Potential Impacts of Land-Use on Climate Variability and Extremes

This study aims at exploring potential impacts of land-use vegetation change （LUC） on regional climate variability and extremes. Results from a pair of Australian Bureau of Meteorology Research Centre （BMRC） climate model 54-yr （1949-2002） integrations have been analysed. In the model experiments, two vegetation datasets are used, with one representing current vegetation coverage in China and the other approximating its potential coverage without human intervention. The model results show potential impacts of LUC on climate variability and extremes. There are statistically significant changes of surface interannual climate variability simulated by the model. Using different vegetation datasets, significant changes in correlation coeiYicients between tropical Pacific Nifio3.4 SST and precipitation and surface temperature over East Asia are identified, which indicate that changes in vegetation coverage may alter ENSO impacts on regional climate variability. Because of the lack of slowly varying surface processes when forests are removed and less rainfall is received following LUC, the ENSO signal simulated by the model becomes stronger. Results furthermore show that land-use could modulate characteristics of decadal variations in this region. When using current vegetation coverage, the model gives better simulation of observed climate variations in the region than the ease using potential vegetation coverage. In addition, results suggest that land-use could be a potential factor contributing to the prolonged drought in central-west China. Changes in local climate extremes, including precipitation and surface temperature maxima and minima, are also identified. Overall, this study has illustrated the importance of further investigation of such important issues in future land-use studies.

AN OPERATIONAL STATISTICAL SCHEME FOR TROPICAL CYCLONE INDUCED RAINFALL FORECAST

A non-parametric method is used in this study to analyze and predict short-term rainfall due to tropical cyclones(TCs) in a coastal meteorological station. All 427 TCs during 1953-2011 which made landfall along the Southeast China coast with a distance less than 700 km to a certain meteorological station- Shenzhen are analyzed and grouped according to their landfalling direction, distance and intensity. The corresponding daily rainfall records at Shenzhen Meteorological Station(SMS) during TCs landfalling period(a couple of days before and after TC landfall) are collected. The maximum daily rainfall(R-24) and maximum 3-day accumulative rainfall(R-72) records at SMS for each TC category are analyzed by a non-parametric statistical method, percentile estimation. The results are plotted by statistical boxplots, expressing in probability of precipitation. The performance of the statistical boxplots is evaluated to forecast the short-term rainfall at SMS during the TC seasons in 2012 and 2013. Results show that the boxplot scheme can be used as a valuable reference to predict the short-term rainfall at SMS due to TCs landfalling along the Southeast China coast.

Prospects for Improving the Operational Seasonal Prediction of Tropical Cyclone Activity in the Southern Hemisphere

Tropical cyclones (TCs) are the most destructive weather phenomena to impact on tropical regions, and reliable predicttion of TC seasonal activity is important for preparedness of coastal communities in the tropics. In investigating prospects for improving the skill of TC seasonal prediction in the South Indian and South Pacific Oceans, including the Australian Region, we used linear regression to model the relationship between the annual number of cyclones and three indices (SOI, NI?O3.4 and 5VAR) describing the strength of the El Ni?o-Southern Oscillation (ENSO). The correlation between the number of Australian Region (90?E - 160?E) TCs and the indices was strong (3-month 5VAR ?0.65, NI?O3.4 ?0.62 and SOI +0.64), and a cross-validation assessment demonstrated that the models which used July-August-September indices and the temporal trend as the predictors performed well. The predicted number of TCs in the Australian Region for 2010/2011 and 2011/2012 seasons was 14 (11 recorded) and 12, respectively. We also found that the correlation between the numbers of TCs in the western South Indian region (30?E to 90?E) and the eastern South Pacific region (east of 170?E) and the indices was weak, and it is therefore not sensible to build linear regression forecast models for these regions. We conclude that for the Australian Region, the new statistical model provides prospects for improvement in forecasting skill compared to the statistical model currently employed at the National Climate Centre, Australian Bureau of Meteorology. The next step towards improving the skill of TC seasonal prediction in the various regions of the Southern Hemisphere will be undertaken through analysis of outputs from the dynamical climate model POAMA (Predictive Ocean-Atmosphere Model for Australia).

东亚水循环中水稳定同位素的GCM模拟和相互比较

利用引入稳定同位素循环的ECHAM4、GISS E和HadCM3模式的模拟,对东亚降水中年平均δD和过量氘d的空间分布以及大气水线(MWL)进行了分析.根据模拟的空间分布,降水同位素在很大程度上反映不同气团的地理背景以及它们之间的相互作用,模拟结果很好地再现了由GNIP实测资料得到的降水稳定同位素的纬度效应、大陆效应和高度效应特征.在对降水d的模拟试验中,3个GCM的模拟均显示降水d具有较显著的纬度分布规律.在东亚地区,根据GCM模拟的MWL斜率与根据71个GNIP站点计算的MWL斜率之间的差值位在合理的估计范围之内.在对具有相对较长取样记录的12个GNIP站点局地大气水线(LMWL)的模拟试验中,LMWL的斜率和截距大致分布在一个合理的范围,但大多被高估.造成斜率和截距被高估可能原因与数据的覆盖范围和长度、模式中水循环过程的细节、模式的精度、降水量的模拟以及对云中冰面过饱和度的假设有关.

青藏高原探空大气水汽偏差及订正方法研究

水汽是大气的主要成分和降水的主要物质来源。青藏高原大气水汽分布对区域天气和气候有很大影响,为了探讨探空观测的大气水汽总量(R)资料的可靠性,本文以地基GPS遥感的大气水汽总量(G)为参照标准,对拉萨(1999～2010年)和那曲(2003年)的R进行对比分析和偏差(R-G)订正。结果表明:近10多年拉萨站R比G明显偏小,偏小程度随使用不同的探空仪而异。GZZ-2型机械探空仪和GTS-1型电子探空仪多年平均的PW偏差分别为-8.8%和-3.9%,随机误差分别为17.6%和13.6%。近10多年PW偏差变化呈减少趋势,这与探空仪性能改进有关。分析发现,青藏高原PW偏差具有明显季节变化和日变化特征,夏季比冬季明显,1200UTC比0000UTC明显。拉萨站GZZ-2型和GTS-1型探空仪在1200UTC多年平均的PW偏差分别为-15.8%和-7.3%,在0000UTC分别为-1.6%和-0.4%。那曲站GZZ-2型探空仪在1200UTC和0000UTC的PW偏差分别为-12.4%和-0.3%。分析还表明,太阳辐射加热与气温的日变化和季节变化是造成高原PW偏差日变化和季节变化的重要原因。据此,提出了高原PW偏差的订正方法,并以拉萨和那曲站为例进行PW偏差订正,订正后的PW系统偏差显著减少,随机误差也相应得到了改善。

晴天地表总辐射和净辐射瞬时值计算方法

以较精确的大气辐射传输模式为基础,建立晴天地表总辐射和净辐射瞬时值的计算方案。与以往的经验计算方法不同,该方案将辐射传输带模式的思路引入地面太阳辐射计算,并考虑大气中吸收和散射物质对太阳辐射的影响,使该方案具有较好的精确性和普适性。采用Kokhanovsky等提出的大气气溶胶反射率和透过率参数化方案,使气溶胶对地面总辐射和净辐射的影响得到较好处理。采用的自变量为数值预报模式或卫星观测提供的气象要素,因此该方案既可用于数值预报模式或陆面过程模式计算地表辐射平衡,又可以利用卫星观测或再分析资料估算地面太阳能资源分布。利用美国能源部3个大气辐射观测站2005年的观测资料及欧洲宇航局提供的卫星反演气溶胶资料对计算方案进行检验。结果表明,该方案十分精确,所有站点的平均相对误差均小于6%,误差的均方差小于0.3 W·m-2。

云天地表总辐射和净辐射瞬时值的计算方法

为减少计算机时,满足实时预报要求,全球数值预报模式中的辐射计算频率通常设定为3 h。这样处理会大大减少计算量,但同时导致较大的辐射日变化偏差,并影响模式对地面能量平衡、对流及降水的模拟。为改进这一缺陷,建立了一种辐射快速计算方案,可用于计算瞬时地面太阳总辐射和净辐射,使到达地面的太阳辐射计算可与模式积分同步进行,改善地面太阳辐射日变化模拟。本文介绍云天情况下的总辐射和净辐射计算方案,该方案所用的输入变量均为预报模式或卫星观测提供。结果表明:该方案既可用于数值预报模式,也可利用观测资料独立计算地面太阳辐射。通过使用美国能源部大气辐射观测资料检验,证明该方案的精度高,地面总辐射瞬时值的平均计算误差小于7%。

TJ-WRF模式对天津地区暴雨和台风个例亮温检验

利用辐射亮温模式对天津地区中尺度预报模式的预报效果进行了检验,分别计算了2012年7月21日和25日天津地区的暴雨过程、8月3—4日台风过程的初始场及预报场的云顶亮温,并与实时卫星观测的亮温进行了比较。结果表明:在有云的区域,TJ-WRF模式模拟的亮温比卫星实况观测的亮温偏高,模式模拟的云覆盖范围偏小,表明天津地区数值预报模式模拟的对流强度偏弱,云未发展到应有的高度;在晴空区域,TJ-WRF模式模拟的亮温也比实况偏高,说明模式模拟的地面温度偏高。TJ-WRF模式对台风路径的模拟效果较好,且模拟的台风中心位置与实况较一致。

ON THE USE OF A SYSTEM-SCALE ASCENT/DESCENT DIAGNOSTIC FOR SHORT-TERM FORECASTING OF TROPICAL CYCLONE DEVELOPMENT,INTENSIFICATION AND DECAY

The Brisbane Tropical Cyclone Warning Centre has used two forms of a thermal advection diagnostic to identify relatively large areas of isentropic ascent and descent for many years. When the thermodynamic conditions are favourable the ascent regions are correlated with significant outbreaks of convection that produce heavy rainfall. The diagnostic is based on the relationship between geostrophic winds that turn with height and flow perpendicular to thickness contours. As the relationship is also valid for the more general case of gradient winds, the diagnostic, in theory, should be useful for most heavy-rain-bearing tropical systems. A climatology of rainfall rate with one form of the diagnostic is presented at two Queensland locations(one tropical and one subtropical) that demonstrates a clear relationship between the isentropic ascent wind distribution and heavy to extreme rainfall.The diagnostics applied to numerical weather prediction models are valuable forecast tools as they identify heavy rainfall threat regions within which the extreme rain is likely to fall, whereas the rainfall from the same models is often under predicted or has large location errors. Applied to tropical lows and tropical cyclones the diagnostics have been used successfully to forecast tropical cyclone formation and rapid intensification and decay. Examples of such intensification and decay from around the world are presented, as well as a climatology of the diagnostic applied to intensifying tropical cyclones in the Australian region.

Estimation of Hourly Solar Radiation at the Surface under Cloudless Conditions on the Tibetan Plateau Using a Simple Radiation Model

In this study, the clear sky hourly global and net solar irradiances at the surface determined using SUNFLUX, a simple parameterization scheme, for three stations （Gaize, Naqu, and Lhasa） on the Tibetan Plateau were evaluated against observation data. Our modeled results agree well with observations. The correlation coefficients between modeled and observed values were 〉 0.99 for all three stations. The relative error of modeled results, in average was 〈 7%, and the root-mean-square variance was 〈 27 W m-2. The solar irradiances in the radiation model were slightly overestimated compared with observation data; there were at least two likely causes. First, the radiative effects of aerosols were not included in the radiation model. Second, solar irradiances determined by thermopile pyranometers include a thermal offset error that causes solar radiation to be slightly underestimated. The solar radiation absorbed by the ozone and water vapor was estimated. The results show that monthly mean solar radiation absorbed by the ozone is 〈 2% of the global solar radiation （〈 14 W m-2）. Solar radiation absorbed by water vapor is stronger in summer than in winter. The maximum amount of monthly mean solar radiation absorbed by water vapor can be up to 13% of the global solar radiation （95 W m-2）. This indicates that water vapor measurements with high precision are very important for precise determination of solar radiation.

Intercomparison of stable isotopes in precipitation simulated by GCMs with GNIP survey over East Asia

Using the isotope enabled ECHAM4, GISS E and HadCM3 GCMs, the spatial distribution of mean 6180 in precipitation, mean seasonality and the correlations of 6180 in precipitation with temperature and precipitation amount are analyzed. The simulated results are in agreement with stable isotopic features by GNIP observations. Over East Asia. the distribution of ~180 in precipita- tion is of marked latitude effect and altitude effect. The latitude effect is covered by the continent effect in some regions. The larg- est seasonality of^lSo in precipitation appears in eastern Siberia controlled by cold high pressure, and the smallest seasonality is in the western Pacific controlled by the subtropical high. Relatively weak seasonality appears in middle latitudes where oceanic and continental air masses frequently interact. However, three GCMs show significant systematic lower ~180 for inland mid-high lati- tudes than GNIP data, which is related to the used isotopic scheme in GCMs. Temperature effect occurs mainly in inland mid-high latitudes. The higher the latitude and the closer the distance to inland is, then the stronger the temperature effect. Amount effect occurs mainly in low-mid latitudes and monsoon areas, with the strongest effect in low-latitude coasts or islands. However, three GCMs provide virtually non-existent amount effect in arid regions over Central Asia. The enrichment action of stable isotopes in falling raindrops under a cloud base, which is enlarged by these modes, is responsible for such a result. A significant difference between spatial distributions of δ^18O statistics by GCMs simulations and by GNIP observations is that the standard deviation of GCMs statistics is greater than that of GNIP statistics. In contrast, by comparing parallel time series at a single station, the standard deviations of GCMs simulations are smaller than that of GNIP observations.

DOWNSTREAM DEVELOPMENT DURING THE EXTRATROPICAL TRANSITION OF TROPICAL CYCLONES:OBSERVATIONAL EVIDENCE AND INFLUENCE ON STORM STRUCTURE

Observational evidence is presented that during Extratropical Transition(ET) of Tropical Cyclones(TCs),Downstream Development(DD) is frequently underway.We show that DD results in rapid changes to the environment.A critical flow change is the development of a low-level trough,sandwiched between two developing anticyclones.The trough appears to merge with the storm,seemingly holding it upright and allowing it to withstand the damaging effects of wind shear.In this way the storm can eventually reach the favourable equatorward entrance region of the upper jet.To evaluate the mechanism,two sets of simulations have been run:one using high-resolution,full physics integrations and another using coarse-resolution with dry physics and the TC removed from the initial condition.We show that the dry dynamics can establish the large scale environment to enable the transition to proceed.The process can produce:(a) the deep,vertically-aligned,low-level pressure trough that merges with the storm,and(b) a partial inhibition through subsidence to embedded convection,allowing the boundary layer to moisten via(i) sustained surface fluxes,and(ii) enhanced horizontal moisture flux convergence from the environmental flow changes.This produces potential for more intense convective activity and vortex resiliency even in relatively strong,deep vertical wind shear.

东亚降水中δ^(18)O的GCM模拟及其与GNIP实测值的比较

本文利用引入稳定同位素循环的ECHAM4,GISS E,HadCM3和MUGCM模式的模拟对东亚降水中平均δ^(18)O的空间分布、季节差异以及δ^(18)O与温度、降水量的关系进行了分析。模拟结果很好地再现了由GNIP实测资料得到的δ^(18)O的变化特征。在东亚,降水中δ^(18)O的分布具有明显的纬度效应和高度效应。降水中δ^(18)O季节差的最大值出现在受冷高压控制的东西伯利亚,最小值出现在受副热带高压控制的西太平洋。在海洋性气团与大陆性气团频繁交绥的中纬度地区,δ^(18)O季节差相对较弱,但经向变化梯度较大。然而,4个GCM的模拟均显示在中高纬度内陆降水中δ^(18)O明显偏低。温度效应主要出现在中高纬度和内陆区,纬度越高、越接近内陆,温度效应越强。降水量效应主要出现在中低纬度和季风区,最强的降水量效应出现在低纬度沿海或海岛。然而,4个GCM均给出实际上并不存在的发生在中亚干旱区的降水量效应。这个结果与雨滴在降落过程中重同位素的富集作用有关,但模式对该机制起到了放大作用。GCM和GNIP降水中δ^(18)O统计量空间分布差异的一个显著特点是,GCM统计量的标准差大于GNIP统计量的标准差。然而,当对单站降水δ^(18)O的时间序列作对比时,GCM模拟值的标准差反而小于GNIP实测值的标准差。

GCM Simulations of Stable Isotopes in the Water Cycle in Comparison with GNIP Observations over East Asia

In this paper, we examine the performance of four isotope incorporated GCMs, i.e., ECHAM4 （Univer- sity of Hamburg）, HadCM3 （Hadley Centre）, GISS E （Goddard Institute of Space Sciences）, and MUGCM （Melbourne University）, by comparing the model results with GNIP （Global Network of Isotopes in Precip- itation） observations. The spatial distributions of mean annual δD and mean annual deuterium excess d in precipitation, and the relationship between δ18O and δD in precipitation, are compared between GCMs and GNIP data over East Asia. Overall, the four GCMs reproduce major characteristics of δD in precipitation as observed by GNIP. Among the four models, the results of ECHAM4 and GISS E are more consistent with GNIP observed precipitation δD distribution. The simulated d distributions are less consistent with the GNIP results. This may indicate that kinetic fractionation processes are not appropriately represented in the isotopic schemes of GCMs. The GCM modeled MWL （meteoric water line） slopes are close to the GNIP derived MWL, but the simulated MWL intercepts are significantly overestimated. This supports that the four isotope incorporated GCMs may not represent the kinetic fractionation processes well. In term of LMWLs （local meteoric water lines）, the simulated LMWL slopes are similar to those from GNIP observa- tions, but slightly overestimated for most locations. Overall, ECHAM4 has better capability in simulating MWL and LMWLs, followed by GISS E. Some isotopic functions （especially those related to kinetic frac- tionation） and their parameterizations in GCMs may have caused the discrepancy between the simulated and GNIP observed results. Future work is recommended to improve isotopic function parameterization on the basis of the high-resolution isotope observations.

OBJECTIVE ESTIMATION OF THE RADIUS OF THE OUTERMOST CLOSED ISOBAR IN TROPICAL CYCLONES

Increased understanding of the importance of TC structure in dynamical,climatological and prediction studies makes determination of TC size important. A new algorithm for the objective estimation of the radius of outermost closed isobar(roci) has been developed. The new method uses storm position and global analyses of mean sea level pressure to compute a mean(axisymmetric) roci. This radius can be used,together with the central pressure,for the construction of a synthetic vortex that is initialized in a numerical prediction model. The method also has important applications in dynamical and climatological studies of TC intensity,size and structure. The algorithm is robust and capable of estimating roci,even in the case of a weak system that may not have a closed isobar in the global analysis. The values produced by the new method are shown to be more consistent than the corresponding operational estimates which are subjective and produced under strong time constraints. Statistical comparison between subjective and objective estimates gives a mean absolute difference of 110 km,which given the difficulty in making a subjective estimate,is satisfactory. In addition,even though limitations exist with the estimates of vortex parameters like the radius to gales(r34),comparison with estimates from an extended best track data set provides independent evaluation of the scheme. Mean absolute difference for r34 for around 3200 cases is near 80 km,even though the best track estimates are subjective and the objective r34 is estimated only from storm central pressure and the objective roci. This validation suggests that the algorithm can be used to obtain useful size estimates of TCs.