Statistics on the vertical structure of rainshowers

A large sample of summertime data from the McGill Radar Weather Observatory was analyzed to determine the variation with altitude of the horizontal extent of individual rainshowers. For echoes defined by a reflectivity factor of 39 dBz (equivalent to a rainfall rate of about 10 mm/h) it was found that the mean area of the total population of echoes decreases linearly with altitude from approximately 20 km² at 2 km to 11 km² at 8 km. Subsets of the total population were investigated, consisting of only the echoes penetrating the altitudes of 6, 7, and 8 km. On the average these relatively tall echoes are much greater in horizontal extent than the total population. Whereas the sizes of the total population of echoes at any altitude are distributed approximately exponentially in terms of the square root of area, the sizes of the “survivors” that extend to high altitudes may be described by the gamma distribution with a mean value decreasing approximately linearly with height above 3 km and a dispersion of 0.55. Some characteristics are also reported for echoes defined by reflectivities of 31 dBz and 47 dBz. Estimates are given of the fraction of the total area in a horizontal plane that contains echoes in each of these categories.     

Statistics on the sizes and internal structures of rainshowers

A large sample of radar reflectivity data from essentially a full summer of operation was analyzed to determine the horizontal extents and internal structure of rain areas observed at altitude levels from 2 to 10 km. Results are given on the size distributions of individual cells or patches defined by reflectivity thresholds approximately 4 dBz apart, on the dependence of mean size on altitude and reflectivity threshold, and on the interior structure of the patches as characterized by the number and sizes of higher-threshold patches contained in each echo. In a more detailed analysis of internal structure, the data were restricted to convective echoes in which certain prescribed reflectivities, ranging from about 30 to 50 dBz, were exceeded. It was found that the dependence of mean quantities, such as patch area, on reflectivity and altitude could be approximately described by simple functions, but that the scatter of observations about the mean was usually large.     

Spatial distribution of upstream magnetospheric ≥50 keV ions

We present for the first time a statistical study of 50 keV ion events of a magnetospheric origin upstream from Earths bow shock. The statistical analysis of the 50–220 keV ion events observed by the IMP-8 spacecraft shows: (1) a dawn-dusk asymmetry in ion distributions, with most events and lower intensities upstream from the quasi-parallel pre-dawn side (4 LT-6 LT) of the bow shock, (2) highest ion fluxes upstream from the nose/dusk side of the bow shock under an almost radial interplanetary magnetic field (IMF) configuration, and (3) a positive correlation of the ion intensities with the solar wind speed and the index of geomagnetic index Kp, with an average solar wind speed as high as 620 km s^–1 and values of the index Kp 2. The statistical results are consistent with (1) preferential leakage of 50 keV magnetospheric ions from the dusk magnetopause, (2) nearly scatter free motion of 50 keV ions within the magnetosheath, and (3) final escape of magnetospheric ions from the quasi-parallel dawn side of the bow shock. An additional statistical analysis of higher energy (290–500 keV) upstream ion events also shows a dawn-dusk asymmetry in the occurrence frequency of these events, with the occurrence frequency ranging between 16%-34% in the upstream region.     

一次局地冰雹的新一代天气雷达回波演变特征分析

2004年5月31日20时40分至21时15分,兵团农六师芳新垦区出现了一次局地冰雹天气,冰雹直径为0.5～0.8cm,但冰雹密度大,致使芳草湖农场353hm2棉花受灾。本文应用雷达的二次产品及回波特征对局地冰雹的发展、演变进行分析,对发展迅速的局地冰雹云的雷达识别和准确预报冰雹落区是有意义的。     

九江地区一次强对流暴雨天气过程分析

从天气形势出发,结合物理量场、卫星云图和雷达回波等演变特征,分析洞庭湖附近生成的两个气旋波东移过程中在九江地区引起的强对流、暴雨天气。     

On turbulence in the quasi-perpendicular bow shock

In this paper we propose to review the fundamental aspects of turbulence theories and their relevance to particle distribution functions observed by the cluster satellites in the quasi-perpendicular shock. The paper focusses on the hierarchical model describing the different levels of plasma turbulence; from the linear theory, through the quasi-linear remedy, to strong turbulence theories in the context of the earth's bow shock. We will discuss very briefly the validity of these approximations, and their relevance as far as satellite observations are concerned. In particular, we will discuss the development of non-Gaussian features in the ion distribution functions through the evaluation of higher order moments such as the kurtosis or flatness and the skewness. We have found that the profile of the kurtosis versus skewness tends to collapse to a parabolic line. This in turn allows us to draw analogies with neutral fluid turbulence where such a collapse of the kurtosis-skewness profile has been observed.     

Ion acceleration and Alfvén wave generation at the Earth’s bow shock

The processes of ion acceleration and Alfvén wave generation by accelerated particles at the Earth’s bow shock are studied within a quasi-linear approach. Steady-state ion and wave spectra are shown to be established in a time of 0.3–4 h, depending on the background level of Alfvénic turbulence in the solar wind. The Alfvén waves produced by accelerated ions are confined within the frequency range 10^?2–1 Hz and their spectral peak with a wave amplitude βB ～ B comparable to the interplanetary magnetic field strength B corresponds to the frequency v = (2–3) × 10^?2 Hz. The high-frequency part of the wave spectrum (v > 0.2 Hz) undergoes damping by thermal ions. The calculated spectra of the accelerated ions and the Alfvén waves generated by them reproduce the main features observed in experiments.     

一次飑线风暴的环境条件和雷达回波特征分析

利用常规气象观测和新一代天气雷达监测资料,对大兴安岭地区加格达奇市2006年8月一次飑线天气的环境条件和雷达回波特征等分析发现：此次过程的影响系统高空槽为前倾结构,飑线发生在地面冷锋前,大气具有较强的不稳定层结,有利于上升运动,水汽充足,低层较强的垂直风切变使对流云团和弓形等线状回波生成并较长时间地维持和发展。对流云团位置与上升运动区域较为一致,产生飑线的强对流云团出现在中低层垂直速度梯度较大并靠近上升运动中心一侧,并位于低层比湿和水汽通量散度梯度较大区域即干锋附近以及K指数能量锋上。飑线弓形回波出现在一个中尺度椭圆状对流云团中心内部发展最强处。当云团向东北偏东方向移过加格达奇雷达站时,雷达反射率因子先后呈现出弓形、＂人＂字形等线状回波特征,当线状回波南端＂人＂字形顶部较强回波和后侧入流急流前方＂V＂型槽口的尖峰移经加格达奇时,带来了雷雨大风天气。     

Kinematic features of a bow echo in southern China observed with doppler radar

A bow echo is a type of mesoscale convective phenomenon that often induces extreme weather and appears with strong reflectivity on radar images. A strong bow echo that developed from a supercell was observed over Foshan City in southern China on 17 April 2011. The intense gusty winds and showers caused huge losses of property and severely affected human lives. This paper presents an analysis of this strong meso- n-scale convective system based on Doppler radar observations. The isolated bow echo exhibited a horizontal scale of about 80 km in terms of reflectivity above 40 dBZ, and a life span of 8 hours. The system originated from the merging of a couple of weakly organized cells in a shear line, and developed into an arch shape as it moved through the shear zone. Sufficient surface moisture supply ensured the convective instability and development of the bow echo. The low-altitude winds retrieved from single Doppler radar observations showed an obvious rear-inflow jet along the notch area. Different from the conventional definition, no book- end anticyclone was observed throughout the life cycle. Very strong slantwise updrafts and downdrafts were recognizable from the retrieved winds, even though the spatial scale of the bow echo was small. Strong winds and induced damage on the surface are considered to have been caused by the mid-level rear-inflow jet and intense convective downdrafts.