利用卫星微波观测亮温与云辐射模拟亮温的台风定位分析北大核心

极轨气象卫星S-NPP、MetOp-A和FY-3B上搭载的微波湿度计观测资料可以反映出台风周围水汽和云雨结构。本文使用权重函数峰值在800 hPa附近的微波湿度计通道观测资料和ERA5再分析资料全天空模拟亮温,以飓风Sandy和Isaac为例,对用方位谱台风中心位置定位方法得到的观测和模拟中心位置进行了比较。利用下午星S-NPP搭载的先进技术微波探测仪(Advanced Technology Microwave Sounder,ATMS)和上午星MetOp-A搭载的微波湿度计(Microwave Humidity Sounder,MHS)观测亮温得到的飓风Sandy(Isaac)中心位置与最佳路径平均相差35.8 km(32.9 km),但用ERA5全天空模拟亮温得到的飓风Sandy(Isaac)中心位置与最佳路径平均相差73.3 km(82.1 km)。若按照热带风暴和台风等级来划分,ATMS和MHS观测和模拟亮温得到的台风中心位置与最佳路径的平均距离对热带风暴分别是36.5 km和105.9 km,对台风分别是25.8 km和56.4 km。若用FY-3B搭载的微波湿度计(以MWHS表示)替换ATMS,所得结果类似。ERA5作为全球大气再分析资料的典型代表,用方位谱台风中心位置定位方法得到的台风中心位置误差较大的原因是ERA5再分析资料全天空模拟亮温在台风中的分布结构与观测亮温相差较大,而模拟亮温与冰水路径分布结构极为相似。研究对台风数值预报中的全天空模拟亮温资料同化具有一定的参考意义。     

对流性降水云微波辐射特性

结合MM5模式和三维微波辐射传输模式, 对2003年7月9日宜昌地区一次典型的中尺度降水中心的对流性降水云微波辐射特性进行研究。结果表明:MM5模式模拟的降水量和落区与实况一致,模拟的水凝物廓线也与TMI反演值接近,85.5 GHz通道辐射亮温与TMI实测情况相近。85.5 GHz通道亮温与地面雨强相关性很弱, 受云中云冰和雪花的散射降温作用显著, 由于其他粒子的综合作用以及斜角观测造成的位置偏移, 霰粒子对该通道亮温散射作用不明显。19.35 GHz通道亮温随雨强增加先升温后降温; 与霰粒子含量表现出明显的负相关关系。37.0 GHz通道亮温随雨强的增加而降温, 雨强大于20 mm/h后达到饱和, 主要受雨水降温作用影响。倾斜观测比天顶垂直观测产生更低的亮温低值中心, 且频率越高, 低值中心的偏移越严重。     

自贡一次短时强降雨过程的多尺度诊断

利用常规天气观测资料、ERA5的0.5°×0.5°再分析资料、地面自动站加密资料和卫星云图产品,对2020年8月12日四川省自贡市短时强降雨天气过程产生的原因和多尺度特征进行研究。结果表明：高空低槽与中低层辐合系统结合低空急流是本次暴雨天气过程的大尺度环流系统。低空急流为强降雨区提供了充足的水汽和能量;辐合上升动力作用增强、水汽含量的迅速增长、不稳定能量的增加,为对流性强降雨提供了条件;强烈发展的两个对流云团合并加强,形成强的中β尺度MCC,强降雨区位于MCC云顶亮温最低值中心附近,云顶亮温最低值达到-82℃;地面的中小尺度辐合线在短时强降雨中起到了对对流系统触发和加强的作用。     

基于TRMM卫星资料对四川盆地降水的三维结构特征分析

利用TRMM卫星多种探测仪器得到的观测资料,分析研究2010年7月15~18日由西南低涡引发的四川盆地区域性暴雨天气过程,重点揭示了该次过程降水的三维结构特征。结果表明:红外和微波亮温数据均能从一定侧面反映低涡云系的降水特征;西南低涡引发的降水属于中尺度系统降水,层云降水对总降水的贡献率超过90%,存在明显的亮带结构;大范围降水区内包含一条主雨带和若干独立的对流性雨团,表现为大范围层云降水围绕对流降水的结构特征,对流性降水云顶最高能发展到17km,局部最大降水率出现在2~5km高度;降水凸起部分为独立的对流降水云团,呈塔状立体结构。      

基于CloudSat、FY-2E资料的中国海域及周边地区深对流和穿透性对流特征

深对流在地-气系统的物质和能量交换中起着至关重要的作用,伴随而来的暴雨、雷电、冰雹等天气会对人类社会产生影响。利用CloudSat/CALIPSO和FY-2E卫星观测数据,研究了中国海域及周边地区非穿透性对流（DCwo）及穿透性对流（CO）的海-陆分布、云顶红外亮温和云团特征（包括对流系统（CS）和对流单体（CC）的面积、活跃性对流比、偏心率、最低亮温、平均亮温梯度）。结果发现:穿透性对流比非穿透性对流的云顶红外亮温更低,垂直高度上的雷达反射率更高;从发生次数来看,非穿透性对流/穿透性对流在海洋比陆地多,低纬度比高纬度多,夏季比其他季节多,冬季海陆差异最大;从云顶亮温的分布来看,海洋比陆地、穿透性对流比非穿透性对流集中分布区间的亮温值更低,穿透性对流的分布区间比非穿透性对流集中;从云团特征来看,对流系统/对流单体的发生频率随面积的增大而降低,穿透性对流比非穿透性对流、海洋比陆地更容易出现较大面积的对流系统/对流单体,海洋穿透性对流的活跃性对流比相对较高;偏心率在0.5以上的发生频率较高,对流系统形状更偏向于圆形,在海洋上更加明显;穿透性对流在海陆上的最低亮温集中分布区间为190-195 K,比非穿透性对流的分布更集中,平均亮温梯度在0.1 K/km以下的发生频率较高。      

辽宁开原强龙卷的卫星云图和雷达回波演变特征分析

利用葵花8号(Himawari-8)卫星资料、沈阳SC天气雷达数据、ERA5再分析资料及常规天气观测资料, 分析了2019年7月3日辽宁开原强龙卷的卫星云图、雷达回波演变及大气环流特征。结果表明: 此次开原强龙卷发生在东北冷涡底部, 低层850 hPa有明显的暖湿气流, 形成了“下湿上干”的垂直结构。3日17:00龙卷初生地0—6 km有22.8 m·s^-1、0—1 km有7.6 m·s^-1强垂直风切变。龙卷生成之前, 初生地西侧比东侧气温偏高, 存在2—5 ℃地面温度差。生成后, 移动路径东侧形成明显冷池, 最低温度19 ℃, 与西侧温差最大达11 ℃。龙卷生成时可见光云图上对流风暴的云砧水平尺度明显增大, 云顶升高、亮温降低。雷达回波演变特征表明, 龙卷对流风暴的发展经历了由多单体非强风暴发展到多单体强风暴再发展到超级单体风暴三个阶段, 龙卷在最强等级时有对流单体的合并。开原龙卷风暴在三个阶段都有中气旋, 17:11中气旋向下伸展到低层, 反射率因子出现指状回波。     

南京对流降水和闪电的TRMM资料分析

用热带降雨测量卫星的测雨雷达2A25产品、微波成像仪1B11产品和闪电成像仪观测资料分析了2007—2008年南京及周边地区五个对流降水云系的降水结构、微波亮温和闪电特性。结果表明,当对流降水云系处于发展阶段时,对流降水是降水量的主要贡献者;而当对流降水云系处于成熟或者逐渐消亡阶段时,对流降水就逐渐向层云降水转变,此时降水量主要由层云降水产生。两种类型降水在降水强度、雨顶高度存在显著区别。微波亮温与近地面2 km处的降水强度呈负相关,基本能反映出近地面的降水强度,尤其是在对流系统的成熟和消散阶段。37.0 GHz亮温最能指示近地表的降水强度。近地面2 km处的降水强度均随闪电次数的增加而增强。     

2012年7月12日鄂东北准静止中尺度对流系统分析

使用高空天气图、NCEP（National Centers for Environmental Prediction）再分析场、新-代天气雷达及地面观测资料分析了导致2012 年7 月12 日鄂东北强降水的准静止中尺度对流系统MCS（Mesoscale Convective System）的演变特征,解释了MCS 维持准静止状态的成因.南支槽、东北冷涡、副热带高压是MCS 形成的主要大尺度天气系统,MCS 形成于副高外围西南气流和冷涡高空槽底部之间,副高位置稳定和冷涡高空槽缓慢南压有利于MCS 稳定少动,且随南支槽加深西南急流的建立有利于持续向MCS 输送水汽和不稳定能量.MCS 表现为单个中α尺度对流云团,成熟时外形呈椭圆型,边缘光滑,亮温低值中心位于MCS 西侧,且有指状突起,亮温低值区域对应中β尺度对流回波带,强降水组合反射率因子为45～55 dBz,回波顶高18 km,中心高度低于6 km,MCS 维持准静止状态.强降水与MCS 亮温低值中心、强回波带相对应,降水效率高,持续时间长.中尺度分析表明,辐合线的维持是MCS 呈准静止状态的主要原因.地形阻挡产生的地面辐合线触发了初始对流,强降水在地面产生冷池、雷暴高压及弧状出流边界,出流边界上风速辐合较强且温度梯度较大区域又触发出新的对流,并在气压梯度力推动下向东南方向传播,抵消了环境风平流运动.MCS 低层主要有西南气流和西北气流,西北气流逐渐从MCS 后部进入,与西南气流形成辐合线,西南气流沿西北气流爬升产生对流,形成自东南向西北倾斜的中尺度锋面,地面出流边界和高空辐合线是中尺度锋面在风场的表现形式,对回波加强、维持有重要作用,且高空辐合线引起的后向传播也抵消了环境风平流运动.     

提高模式分辨率后热带气旋生成迟缓的原因分析

利用目前国际上最先进的中尺度WRF模式模拟热带气旋生成,网格分辨率从9 km增加到3 km,3 km网格中积云参数化方案不起作用,依靠微物理方案来模拟对流尺度系统特征,模式中热带气旋的生成过程变得迟缓。当低压扰动发展到一定程度后再加入3 km网格,生成过程有加快趋势。本研究针对该现象进行分析。结果表明:只用微物理方案使低层(950～700 hPa)风速的垂直切变减小,不利于对流发展;切变减小主要是由于动量垂直输送项的差异所致。在加入细网格的6 h内,低层对流尺度(减去区域平均)的动量垂直输送量平均增加了一倍,某些时刻达到了5倍以上;动量混合增加是由于微物理方案模拟的垂直速度增加所致。此外,只用微物理方案导致对流有效位能迅速被消耗。低层垂直切变和对流有效位能的减小都不利于对流发展,从而导致热带气旋生成发展过程迟缓。本研究表明,目前WRF中的微物理方案在模拟热带气旋生成过程中的对流发展时仍然存在问题。     

襄樊特大暴雨FY-3A微波资料分析

搭载在中国新一代极轨卫星FY-3A上的微波湿度计对于大气湿度以及云雨分布特征具有较好的探测能力。利用其150 GHz极化波段亮温和183.31 GHz附近的水汽吸收波段亮温及其反演产品,对2008年7月22日襄樊特大暴雨中10：00—11：00强降水成因进行了分析。结果表明：襄樊10：00—11：00强降水产生时,微波湿度计5个波段亮温均处于低值区的大梯度带;极化比、极化差出现明显的大、小值分裂;微波湿度计的高频通道资料表明,襄樊处于深对流中心地带,正是这种深厚的对流系统导致了襄樊的强降水。     

Frontal modification and lee cyclogenesis in the Alps: A case study using the ALPEX reanalysis data set

Summary The interaction of a cold front with the Alps is studied by means of real-case numerical simulations for a case occurring at the end of the Alpine Experiment (ALPEX) on 28 April–2 May, 1982. Simulations are performed with the numerical weather prediction model chain Europa-Modell (EM) and its one-way nested high-resolution model (HM) of the German and Swiss Weather Services. The outer EM simulation (56 km horizontal resolution) uses initial and lateral boundary fields taken from the ALPEX-IIIb reanalysis data set. This reanalysis data set is based on the operational EM analysis scheme, but takes into consideration a wide range of field phase data taken during the ALPEX field campaign. A comparison of simulations driven by the ALPEX and ECMWF reanalysis (ERA) data is performed. The simulation driven by the former captures the intensity and vertical depth of the developing lee cyclone substantially better than the corresponding run driven by the ERA. The transient development of the impingement of a cold front on the Alps induces a wide range of mesoscale phenomenon such as flow splitting, mistral, north foehn, cyclogenesis, anticyclonic vortex and bora. These flow evolutions are analyzed using HM simulations with a horizontal resolution of 14 km and visualized performing comprehensive trajectory calculations. Received September 9, 2000/Revised November 28, 2000     

Simulation of the atmospheric circulation in the Weddell Sea region using the limited-area model REMO

Summary ?To analyse the applicability of a limited-area atmosphere model to the Southern Ocean, a one-year simulation for 1985 is performed using the REgional MOdel REMO at 55-km horizontal grid-spacing implemented for the Antarctic regions of the Weddell, Bellingshausen and Amundsen Seas. To evaluate the performance of REMO, a comparison of model results to observations and to reanalysis/analysis data sets is carried out. REMO is initialized and driven at the lateral and lower boundaries by data of the European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis (ERA15). Overall, REMO is an appropriate tool for further climate studies in Antarctic regions. It reproduces reasonably well basic spatial patterns and the seasonal cycle of the atmospheric circulation. However, the simulated mean sea level pressure (MSLP) is predominantly lower than the MSLP provided by observations and by ERA. Considerable temperature differences in the lower troposphere over sea ice in winter cause discrepancies between the REMO and ERA pressure fields in the mid-troposphere too. The precipitation rate P of the REMO simulation agrees qualitatively well with main features of the observed climatological spatial distribution described in literature. The seasonal cycle of P in the inner Weddell Sea reflects the Antarctic semi-annual oscillation. Concerning the forcing fields, the ERA sea ice surface temperatures in winter are generally higher than satellite derived surface temperatures. Although the differences are 10 to 15 K in the southern Weddell Sea, this deficiency of the ERA data hardly influences the mean large-scale circulation. Received October 10, 2001; revised April 22, 2002; accepted May 12, 2002     

Precipitation,temperature and wind in Norway: dynamical downscaling of ERA40

A novel downscaling approach of the ERA40 (ECMWF 40-years reanalysis) data set has been taken and results for comparison with observations in Norway are shown. The method applies a nudging technique in a stretched global model, focused in the Norwegian Sea (67°N, 5°W). The effective resolution is three times the one of the ERA40, equivalent to about 30 km grid spacing in the area of focus. Longer waves (<T42) in the downscaled solution are nudged towards the ERA40 solution, and thus the large-scale circulation is similar in the two data sets. The shorter waves are free to evolve, and produce high intensities of winds and precipitation. The comparison to observations incorporate numerous station data points of (1) precipitation (#357), (2) temperature (#98) and (3) wind (#10), and for the period 1961–1990, the downscaled data set shows large improvements over ERA40. The daily precipitation shows considerable reduction in bias (from 50 to 11%), and twofold reduction at the 99.9 percentile (from −59 to −29%). The daily temperature showed a bias reduction of about a degree in most areas, and relative large RMSE reduction (from 7.5 to 5.0°C except winter). The wind comparison showed a slight improvement in bias, and significant improvements in RMSE.     

Validation of climate-mode MM5-simulations for the European Alpine Region

Climate mode simulations using MM5 have been conducted over Western Europe on a 45-km grid driven by ECMWF’s ERA40 reanalysis data. We focus our validation on the Alpine region and the Alpine foreland. A reference experiment comprising the years of 1991 to 2000 shows reasonable correspondence to station measurements and a gridded precipitation climatology of the Alps. Also, the mean monthly diurnal cycle of near-surface temperature and dew point temperature verified in the Alpine foreland compares quite well to station data showing some minor discrepancies mainly in the afternoon that seem to be common to regional models. Furthermore, a set of sensitivity experiments was conducted for the years of 1996 to 1999. This set was spanned on the one hand by three convection schemes to get an estimate of the possible range of simulated precipitation amounts inherent to the MM5-system. On the other hand, two different formulations of the horizontal numerical diffusion were investigated with respect to their influence on simulated precipitation in mountainous terrain. It was found that the impact of the formulation of numerical diffusion is similarly large as the sensitivity to the convection scheme, with computing diffusion along the terrain-following coordinate surfaces being clearly worse than computing it in a truly horizontal manner.     

Simulation of Indian summer monsoon circulation and rainfall using RegCM3

Summary The Regional Climate Model RegCM3 has been used to examine its suitability in simulating the Indian summer monsoon circulation features and associated rainfall. The model is integrated at 55 km horizontal resolution over a South Asia domain for the period April–September of the years 1993 to 1996. The characteristics of wind at 850 hPa and 200 hPa, temperature at 500 hPa, surface pressure and rainfall simulated by the model over the Indian region are examined for two convective schemes (a Kuo-type and a mass flux scheme). The monsoon circulation features simulated by RegCM3 are compared with those of the NCEP/NCAR reanalysis and the simulated rainfall is validated against observations from the Global Precipitation Climatology Centre (GPCC) and the India Meteorological Department (IMD). Validation of the wind and temperature fields shows that the use of the Grell convection scheme yields results close to the NCEP/NCAR reanalysis. Similarly, the Indian Summer Monsoon Rainfall (ISMR) simulated by the model with the Grell convection scheme is close to the corresponding observed values. In order to test the model response to land surface changes such as the Tibetan snow depth, a sensitivity study has also been conducted. For such sensitivity experiment, NIMBUS-7 SMMR snow depth data in spring are used as initial conditions in the RegCM3. Preliminary results indicate that RegCM3 is very much sensitive to Tibetan snow. The model simulated Indian summer monsoon circulation becomes weaker and the associated rainfall is reduced by about 30% with the introduction of 10 cm of snow over the Tibetan region in the month of April.     

浙江夏季弱天气尺度强迫背景下午后局地热对流特征分析

利用常规高空观测资料、浙江多普勒雷达产品、ERA5逐小时再分析资料(0.25°×0.25°),统计分析了2015—2021年浙江夏季(6—8月)弱天气尺度强迫背景下午后局地热对流(根据移动速度分为移动型和少动型两类)的时空分布特征、生命周期特征、环境参数特征等。(1)两类热对流触发频次相当,年际变化均较大,但年、月、日变化趋势基本一致,午后出现少动型局地热对流的可能性更大,而傍晚出现移动型局地热对流的可能性更大。(2)局地热对流的触发与地形密切相关,对流高频区位于浙东沿海、浙西北、浙西南山区,浙中北平原和盆地均为低频区,热对流触发频次均随高度增加而减少,多触发于山脉及其附近的中低海拔处,中高海拔地区更容易触发少动型局地热对流。(3)两类对流均在弱垂直风切变环境下即可触发,移动型单体更容易得到发展。     

采用不同背景场的龙卷模拟结果对比分析

采用常规气象观测资料,对2014年5月21日17:40广州市黄埔区九龙镇一次西风带龙卷的环流背景场进行分析,并利用FNL和ERA5再分析资料作为背景场模拟该龙卷过程,研究了不同再分析场对模拟效果的影响,结果表明:对于雷达回波、垂直速度、水平风场及水成物,FNL资料作为初始场对西风带龙卷的模拟优于ERA5资料。FNL再分析资料作为初始场、边界场能有效地模拟出西风带龙卷的超级单体结构。5月21日17:45对流风暴与经典超级单体结构相似,出现了钩状回波特征与入流缺口,其垂直结构上强回波向东倾斜,形成穹窿结构。17:50超级单体钩状回波处有明显的强上升气流与后侧下沉气流,具备生成龙卷的必要条件;从垂直涡度的低层平面发现涡旋特征十分明显,其高度从250m伸展至2km,龙卷有可能发生在从地面一直延伸至2km向西北倾斜的小尺度气旋内。      

Decadal variation of surface solar radiation in the Tibetan Plateau from observations, reanalysis and model simulations

In this study, the annual and seasonal variations of all-sky and clear-sky surface solar radiation (SSR) in the eastern and central Tibetan Plateau (TP) during the period 1960–2009 are investigated, based on surface observational data, reanalyses and ensemble simulations with the global climate model ECHAM5-HAM. The mean annual all-sky SSR series shows a decreasing trend with a rate of ?1.00 Wm^?2 decade^?1, which is mainly seen in autumn and secondly in summer and winter. A stronger decrease of ?2.80 Wm^?2 decade^?1 is found in the mean annual clear-sky SSR series, especially during winter and autumn. Overall, these results confirm a tendency towards a decrease of SSR in the TP during the last five decades. The comparisons with reanalysis show that both NCEP/NCAR and ERA-40 reanalyses do not capture the decadal variations of the all-sky and clear-sky SSR. This is probably due to a missing consideration of aerosols in the reanalysis assimilation model. The SSR simulated with the ECHAM5-HAM global climate model under both all-sky and clear-sky conditions reproduce the decrease seen in the surface observations, especially after 1980. The steadily increasing aerosol optical depth (AOD) at 550 nm over the TP in the ECHAM5-HAM results suggests transient aerosol emissions as a plausible cause.     

Sensitivity studies of high-resolution RegCM3 simulations of precipitation over the European Alps: the effect of lateral boundary conditions and domain size

A suite of high-resolution (10 km) simulations were performed with the International Centre for Theoretical Physics (ICTP) Regional Climate Model (RegCM3) to study the effect of various lateral boundary conditions (LBCs), domain size, and intermediate domains on simulated precipitation over the Great Alpine Region. The boundary conditions used were ECMWF ERA-Interim Reanalysis with grid spacing 0.75^°, the ECMWF ERA-40 Reanalysis with grid spacing 1.125 and 2.5^°, and finally the 2.5^° NCEP/DOE AMIP-II Reanalysis. The model was run in one-way nesting mode with direct nesting of the high-resolution RCM (horizontal grid spacing Δx = 10 km) with driving reanalysis, with one intermediate resolution nest (Δx = 30 km) between high-resolution RCM and reanalysis forcings, and also with two intermediate resolution nests (Δx = 90 km and Δx = 30 km) for simulations forced with LBC of resolution 2.5^°. Additionally, the impact of domain size was investigated. The results of multiple simulations were evaluated using different analysis techniques, e.g., Taylor diagram and a newly defined useful statistical parameter, called Skill-Score, for evaluation of daily precipitation simulated by the model. It has been found that domain size has the major impact on the results, while different resolution and versions of LBCs, e.g., 1.125^° ERA40 and 0.7^° ERA-Interim, do not produce significantly different results. It is also noticed that direct nesting with reasonable domain size, seems to be the most adequate method for reproducing precipitation over complex terrain, while introducing intermediate resolution nests seems to deteriorate the results.