A Numerical Investigation on Microphysical Properties of Clouds and Precipitation over the Tibetan Plateau in Summer 2014

In order to improve our understanding of microphysical properties of clouds and precipitation over the Tibetan Plateau (TP), six cloud and precipitation processes with different intensities during the Third Tibetan Plateau Atmospheric Science Experiment (TIPEX-Ⅲ) from 3 July to 25 July 2014 in Naqu region of the TP are investigated by using the high-resolution mesoscale Weather Research and Forecasting (WRF) model. The results show unique properties of summertime clouds and precipitation processes over the TP. The initiation process of clouds is closely associated with strong solar radiative heating in the daytime, and summertime clouds and precipitation show an obvious diurnal variation. Generally, convective clouds would transform into stratiform clouds with an obvious bright band and often produce strong rainfall in midnight. The maximum cloud top can reach more than 15 km above sea level and the velocity of updraft ranges from 10 to 40 m s-1. The simulations show high amount of supercooled water content primarily located between 0 and -20℃ layer in all the six cases. Ice crystals mainly form above the level of -20℃ and even appear above the level of -40℃ within strong convective clouds. Rainwater mostly appears below the melting layer, indicating that its formation mainly depends on the melting process of precipitable ice particles. Snow and graupel particles have the characteristics of high content and deep vertical distribution, showing that the ice phase process is very active in the development of clouds and precipitation. The conversion and formation of hydrometeors and precipitation over the plateau exhibit obvious characteristics. Surface precipitation is mainly formed by the melting of graupel particles. Although the warm cloud microphysical process has less direct contribution to the formation of surface precipitation, it is important for the formation of supercooled raindrops, which are essential for the formation of graupel embryos through heterogeneous freezing process. The growth of graupel particles mainly relies on the riming process with supercooled cloud water and aggregation of snow particles.     

An Operational Precipitable Water Vapor Retrieval Algorithm for Fengyun-2F/VISSR Using a Modified Three-Band Physical Split-Window Method

The Visible and Infrared Spin-Scan Radiometer(VISSR) onboard the Fengyun-2(FY-2) satellite can provide valuable thermal infrared observations to help create a precipitable water vapor(PWV) product with high spatial and temporal resolutions. The current FY-2/VISSR PWV product in operation is produced by using a traditional two-band physical split-window(PSW) method, which produces low quality results under dry atmospheric conditions. Based on the sensitivity characteristics of FY-2 F/VISSR water vapor channel and two split-window channels to atmospheric water vapor, this study developed a new, robust operational PWV retrieval algorithm for FY-2 F to improve the operational precision of the current PWV product. The algorithm uses a modified three-band PSW method, which adds a scale for the water vapor channel in the improved three-band PSW method. Integrated PWV products from the radiosonde data in 2016 are used here to validate the precision of the PWV retrieved by the modified three-band and traditional two-band PSW methods. The mean bias, root mean square error(RMSE), and correlation coefficient of the PWV retrieved by the modified three-band PSW method are 0.28 mm, 4.53 mm, and 0.969, respectively. The accuracy is much better than the PWV retrieved by the two-band method, whose mean bias, RMSE, and correlation coefficient are 12.67 mm, 29.35 mm, and 0.23. Especially, in mid-or high-latitude regions, the RMSE of the PWV is improved from 10 to 2 mm by changing the inversion in the two-band method to the modified three-band PSW method. Furthermore, the modified three-band PSW results show a better consistency with the radiosonde PWV at any zonal belt and season than the two-band PSW results. This new algorithm could significantly improve the quality of the current FY-2 F/VISSR PWV product, especially at sites where the actual PWV are lower than 15 mm.     

感热加热对华北及其周边对流活动的影响

利用2006～2017年风云气象卫星资料和气象再分析资料,对华北及周边5～8月对流活动和地面感热加热进行统计分析。分析表明,华北及周边白天平均感热加热和地形关系密切,内蒙古中部和东南部、华北北部和华北西部山区感热加热较强,最强感热加热出现在5月和6月,7月和8月明显减弱。和感热加热强度相对应,对流活动频率较高的月份同样出现在5月和6月,其中5月以弱对流为主,6月华北中北部强对流最活跃,另外,环渤海区域6～7月强对流相对频繁。5～8月日平均感热加热和对流频率趋势呈现一致的减弱对应关系。上午,感热加热引起河北西部和北部对流层低层出现辐合气流,700 hPa以下出现不同程度的增温,上升气流可达对流层中层,东侧的平原地区出现补偿下沉运动,升温和上升运动触发对流,在有利条件下发展东移。不同月份和区域对流频率日变化呈现明显差异,6月对流频率日变化显著,8月最弱,山区对流频率日变化显著,东部渤海及周边对流频率日变化较小。对流频率的月平均分布和日变化均表现出和地形相关的感热加热差异的特征。     

华南地区春季平流层入侵对对流层低层臭氧影响的模拟研究

利用中尺度大气化学模式WRF/Chem对2013年3月6日华南地区一次平流层入侵事件及其对对流层低层臭氧的影响进行模拟研究。通过加入UBC（Upper Boundary Condition）上边界处理方案,弥补WRF/Chem模式未考虑平流层臭氧化学反应的不足。结合臭氧探空廓线资料、地面O₃、CO、NO_x、相对湿度、温度和风速等观测资料以及再分析资料对模拟结果进行定量评估,结果表明模式能较为真实地模拟本次平流层入侵过程。模拟分析进一步揭示:（1）副热带高空急流是本次平流层入侵的主要原因。当华南地区处在副热带急流入口区左侧下沉区域时,平流层入侵将富含臭氧的干燥空气输送到对流层中低层。（2）本次平流层入侵对对流层低层臭氧收支有重要影响,导致香港地区近地层臭氧体积混合比浓度明显上升,如塔门站夜间臭氧浓度升高21.3 ppb（1 ppb=1×10-9）。地面气象场和化学物种的分析进一步确认了平流层入侵的贡献。（3）采用动力学对流层顶高度时零维箱式模型和Wei公式计算得到的平流层入侵通量相当,分别为-1.42×10-3 kg m-2 s-1和-1.59×10-3 kg m-2 s-1,这一结果与前人研究相吻合,且与采用热力学对流层顶高度计算所得到的结果具有可比性。     

两个相似路径台风残余造成局地特大暴雨的成因机制和能量收支对比分析

本文利用ERA-Interim 0.5°×0.5°再分析资料、自动站小时和分钟加密资料、风云2G（FY-2G）卫星红外云图及多普勒雷达和风廓线雷达资料对2015年路径高度相似的“苏迪罗”和“杜鹃”台风在浙江沿海引发的局地特大暴雨进行对比分析。这两次降水过程都是在台风减弱为热带低压甚至残压并深入内陆远离浙江沿海后发生的。结果表明,“苏迪罗”降水过程是由低层强东南和偏南急流长时间辐合加上有利地形共同作用导致的;经向环流背景下来自季风持续的水汽输送有利于“苏迪罗”维持较长的生命史和稳定的降水。“杜鹃”残压特大暴雨的触发系统则是高纬地面冷高压底部的东东北出流南下与“杜鹃”北象限的东东南风交汇形成的中尺度倒槽;纬向环流和强盛副热带高压造成的弱引导气流及夏季风南撤和低涡卷挟造成的水汽通道断裂是“杜鹃”登陆后快速减弱为残压和降水维持时间较短的原因。两次台风降水过程中均无外部动能输送和来自有效位能的动能转换。动能收支的主要影响因子为中低层局地次网格运动间的能量转换、旋转风和散度风效应及下垫面的摩擦耗散。所以,虽然“杜鹃”的对流有效位能很小,但仍可造成强对流和特大暴雨。此外,降水过程中释放的凝结潜热造成的局地非绝热加热使气柱中显热能大量累积,促使地面中小尺度涡旋和倒槽不断加深,造成降水的增幅。     

基于多源卫星数据的云底高度反演算法在中国近海的应用研究

本文利用直减率反演云底高度的计算方法,联合星载主、被动探测资料开展了中国东海、南海上空暖云云底高度反演研究,同时对暖云的分布特征进行了统计。结果表明受大气层结稳定程度影响,夜间的反演效果优于白天。两种资料的云顶高度较一致时,反演效果好,该方法具有可行性。     

94/220 GHz星载雷达双波长比对非球形冰晶云参数敏感性分析

利用不同形状冰晶的散射特性,获得了非球形冰晶云的94/220 GHz测云雷达双波长比,探讨了非球形冰晶云的双波长比与云内微物理参数的关系,分析了衰减前后的星载雷达反射率因子及双波长比的垂直廓线。结果表明:（1）双波长比可以反映小到0.1 mm中值尺度的冰粒子,对粒子总数、谱的形状参数不敏感,对粒子大小、形状、云衰减较敏感。（2）雷达灵敏度一定时,星载雷达可测云厚与雷达波长、冰含水量（IWC）的垂直分布、云厚及衰减有关;没有进行衰减订正时,双波长比和衰减有关,冰含水量越大,波长越短,衰减越大,双波长比最大值与可探测云厚有关。两部雷达可探测冰含水量为0.001—0.1 g/m3、厚2 km的冰云;当云厚5 km、冰含水量垂直分布在0.001—0.2 g/m3时,云厚的94%基本可以被220 GHz云雷达探测到。（3）如果两部雷达气象方程中用水的介电因子,测量回波强度应进行介电因子的订正后再计算双波长比。      

WRF中土壤图及参数表的更新对华北夏季预报的影响研究

土壤质地及其物理性质的参数化对陆面过程模拟具有明显的影响。研究了土壤质地和土壤水文参数表的更新对WRF（Weather Research and Forecasting）模拟性能的影响。使用北京师范大学土壤属性数据集和修正后的土壤水文参数表替换WRF默认数据,对2017年6—8月华北地区开展数值模拟试验和评估验证。结果表明,模拟结果对土壤类型数据集和水文参数表的更新较为敏感,对地面要素预报有正效果。WRF默认土壤数据集中,中国东部以粘壤土为主,而在北京师范大学土壤数据集里则以壤土为主;修正后的土壤水文参数在Noah陆面过程中增强了裸土潜热蒸发能力。数值模拟试验表明,土壤输入数据和土壤水文参数的更新能够增强陆面向大气的潜热同时减弱感热输送,致使大气底层温度降低而湿度增大。利用华北区域748个地面气象观测站的2 m温度和2 m湿度对2017年夏季的模拟结果进行验证,结果显示更新试验对地面要素的预报偏差有较好的修正作用,能够将2 m温、湿度的预报技巧分别提高3.4%和2.9%。      

基于SRTM数据的天气雷达探测环境分析研究

利用SRTM高程数据作为选址基础数据,结合天气雷达工作方式和探测方法,计算得到天气雷达在0.5°、1.0°、2.4°仰角上地物遮挡情况;利用高程格点数据获得3个仰角的地物剖面数据,提高了SRTM数据利用精度和运算速度;分析中结合地球曲率和电磁波折射影响,改进算法获得站点遮蔽角图,站点上空1 km、海拔3和6 km等射束高度图及数据,该分析结果充分体现了SRTM数据的高分辨率特点。最后将结果数据与GIS地图结合,完成了四川省天气雷达网探测环境分析,并给出了各个台站评估结果。     

江苏省夏季浓雾的时空分布特征及气象影响因子分析

利用江苏省70个国家基本站逐10 min连续观测资料,对江苏省夏季浓雾的时空分布特征及影响因子进行分析研究。结果表明:(1)夏季浓雾易在气温小于29℃、风速低于3 m·s~(-1),且盛行偏东风的条件下形成;低温高湿的梅雨期是夏季浓雾在6月高发(42.4%)的可能原因。(2)夏季浓雾生消时间与秋、冬季显著不同,主要发生于00—06时,消散集中于05—08时,持续时间主要在6 h以内。(3)夏季浓雾以辐射雾为主,辐射雾、平流雾和锋面雾分别占58. 1%、35. 5%和6.4%。(4)夏季浓雾发生频次呈现从东北部沿海地区向西南部内陆地区递减的趋势,淮北地区夜间降温幅度高于苏南地区是出现这一现象的主要原因。(5)成雾前6～24 h出现的弱降水为近地层提供水汽,此后天气转晴,静稳的大气层结下有利于夏季浓雾的出现。