BL Lacertae objects and the extragalactic γ-ray background

A tight correlation between γ-ray and radio emission is found for a sample of BL Lacertae(BL Lac) objects detected by the Fermi Gamma-ray Space Telescope(Fermi) and the Energetic Gamma-Ray Experiment Telescope(EGRET) . The γ-ray emission of BL Lac objects exhibits strong variability,and the detection rate of γ-ray BL Lac objects is low,which may be related to the γ-ray duty cycle of BL Lac objects. We estimate the γ-ray duty cycle,δγ 0.11,for BL Lac objects detected by EGRET and Fermi. Using the empirical r...     

一次冰雹云演变过程的卫星遥感监测与分析

针对2003年9月21日的一次区域性冰雹云强对流天气过程,结合天气实况、环流场特征和雷达资料,利用NOAA卫星数据,从天气分析、能量转换以及云在可见光、中红外波段的反射特性,分析了冰雹云的演变及特征。结果表明,伴随着大气能量的转换,当日青海湖以北的对流冷云单体在东移过程中,结合了西南部暖舌带来的暖湿气流,午后受地面增温出现了爆发性发展。(1)17：00云团发展最旺盛,云顶温度最低值达到-44℃,并确定冰雹发生的云顶温度临界值为-39．2℃,云顶亮温为221K;(2)冰雹云温度水平梯度介于20～35℃／50km之间,表现为有雹无灾的冰雹强度特征;(3)卫星数据表明,此时雹云与其它云团不同,具有粒子有效半径较大,气溶胶光学厚度也大的特点。     

我国西南地区一次暴雨过程特征及成因

利用卫星云图、多普勒天气雷达资料和高空风等各种天气学资料,对2009年6月8—9日广西、贵州、以及和湖南交界地带的一次暴雨过程进行了综合分析。结果表明,暴雨是由中尺度对流复合体东移、β中尺度强对流云团发展、以及二者合并造成的;地面α中尺度低压带配合α中尺度纬向切变线的生成,为中尺度对流复合体（mesoscale convective complex,MCC）的东移发展、β中尺度强对流云团的发展、以及二者的合并创造了有利条件;地面能量比低值舌的活动是MCC和β中尺度强对流云团生成和发展的触发机制之一;在多普勒雷达径向速度图上,MCC的生成和发展,伴随西南低空急流的建立和维持,大范围的逆风区的生成;MCC的消亡,伴随西南低空急流的减弱和消失,对应西北气流建立和东扩。MCC发展期和β中尺度强对流云团发展期、MCC消散期和β中尺度强对流云团消散期的涡度收支以及视热源和视水汽汇有很大的不同。     

EFFECT OF THE LINEAR BETA TERM ON MOVEMENT AND DEVELOPMENT OF TROPICAL CYCLONE

The dynamics of tropical cyclone is investigated in a nondivergent,barotropic model with nobasic flow.The effect of linear beta term on the movement and development of tropical cyclone isemphatically demonstrated.The streamfunction tendency due to the symmetric component of linearbeta term appears in a dipole-like pattern with an east-west symmetry,which maintains andintensifies the large-scale beta gyres and causes the tropical cyclone to have a westerly movingcomponent.The streamfunction tendency due to the asymmetric component of linear beta termarises in an ellipse pattern with a north-south major axis,which weakens the tropical cyclone.Thestreamfunction tendency due to the asymmetric component of linear beta term and the intensity oflarge-scale cyclonic beta gyre synchronously vary in a fluctuating manner with time.