显著加强,该低涡系统在空间结构上,从倾斜涡柱逐渐发展成近乎直立的、贯穿整个对流层的深厚低涡系统;（2）中低层低涡系统快速发展过程与高低空系统构成耦合作用有关：低层低涡系统显著加强之前,对流层上层（300-200 hPa）首先出现高空槽异常加深并向南发展,该高空槽发展的开始阶段与其本身冷暖平流造成的斜压发展过程对应;而后,随着高纬度平流层高位涡沿等熵面向南运动,造成华北地区对流层上层涡度增强,形成正位涡异常区;当这一正位涡异常区叠加在对流层中低层锋区上空时,造成对流层中低层气旋快速发展并向下伸展,诱发河南西北部的新生气旋;低涡系统的发展进一步强化了低空暖平流,促使低空气旋向东北方向发展\     

陆地上爆发性温带气旋的暖锋后弯结构分析

温带气旋的暖锋后弯是20世纪80年代末从北大西洋爆发性气旋现场科学试验中发现的一个重要科学成果, 修正了 经典温带锋面气旋模式中锢囚锋的概念。在地球同步卫星云图上, 经常可以观测到欧亚大陆的气旋云系也存在与暖锋后弯 非常类似的演变过程。利用常规的高空、地面观测, NCEP的1°×1°再分析场和FY-2E 水汽图像等资料, 通过分析2012年5 月11-14日一次蒙古气旋强烈发展的结构特征, 揭示了在内陆地区温带气旋发展过程中若暖锋锋区增强而冷锋锋区减弱, 则 该类气旋也存在暖锋后弯和暖核被隔离的事实。分析结果还表明该类蒙古气旋中暖核强度虽不如海洋爆发性气旋, 但其厚 度可伸展到600hPa,在对流层低层850hPa低压中心附近不仅有暖核, 而且存在与暖核近乎重合的干中心, 但干暖中心与气 旋中心并不重合。另外, 单站探空及垂直剖面的分析显示, 在后弯的暖锋锋区附近存在强的湿上升运动, 干暖核内以干空气 和下沉运动为主, 在边界层内后弯暖锋锋区比冷锋锋区强;在冷锋后部的干输送带中对流层中高层锋区明显, 且总体上相对 湿度较小、上升运动微弱, 最显着的干区位于锋区上方下沉的暖空气中。     

Radar observations of snow formation in a warm pre‐frontal snowband

Abstract

Radar reflectivity measurements and sounding data were analyzed to investigate snowfall production in a long‐lasting snowband that formed in advance of a warm surface front moving across Alberta. The sounding data indicated that the band could have been forced by slantwise overturning during the release of moist symmetric instability combined with frontogenesis. The stability analysis presented here is novel in that it includes ice phase thermodynamics, neglected in previous studies of slantwise convection.

Radar reflectivity fields were analyzed to determine the total snow content and the mass outflow rate as factors of time. The peak value of total snow content was 17 kilotons per km of snowband, and the peak mass outflow rate was 10 tons s^‐1 km^‐1. The snowfall rate averaged across the cloud base was about 0.8 cm h^‐1, and the average snow content remained close to 0.2 g m^‐1. The characteristic time (defined as the ratio of total snow content over mass outflow rate) was about 30 minutes, which is approximately the time needed for the growth of snowflakes by aggregation in the observed temperature range. The precipitation efficiency of the snowband, defined as the ratio of snow mass outflow to water vapour inflow was estimated to be 14%. The precipitation production values observed in the Alberta snowband are compared with previous estimates reported for frontal rainbands and Alberta thunderstorms.     

A numerical study on the effect of an extratropical cyclone on the evolution of a midlatitude front

The extratropical transition (ET) of tropical cyclone (TC) Haima (2004) was simulated to understand the impact of TC on midlatitude frontal systems. Two experiments were conducted using the Advanced Research version of the Weather Research and Forecast (WRF) model. In the control run (CTL), a vortex was extracted from the 24-hour pre-run output and then inserted into the National Centers for Environmental Prediction (NCEP) global final (FNL) analysis as an initial condition, while TC circulation was removed from the initial conditions in the sensitivity run (NOTC). Comparisons of the experiments demonstrate that the midlatitude front has a wider meridional extent in the NOTC run than that in the CTL run. Furthermore, the CTL run produces convection suppression to the southern side of the front due to strong cold advection related to the TC circulation. The easterly flow north of the TC not only decelerates the eastward displacement of the front and contracts its zonal scale but also transports more moisture westward and lifts the air along equivalent potential temperature surfaces ahead of the front. As a result, the ascending motion and diabatic heating are enhanced in the northeastern edge of the front, and the anticyclonic outflow in the upper-level is intensified. The increased pressure gradient and divergent flow aloft strengthen the upper-level jet and distort the trough axis in a northwest-southeast orientation. The thermal contrast between the two systems and the dynamic contribution related to the TC circulation can facilitate scalar and rotational frontogenesis to modulate the frontal structure.     

Development of particle size and composition distributions with a novel aerosol dynamics model

An aerosol dynamics model, AEROFOR2, is developed in the context of the BIOFOR project focussing on boreal forest aerosol. It is the second version of a Lagrangian type box model AEROFOR for investigating the formation and growth of particles under clear sky atmospheric conditions. Particles can consist of soluble and insoluble material and the particle population can be externally or internally mixed. AEROFOR2 includes gas phase chemistry and aerosol dynamics, and calculates the number and composition distributions of particles as functions of time. Observed growth rates of the nucleation mode particles after a typical nucleation event are 2–3 nm/h. The model simulations predict that 3·10⁷ molecules cm⁻³ of insoluble organic vapour and less than 6·10⁶ molecules cm⁻³ of soluble vapour condensing onto particles are enough to make them grow in good agreement with the observed growth rates. Then the source rate of the organic vapour must be an order of 10⁵ molecules cm⁻³ s⁻¹, and its saturation vapour density should be below 10⁶ molecules cm⁻³. If the aerosol was initially an internal mixture of soluble (70%) and insoluble (30%) constituents it transformed to an externally mixed aerosol during the simulation. By applying the externally‐mixed aerosol based on measured soluble volume fractions, it was concluded that the modelled soluble fraction of the nucleation mode was too low in comparison with the measurements, and thus, a part of the condensable organic vapour must be water soluble.     

2018年1月北大西洋上一个具有“T”型锋面结构的超强爆发性气旋的分析

利用欧洲中期天气预报中心（ECMWF）提供的0.5°×0.5° ERA-Interim再分析资料，麦迪逊-威斯康星大学气象卫星研究所（CIMSS）提供的地球静止环境业务卫星（GOES-EAST）红外卫星云图和天气预报模式（WRF）的模拟结果，对2018年1月3—6日发生在北大西洋上的一个具有“T”型（T-bone）锋面结构的超强爆发性气旋进行分析。该爆发性气旋在较暖的湾流上空生成，沿海表面温度大值区向东北方向快速移动，生成后6 h内爆发性发展，24 h中心气压降低48.7 hPa。高空槽加深、涡度平流加强和低层较强的大气斜压性为气旋快速发展提供了有利的环流背景场。由于气旋发展迅速，低层相对涡度急剧增大，低压中心南部来自西北方向的干冷空气随气旋式环流快速向东推进，与东南暖湿气流汇合，锋生作用较强。较暖的洋面对西北冷空气的加热作用使得交汇的冷、暖空气温度梯度较小。减弱东移的冷锋与暖锋逐渐形成近似垂直的“T”型结构。用Zwack-Okossi方程诊断分析表明，非绝热加热、温度平流和正涡度平流是该爆发性气旋发展的主要影响因子。气旋初始爆发阶段，西北冷空气进入温暖的洋面，海洋对上层大气感热输送和潜热释放较强，非绝热加热对气旋快速发展有较大贡献。气旋进一步发展，“T”型锋面结构显著，温度平流净贡献较大，对气旋的发展和维持起重要作用。      

阿拉伯海夏季季风气旋气流结构和次天气尺度现象的一些特征

本文利用1979年夏季季风实验期间稠密的飞机下投探空仪资料,对一次阿拉伯海季风气旋的垂直气流分布、云层分布、水平螺旋辐合气流和强风环等结构以及它们之间的关系等进行了分析研究。并指出了高空次天气尺度和中尺度的一些现象。垂直气流速度是根据探空仪落速变化算出,把它用在天气系统分析是初次尝试。     

Lidar and Sun photometer observations of atmospheric boundary-layer characteristics over an urban area in a mountain valley

We present measurements of the vertical aerosol structure and the aerosol optical depth in the lower troposphere performed above the city of Sofia (an urban area situated in a mountain valley), western Bulgaria by means of a ground-based aerosol lidar operating continuously for a number of years. The lidar measurements were accompanied by measurements of the aerosol optical depth (AOD) in the visible and near infrared regions of the spectrum performed in October 2004 using Microtops II radiometers. The maximum values of the AOD were found to occur 1–2 h before the complete development of the atmospheric boundary layer, i.e. during the residual layer destruction, which confirms our hypothesis concerning the slope circulation effect on the processes taking place in the atmospheric boundary layer. The AOD values obtained by the lidar are lower than those taken by the sun photometer. Further, the AOD exhibits two different types of behaviour. In the case of a ‘clear atmosphere’ (i.e. in the absence of volcanic eruptions and/or dust transport from the Sahara) most of the aerosol accumulated within the atmospheric boundary layer over the urban area considered. The combined use of the two instruments allows the comparison between the optical characteristics of the atmospheric aerosol (e.g. aerosol extinction coefficient, etc.) obtained by the lidar and through an independent method (sun photometer).