GRAPES全球集合预报系统不同随机物理扰动方案影响分析

为了更好地理解不同随机物理扰动方案对全球中期集合预报的影响差异,本研究基于GRAPES全球集合预报系统（GRAPES-GEPS）对比分析了随机物理倾向扰动（Stochastically Perturbed Parameterization Tendencies,SPPT）、随机动能补偿（Stochastic Kinetic Energy Backscatter,SKEB）及联合使用SPPT与SKEB三种模式扰动方案所产生的扰动特征及其对集合预报的影响。为避免初值扰动影响,考察随机物理方案所产生的扰动特征时,不使用初值扰动。通过扰动与误差相关性分析（PECA）发现,不同随机物理扰动方案所产生的扰动对预报误差均具有一定的描述能力,而且联合使用SPPT与SKEB方案时,扰动对误差的描述能力最好。对所有扰动方案来说,扰动总能量最初主要集中在热带地区对流层中高层以及平流层低层。随着预报时效的延长,扰动总能量不断增大,其大值区不断向热带外地区转移。从扰动总能量的谱结构来看,扰动能量均呈现升尺度发展的特征。在基于奇异向量初值扰动的GRAPES-GEPS中,随机物理扰动方案的使用均能够显著增加不同地区等压面要素的集合离散度,并在一定程度上改善集合平均误差。由于集合离散度的增大,预报失误率显著减小。连续分级概率评分也有所减小,尤其是在热带地区,改进更为明显。此外,中国地区不同量级（小雨、中雨、大雨和暴雨）降水概率预报技巧在一定程度上得到改善。上述改进均在联合使用SPPT与SKEB方案时最好,这与扰动总能量、扰动与误差相关分析结果一致。      

随机物理倾向扰动在风暴尺度集合预报中的影响研究

为深入探究随机物理倾向扰动（Stochastically Perturbed Parameterization Tendencies,SPPT）方案在风暴尺度集合预报中的影响,基于WRF模式利用FNL资料对SPPT方案中的3个参量分别进行敏感性试验,得到SPPT方案的最佳参数配置,并在此基础上分析SPPT方案模拟的降水分布特征。结果表明:SPPT方案敏感性试验中,去相关时间选择6 h时构造的集合成员可信度更高,逐时降水评分效果在积分中后期较高,对于暴雨及以上量级的评分技巧最优;造成降水主要天气系统的维持时间对该变量的选取有较大的影响。去相关空间尺度选择100 km的集合试验更为可靠,对降水预报技巧较高;同时该变量的选取与天气过程中的大尺度信息、中小尺度系统的活跃以及模式的空间分辨率有密切联系。通过对离散度和离群值分析认为扰动振幅选择0.525最为合理。SPPT方案集合成员在局部地区可以较大幅度地改变降水量,对降水落区的准确模拟存在一定的局限性。      

GRAPES区域集合预报系统模式不确定性的随机扰动技术研究

为进一步描述GRAPES(Global/Regional Assimilation and Prediction System)区域集合预报系统(GRAPES Me—soscale Ensemble Prediction System,GRAPES—MEPS)中GRAPES—Meso模式的不确定性特征,本研究在GRAPES—MEPS系统中引人了模式物理参数化倾向随机扰动方案(Stochastically Perturbed Parameterization Tendencies,SPPT),随机扰动型的产生是基于-阶马尔科夫链,其具有时间相关性特征,并服从正态分布,另外经过谱展开随机场具有空间结构特征,在水平结构上较平滑和连续。本文开展了基于SPPT方案的GRAPES—MEPS集合预报试验,针对SPPT方案中随机场的扰动幅度和时间相关尺度参数开展了一系列敏感性试验,并对试验结果进行了较全面的集合预报客观检验,此外,针对一次强降水过程,分析了SPPT方案对降水预报的影响。试验结果表明,引入SPPT方案能在一定程度上提高GRAPES—MEPS系统的预报技巧,降低系统的漏报率;且能显著改进预报后期大雨量级降水的预报技巧。通过敏感性试验发现,对于GRAPES—MEPS系统,SPPT方案的效果与随机扰动场幅度的范围,及扰动场的时间相关尺度选择相关,需经过敏感性试验确定出较适合的参数。     

不同模式扰动方案在风暴尺度集合预报中的对比试验研究

由于适用于中长期集合预报的模式扰动技术在风暴尺度集合预报系统中的影响并不明确,为探究不同模式扰动方案在风暴尺度集合预报中的效果,基于WRF模式设计了3组模式扰动方案:多物理扰动（MP）方案、随机物理倾向扰动（Stochastically Perturbed Parameterization Tendencies,SPPT）方案以及由MP方案与SPPT方案组合构建的一种新混合扰动（SPMP）方案。对2013年7月5—6日发生在江淮流域的一次强对流天气过程进行了数值模拟。结果表明:MP方案在积分前期的降水概率评分较高,对高层大气的扰动效果更为合理;SPPT方案主要作用于积分中后期,对大气低层及近地面的扰动效果最为理想,尤其是对于地面水汽场的模拟;SPMP方案能显著提高大气中高层各预报变量的离散度,降低均方根误差,提升集合成员的可信度,有效弥补降水预报评分在单独使用MP方案和SPPT方案不同积分时段的不足。在扰动水平传播方向上,SPMP方案的扰动形态主要受MP方案主导;垂直方向上,SPMP方案在低层的扰动形态与SPPT方案一致,在高层受MP方案控制。波谱能量分析表明3组方案的扰动能量随积分时间均有向大尺度传播的趋势,SPMP方案能有效补偿两种方案能量在各尺度的耗散。      

Representing Model Uncertainty by Multi-Stochastic Physics Approaches in the GRAPES Ensemble

To represent model uncertainties more comprehensively,a stochastically perturbed parameterization(SPP)scheme consisting of temporally and spatially varying perturbations of 18 parameters in the microphysics,convection,boundary layer,and surface layer parameterization schemes,as well as the stochastically perturbed parameterization tendencies(SPPT)scheme,and the stochastic kinetic energy backscatter(SKEB)scheme,is applied in the Global and Regional Assimilation and Prediction Enhanced System-Regional Ensemble Prediction System(GRAPES-REPS)to evaluate and compare the general performance of various combinations of multiple stochastic physics schemes.Six experiments are performed for a summer month(1-30 June 2015)over China and multiple verification metrics are used.The results show that:(1)All stochastic experiments outperform the control(CTL)experiment,and all combinations of stochastic parameterization schemes perform better than the single SPP scheme,indicating that stochastic methods can effectively improve the forecast skill,and combinations of multiple stochastic parameterization schemes can better represent model uncertainties;(2)The combination of all three stochastic physics schemes(SPP,SPPT,and SKEB)outperforms any other combination of two schemes in precipitation forecasting and surface and upper-air verification to better represent the model uncertainties and improve the forecast skill;(3)Combining SKEB with SPP and/or SPPT results in a notable increase in the spread and reduction in outliers for the upper-air wind speed.SKEB directly perturbs the wind field and therefore its addition will greatly impact the upper-air wind-speed fields,and it contributes most to the improvement in spread and outliers for wind;(4)The introduction of SPP has a positive added value,and does not lead to large changes in the evolution of the kinetic energy(KE)spectrum at any wavelength;(5)The introduction of SPPT and SKEB would cause a 5%-10%and 30%-80%change in the KE of mesoscale systems,and all three stochastic schemes(SPP,SPPT,and SKEB)mainly affect the KE of mesoscale systems.This study indicates the potential of combining multiple stochastic physics schemes and lays a foundation for the future development and design of regional and global ensembles.     

Environmental Influences on the Intensity Change of Tropical Cyclones in the Western North Pacific

The atmospheric and oceanic conditions are examined during different stages of the lifecycle of western North Pacific tropical cyclones (TCs), with the intention to understand how the environment affects the intensity change of TCs in this area. It is found that the intensification usually occurs when the underlying sea surface temperature (SST) is higher than 26℃. TCs usually experience a rapid intensification when the SST is higher than 27.5℃ while lower than 29.5℃. However, TCs decay or only maintain its intensity when the SST is lower than 26℃. The intensifying TCs usually experience a low-to-moderate vertical wind shear (2-10 ms-1 ). The larger the vertical wind shear, the slower the TCs strengthen. In addition, the convective available potential energy (CAPE) is much smaller in the developing stage than in the formation stage of TCs. For the rapidly intensifying TCs, the changes of SST, CAPE, and vertical wind shear are usually small, indicating that the rapid intensification of TCs occurs when the evolution of the environment is relatively slow.     

西北太平洋热带气旋强度变化的若干特征

使用NOAA海表温度资料、ECMWF再分析资料和JTWC台风最佳路径数据,对1984—2013年30年西北太平洋热带区域（100 °E~180 °,0~60 °N）内热带气旋（TC）的强度变化特征及其与环境风垂直切变（VWS）、海表温度（SST）、最大风速半径（RMW）的关系作了统计分析,尤其关注TC强度突变。结果表明:（1）在研究区域内,TC样本中35.2%强度稳定,52.8%强度变化缓慢,仅12.0%强度突变,约92.7%的迅速加强TC样本发生在其台风及以上强度等级;（2）2000年以来,TC强度稳定样本减少,强度迅速变化样本增多。5月和9—10月是TC强度突变的高频期;（3）超过12 m/s的环境VWS下TC迅速加强较少,且只有台风及以上强度TC才能在大于12 m/s的VWS下迅速加强;（4）TC加强和迅速加强主要在28.5~30.0 ℃的SST洋面上发生,在较低SST下仍迅速加强的TC强度等级较高;（5）TC样本的RMW多小于100 km,其中强度突变TC RMW峰值区在20~40 km;（6）加强TC的RMW的24 h变化一般减小,减弱TC的RMW则增大;其中强度突变TC尤其明显,超强台风发生迅速加强时,RMW减小的比率达84.6%,但仍有15.4%比率的RMW增大。      

The sensitivity of sub-Saharan precipitation to Atlantic SST

The climate model of the Goddard Institute for Space Studies (Hansen et al., 1983) is used to study the sensitivity of sub-Saharan rainfall to Atlantic Ocean SST. Initial changes of SST in the South Atlantic Ocean on March 1st are shown to reduce the June–August sub-Saharan precipitation totals using the model version with an interactive ocean that updates SST. Evidence is offered in support of theories that link Sahel drought with anomalously warm SST in the eastern South Atlantic and the study compares the model's response to spatially coherent SST anomalies with the response to random SST perturbations. The physical processes whereby SST and sea-level pressure synoptics influence the African summer monsoon are discussed in reference to the simulations. Predictibility of Sahel summer rainfall based on spring SST patterns or spring atmospheric circulation patterns is implied by the results. The SST/Sahel drought links are discussed for projections of future climate characteristics.     

Contribution of tropical cyclones to abnormal sea surface temperature warming in the Yellow Sea in December 2004

Unusual sea surface temperature (SST) warming occurred over the Yellow Sea (YS) in December 2004. To identify the causes of the abnormal SST warming, we conducted an analysis on atmospheric circulation anomalies induced by tropical cyclones (TCs) and their impacts on upper ocean characteristics using multiple datasets. With the analysis of various datasets, we explored a new aspect of the relationship between TC activity and SST. The results show that there is a significant link between TC activity over the Northwest Pacific (NWP) and SST in the YS. The integrated effect of consecutive TCs activity induces a large-scale atmospheric cyclonic circulation anomaly over the NWP and consequently anomalous easterly winds over the YS and East China Sea. The mechanism of the unusually warm SST in the YS can be explained by considering TCs acting as an important source of Ekman heat transport that results in substantial intrusion of relatively warm surface water into the YS interior. Furthermore, TC-related circulation anomalies contribute to the retention of the resulting warm SST anomalies in the entire YS.     

Probabilistic discrimination between large-scale environments of intensifying and decaying African Easterly Waves

About 50?C60% of Atlantic tropical cyclones (TCs) including nearly 85% of intense hurricanes have their origins as African Easterly Waves (AEWs). However, predicting the likelihood of AEW intensification remains a difficult task. We have developed a Bayesian diagnostic methodology to understand genesis of North Atlantic TCs spawned by AEWs through the examination of the characteristics of the AEW itself together with the large-scale environment, resulting in a probabilistic discrimination between large-scale environments associated with intensifying and decaying AEWs. The methodology is based on a new objective and automatic AEW tracking scheme used for the period 1980 to 2001 based on spatio-temporally Fourier-filtered relative vorticity and meridional winds at different levels and outgoing long wave radiation. Using the AEW and Hurricane Best Track Files (HURDAT) data sets, probability density functions of environmental variables that discriminate between AEWs that decay, become TCs or become major hurricanes are determined. Results indicate that the initial amplitude of the AEWs is a major determinant for TC genesis, and that TC genesis risk increases when the wave enters an environment characterized by pre-existing large-scale convergence and moist convection. For the prediction of genesis, the most useful variables are column integrated heating, vertical velocity and specific humidity, and a combined form of divergence and vertical velocity and SST. It is also found that the state of the large-scale environment modulates the annual cycle and interannual variability of the AEW intensification efficiency.     

A mechanism for long-term changes of Atlantic tropical cyclone intensity

Although previous studies reported upward trends in the basin-wide average lifetime, annual frequency, proportion of intense hurricanes and annual accumulated power dissipation index of Atlantic tropical cyclones (TCs) over the past 30?years, the basin-wide intensity did not increase significantly with the rising sea surface temperature (SST). Observational analysis and numerical simulation conducted in this study suggest that Sahel rainfall is the key to understanding of the long-term change of Atlantic TC intensity. The long-term changes of the basin-wide TC intensity are generally associated with variations in Sahara air layer (SAL) activity and vertical wind shear in the main development region (MDR), both of which are highly correlated with Sahel rainfall. The drying Sahel corresponds to an equatorward shift in the African easterly jet and African easterly wave activity, introducing the SAL to lower latitudes and increasing the MDR vertical wind shear. As a result, Atlantic TCs are more vulnerable to the suppressing effects of the SAL and vertical wind shear. Since the SST warming, especially in the tropical Indian Ocean, is a dominant factor for the Sahel drying that occurred over the past 30?years, it is suggested that the remote effect of SST warming is important for the long-term change of Atlantic TC intensity. Although influence of the AMO warm phase that started in the early 1990s alone can provide a favorable condition for TC intensification, its influence may have been offset by the influence of the ongoing SST warming, particularly in the Indian Ocean. As a result, there was no significant trend observed in the basin-wide average and peak intensity of Atlantic TCs.     

NUMERICAL SIMULATIONS OF THE PACIFIC MERIDIONAL MODE IMPACTS ON TROPICAL CYCLONES ACTIVITY OVER THE WESTERN NORTH PACIFIC

Based on analyses of the relationship between Pacific Meridional Mode (PMM) and number of tropical cyclones (TCs) activity over the western North Pacific, the impacts of the PMM on Tc activity over the western North Pacific are studied using numerical simulations with an Atmospheric General Circulation Model (CAM3) of National Center for Atmospheric Research (of USA). The result shows that the PMM has impacts on the large-scale generating environment of TCs, thus affecting their number and strength. The numerical simulations using the NCAR CAM3 indicate that with the inclusion of the forcing from sea surface temperature (SST) of the PMM, there appears a decreased magnitude of the vertical zonal wind shear, large proportion of relative humidity, anomalous westerly wind at low levels and anomalous easterly wind at high levels, in association with anomalous cyclonic circulation at low levels and anomalous anti-cyclonic circulation at high levels over the tropical western Pacific. Thus, the PMM provides favorable environment for the typhoon genesis. In the sensitivity experiment, TCs have larger strength, lower SST at the center, stronger tangential wind at 850 hPa and intensified warm cores at high levels. In this paper, the simulation results are similar to that in the data analyses, which reveals the important impact of the PMM on TC activity over the western North Pacific.     

Spring asymmetric mode in the tropical Indian Ocean: role of El Niño and IOD

The spring asymmetric mode over the Tropical Indian Ocean (TIO) is characterized by contrasting patterns of rainfall and surface wind anomalies north and south of Equator. The asymmetric pattern in rainfall has evolved as a leading mode of variability in the TIO and is strongly correlated with El Niño-Southern Oscillation (ENSO) and positive Indian Ocean Dipole (IOD). The evolution of the asymmetric pattern in rainfall and surface wind during pure El Niño/IOD and co-occurrence years are examined in the twentieth century reanalysis for the period of 1871–2008 and atmospheric general circulation model (AGCM) simulations. The study revealed that spring asymmetric mode is well developed when El Niño co-occurred with IOD (positive) and is driven by the associated meridional gradients in sea surface temperature (SST) and sea level pressure (SLP). The pure El Niño composites are characterized by homogeneous (spatially) SST anomalies (positive) and weaker SLP gradients and convection, leading to weak asymmetric mode. The asymmetric mode is absent in the pure IOD (positive) composites due to the persistence of east west SST gradient for a longer duration than the co-occurrence years. The meridional gradient in SST anomalies over the TIO associated with the ENSO-IOD forcing is therefore crucial in developing/strengthening the spring asymmetric mode. The northwest Pacific anticyclonic circulation further strengthen the asymmetric mode in surface winds by inducing northeasterlies in the north Indian Ocean during pure El Niño and co-occurrence years. The simulations based on AGCM, forced by observed SSTs during the period of 1871–2000 supported the findings. The analysis of available station and ship track data further strengthens our results.     

AN OBSERVATIONAL STUDY ON DISTRIBUTION OF PRECIPITATION ASSOCIATED WITH LANDFALLING TROPICAL CYCLONES AFFECTING CHINA

In order to provide an operational reference for tropical cyclone precipitation forecast,this study investigates the spatial distributions of precipitation associated with landfalling tropical cyclones(TCs) affecting China using Geostationary Meteorological Satellite 5(GMS5)-TBB dataset.All named TCs formed over the western North Pacific that made direct landfall over China during the period 2001-2009 are included in this study.Based on the GMS5-TBB data,this paper reveals that in general there are four types of distribution of precipitation related to landfalling TCs affecting China.(a) the South-West Type in which there is a precipitation maximum to the southwestern quadrant of TC;(b) the Symmetrical South Type in which the rainfall is more pronounced to the south side of TC in the inner core while there is a symmetrical rainfall distribution in the outer band region;(c) the South Type,in which the rainfall maxima is more pronounced to the south of TC;and(d) the North Type,in which the rainfall maxima is more pronounced to the north of TC.Analyses of the relationship between precipitation distributions and intensity of landfalling TCs show that for intensifying TCs,both the maximum and the coverage area of the precipitation in TCs increase with the increase of TC intensity over northern Jiangsu province and southern Taiwan Strait,while decreasing over Beibu Gulf and the sea area of Changjiang River estuary.For all TCs,the center of the torrential rain in TC shifts toward the TC center as the intensity of TC increases.This finding is consistent with many previous studies.The possible influences of storm motion and vertical wind shear on the observed precipitation asymmetries are also examined.Results show that the environmental vertical wind shear is an important factor contributing to the large downshear rainfall asymmetry,especially when a TC makes landfall on the south and east China coasts.These results are also consistent with previous observational and numerical studies.     

Influences of Tidal Fronts on Coastal Winds Over an Inland Sea

A regional numerical model of the atmosphere was applied to an inland sea, the Seto Inland Sea in Japan, to study the influence of sea-surface temperature (SST) variations, accompanied by a tidal front, on the coastal winds in summer when tidal fronts fully develop. After confirmation of the model performance, two sensitivity simulations, which used spatially uniform SST with the highest and lowest values over the study area, were performed. The control and sensitivity simulations show that the mean wind speeds were apparently reduced by the low SST and the SST gradient accompanying the tidal front. The comparison of the terms in the momentum equations in control and sensitivity simulations indicates that the change of the perturbation pressure gradient force with the SST gradient is the most important factor in the modification of near-surface winds with SST variations. When the air flows across a tidal front, the air cools over the low SST area and warms over the high SST area. Consequently, the surface perturbation pressure increases over the low SST area and decreases over the high SST area. This adjustment in surface perturbation pressure produces an additional pressure gradient force with direction from the low SST area to the high SST area that decelerates the surface wind in the area upwind of the tidal front and accelerates the surface wind downwind of the tidal front.     

Which Features of the SST Forcing Error Most Likely Disturb the Simulated Intensity of Tropical Cyclones?

Among all of the sources of tropical cyclone(TC) intensity forecast errors, the uncertainty of sea surface temperature(SST) has been shown to play a significant role. In the present study, we determine the SST forcing error that causes the largest simulation error of TC intensity during the entire simulation period by using the WRF model with time-dependent SST forcing. The SST forcing error is represented through the application of a nonlinear forcing singular vector(NFSV)structure. For the selected 12 TC cases, the NFSV-type SST forcing errors have a nearly coherent structure with positive(or negative) SST anomalies located along the track of TCs but are especially concentrated in a particular region. This particular region tends to occur during the specific period of the TCs life cycle when the TCs present relatively strong intensity, but are still intensifying just prior to the mature phase, especially within a TC state exhibiting a strong secondary circulation and very high inertial stability. The SST forcing errors located along the TC track during this time period are verified to have the strongest disturbing effect on TC intensity simulation. Physically, the strong inertial stability of TCs during this time period induces a strong response of the secondary circulation from diabatic heating errors induced by the SST forcing error. Consequently, this significantly influences the subsidence within the warm core in the eye region, which,in turn, leads to significant errors in TC intensity. This physical mechanism explains the formation of NSFV-type SST forcing errors. According to the sensitivity of the NFSV-type SST forcing errors, if one increases the density of SST observations along the TC track and assimilates them to the SST forcing field, the skill of TC intensity simulation generated by the WRF model could be greatly improved. However, this adjustment is most advantageous in improving simulation skill during the time period when TCs become strong but are still intensifying just prior to reaching full maturity. In light of this, the region along the TC track but in the time period of TC movement when the NFSV-type SST forcing errors occur may represent the sensitive area for targeting observation for SST forcing field associated with TC intensity simulation.     

中尺度降水集合预报随机参数扰动方法敏感性试验

中尺度降水模式预报具有很大的不确定性,为更好地描述与模式降水预报密切相关的物理过程关键参数的不确定性,基于中国气象局GRAPES（Global/Regional Assimilation and Prediction System）中尺度区域集合预报模式,从对模式降水预报不确定性有较大影响的积云对流、云微物理、边界层及近地面层等4个参数化方案中选取了18个关键参数,设计了一种随机参数扰动方案（Stochastically Perturbed Parameterization,SPP）,并通过2015年6—7月总计10 d的随机扰动集合预报试验,对比分析了SPP方案对不同物理过程参数扰动敏感性、随机场时、空尺度敏感性、能量变化特征及其集合预报效果。结果显示,对所选择的任一物理过程参数化方案增加SPP扰动后,降水及等压面要素的概率预报技巧优于无SPP扰动的预报,而扰动积云对流和边界层过程中的参数较扰动云微物理过程中的参数影响更显著,且同时扰动积云对流、云微物理、边界层及近地面层参数化方案中的18个参数的集合预报效果优于扰动任何单一物理过程中的部分参数,表明SPP方案能够有效地提高中尺度降水概率预报技巧;从能量变化特征可知,不同物理过程的参数扰动对动能、内能和总能量的影响层次和特征有所不同,但总体而言,扰动前后各项能量基本相同;随机场时、空尺度敏感性试验发现,SPP扰动随机场时间、空间相关尺度对集合预报效果有明显影响,当扰动随机场选用12 h抗相关时间及截断波数20时,集合预报结果最优。上述结果表明,SPP随机参数扰动方案不仅能够有效提高集合概率预报效果,还能够提高集合降水概率预报技巧,具有良好的业务应用与发展前景。      

GRAPES全球集合预报系统模式扰动随机动能补偿方案初步探究

为了体现次网格尺度能量升尺度转换过程中存在的不确定性, 文中将随机动能补偿(Stochastic Kinetic Energy Backscatter, SKEB)方案应用于GRAPES(Global/Regional Assimilation and Prediction System)全球集合预报系统(GRAPES-GEPS), 以更好地表征模式误差并且增大集合离散度。使用的SKEB方案基于具有一定时、空相关特征的随机型以及由数值扩散导致的局地动能耗散率来构造随机流函数强迫。并根据流函数与水平风速旋转分量的关系, 将SKEB方案中的流函数强迫转化为适用于GRAPES全球模式的水平风速扰动。结果表明, SKEB方案的使用一方面能够提高GRAPES对大气动能谱的模拟能力; 另一方面能够改善GRAPES-GEPS的集合离散度与集合平均误差的关系, 增加了集合离散度, 并在一定程度上减小了集合平均误差, 尤其是在热带地区这种改进更为显著。而且该方案使得热带地区连续分级概率评分(CRPS评分)显著减小。就降水预报而言, 从Brier评分与相对作用特征面积(AROC, Area under the Relative Operating Characteristics)的结果来看, SKEB方案有助于改善中国地区小雨[0.1 mm, 10 mm)、中雨[10 mm, 25 mm)与大雨[25 mm, 50 mm)量级降水的概率预报技巧, 而对暴雨[50 mm, ∞)量级降水预报技巧影响很小(24 h降水量)。总体上, 模式扰动随机动能补偿方案提高了GRAPES-GEPS的概率预报技巧。      

A study of climate and weather variability over the tropical southwest Indian Ocean

Summary The climatology and variability of summer convection and circulation over the tropical southwest Indian Ocean is investigated using satellite imagery, routine synoptic observations, outgoing longwave radiation (OLR) data, sea surface temperatures (SST) and areal averaged rainfall departures. OLR has a –0.90 correlation with rainfall departures and the OLR minimum (ITCZ) in January and February lies across the 10°S latitude, extending further south near Madagascar. The intensity of ITCZ convection is greatest in the longitudes 20–35°E over northern Zambia and is considerably reduced over the SW Indian Ocean. Spatial correlations are analyzed for standardized departures of OLR, rainfall and SST. The correlations change sign in a coherent fashion, creating a climatic dipole between southern Africa and the SW Indian Ocean. Interannual trends are examined through analysis of January–February zonal and meridional wind indices constructed from significantly correlated variables at Zimbabwe, Madagascar and Mauritius. Circulation variability is dominated by quasi-decadal cycles and a trend of inereasing westerly winds. Zonal wind shear alternates from easterly (barotropic) to westerly and together with SST appears to regulate the frequency and intensity of tropical cyclogenesis. Areally averaged rainfall departures exhibit 6.25 year cycles in NE Madagascar and 12.5 and 18.75 year cycles in SW Madagascar and Zimbabwe, respectively. Summer rainfall and meridional winds in NE Madagascar and Zimbabwe are out of phase and negatively correlated in most summers. The presence of synoptic weather systems is assessed using daily Hovmoller-type satellite imagery composites. Convective structure is dominated by transient waves in the 10°–20°S latitude band, with periods of 15–20 days common. The waves are more prominent in summers with increased easterly shear and contribute to fluctuations in rainfall over SE Africa.With 8 Figures     

浙江北部近海边界层风廓线特征分析

利用2010年12月至2014年5月宁波近海凉帽山370m高塔气象梯度风观测和浙江北部沿海自动气象站测风资料,对浙江北部近海风速垂直廓线进行分析,结果发现:受地形影响,偏南、偏北风时塔基风速一般比上一层风速大。不同天气系统影响下近地边界层风廓线不同,南风型320m以下风速基本遵从对数律。热带气旋影响型和北风型时风廓线可分为3段,常通量层内基本满足对数律,该层向上一段高度热带气旋影响型风速变化不大,北风型反而减小,再往上风速又继续增大。北风型风廓线的这种3段结构表现比热带气旋影响型更为清楚,约80~109m风速出现相对极大值,200~250m间存在风速极小值。满足对数律的近地边界层内小风比大风具有更好的拟合优度。浙江北部沿海自动气象站测风资料不同风型统计分析与高塔风廓线表现基本一致。