The sublimation of falling snow over the Mackenzie River Basin

The sublimation of falling snow may be an important component of the atmospheric water budget of the Mackenzie River Basin and many parts of the Arctic. To investigate this issue, a simple sublimation model is used along with surface precipitation observations and sonde data obtained during the autumn 1994 Beaufort and Arctic Storms Experiment (BASE). Model results are then compared with actual precipitation measurements at Inuvik and Tuktoyaktuk, sites in Northern Canada, to approximate mass loss due to sublimation. The sublimation results are found to vary in concert with cloud base height, precipitation intensity aloft and the nature of the precipitation. Atmospheric conditions are furthermore examined over a wide range of the Arctic, especially the Mackenzie River Basin, to assess to what degree the results can be generalized. The presence of a relatively dry near-surface layer, a favourable environment for sublimation, is a key feature of most sites during the early autumn storm period. Estimates of sublimational mass losses are found over Inuvik and Tuktoyaktuk using sonde derived cloud base heights and temperature and humidity profiles. Sublimation losses for such sites are found to be of the order of 40–60%, which shows that sublimation is indeed a significant process over the Mackenzie Basin and needs to be well handled in climate models. However, increasing the vertical resolution of the sublimation model to that of climate scales can dramatically affect predicted sublimation amounts; how to properly account for sublimation then remains a difficult task.     

Evaluation of TIGGE Daily Accumulated Precipitation Forecasts over the Qu River Basin,China

Quantitative precipitation forecasts(QPFs) provided by three operational global ensemble prediction systems(EPSs) from the THORPEX(The Observing System Research and Predictability Experiment) Interactive Grand Global Ensemble(TIGGE) archive were evaluated over the Qu River basin, China during the plum rain and typhoon seasons of 2009–13. Two post-processing methods, the ensemble model output statistics based on censored shifted gamma distribution(CSGD-EMOS) and quantile mapping(QM), were used to reduce bias and to improve the QPFs.The results were evaluated by using three incremental precipitation thresholds and multiple verification metrics. It is demonstrated that QPFs from NCEP and ECMWF presented similarly skillful forecasts, although the ECMWF QPFs performed more satisfactorily in the typhoon season and the NCEP QPFs were better in the plum rain season. Most of the verification metrics showed evident seasonal discriminations, with more satisfactory behavior in the plum rain season. Lighter precipitation tended to be overestimated, but heavier precipitation was always underestimated. The post-processed QPFs showed a significant improvement from the raw forecasts and the effects of post-processing varied with the lead time, precipitation threshold, and EPS. Precipitation was better corrected at longer lead times and higher thresholds. CSGD-EMOS was more effective for probabilistic metrics and the root-mean-square error. QM had a greater effect on removing bias according to bias and categorical metrics, but was unable to warrant reliabilities. In general, raw forecasts can provide acceptable QPFs eight days in advance. After post-processing, the useful forecasts can be significantly extended beyond 10 days, showing promising prospects for flood forecasting.     

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采用欧拉方法分析盆地东北部夏季3例持续性暴雨过程的水汽输送及其异常特征,并利用拉格朗日轨迹模式模拟计算了影响川东北的主要水汽输送通道,结果表明：（1）不同纬度多系统相互作用对水汽输送的加强,是造成2007年和2010年两例持续性暴雨水汽强度异常大的重要原因。2012年持续性暴雨个例中异常水汽主要源于孟加拉湾和南海两支水汽的共同作用。（2）水汽源地可追溯至孟加拉湾、南海、西太平洋和阿拉伯海。边界层的水汽输送更多是自南海沿华南地区进入川东北;中低层水汽输送路径,或以孟加拉湾进入川东北,或以南海进入川东北,或两者共同作用。在垂直方向上,多条水汽输送通道的相互叠加,促使川东北产生强的水汽通量。（3）每条水汽输送通道在不同的个例中主次作用不一致。同一个例的水汽输送通道并非固定不变,不同降雨阶段可能与不同水汽输送通道对应。

     

The synoptic‐ and planetary‐scale signatures of precipitating systems over the Mackenzie River Basin

Abstract

The synoptic‐ and planetary‐scale signatures of precipitating systems over the Mackenzie River Basin (MRB) are elucidated using composites based on a 28‐year sample of widespread precipitation events. These wet events are defined as days on which 5 or more of 12 surface stations in the MRB receive at least 2.5 mm of precipitation. Seasonal composites based on a total of 600 wet events reveal a sequence of statistically significant flow anomalies. Examination of individual wet events motivates stratification of the seasonal samples according to sea‐level pressure distribution. One evolution that is particularly common during fall, winter and spring involves lee cyclogenesis over the southern MRB in association with a strong cyclone over the Gulf of Alaska; such events are dubbed Gulf Redevelopment (GR) cases. A composite based on 59 wintertime GR events indicates upslope flow north of the lee cyclone and warm advection along an east‐west oriented warm front during the precipitation event. Composites of the Q‐vector and the divergence of this field confirm the presence of quasigeostrophic (QG) forcing for ascent over the MRB during this period. A thermally indirect “topographic tilting” mechanism, involving downs‐lope warming over the southern MRB and upslope cooling to the north, is hypothesized to increase warm‐frontal baroclinicity over the MRB. The GR composite 500 hPa geopotential height anomaly pattern is characterized by a series of anomalies extending from the Bering Sea to the Gulf of Mexico. The western (eastern) anomalies tend to decay (amplify) with time. The composite exhibits a positive anomaly over the Bering Sea, a negative anomaly over the Gulf of Alaska that moves eastward into the MRB during the precipitation event, and a positive anomaly that moves eastward over western and central North America. The presence of large, slow‐moving flow anomalies and an extended period of enhanced southwesterly geostrophic flow over the MRB in the composite suggests that a persistent influx of Pacific moisture is required to moisten the atmosphere over the MRB sufficiently for widespread precipitation. An independent composite of dry MRB cyclone events exhibits substantially weaker southwesterly geostrophic flow into the MRB relative to the wet GR composite.     

Variations in hydrological parameters of the Zeravshan River and its tributaries depending on meteorological conditions

According to the data of the Agency for Hydrometeorology of the Republic of Tajikistan, the volume of the Zeravshan glacier significantly reduced in 1927–1991 (by more than 2 km³), and its further degradation by 30–35% is expected by 2050. To monitor the meteorological conditions in the Zeravshan River basin, in particular, the area of the Zeravshan glacier, air temperature variations in 1931–1961 and 1981–2011 are analyzed. It was found that the period of 1931–1961 is characterized by stable air temperature and its significant rise began in 1981. The trend towards the decrease in water discharge of the Zeravshan River is observed in 1931–1961. It is demonstrated that the average long-term runoff decreased from 6.08 km³ in 1931–1961 to 5.36 km³ in 1981–2011. The similar measurements were carried out in the basin of the Yaghnob River being the tributary of the Zeravshan River. It was revealed that the difference in average long-term runoff between the periods of 1931–1961 and 1981–2011 is insignificant and makes up not more than 2%. Besides, it was found that the results of meteorological observations in 1931-1961 do not agree with the real picture of the Zeravshan glacier degradation.     

长江流域一次暴雨过程的模拟对行星边界层参数化的敏感性研究

用MM5对长江流域的一次暴雨进行模拟考察其对行星边界层参数化的敏感性。不同的边界层参数化表现在不同的地表通量和垂直混合设计上，本文分析了MM5中4个主要方案地表通量和垂直混合参数化方案的不同以及它们对降水强度、落区和时间的影响。研究还发现，地表通量对暴雨模拟结果的影响比垂直混合方案要大。     

Sensitivity of the last glacial inception to initial and surface conditions

We investigate the sensitivity of simulations of the last glacial inception (LGI) with respect to initial (size of the Greenland ice sheet) and surface (state of ocean/vegetation) conditions and two different CO₂ reconstructions. Utilizing the CLIMBER-2 Earth system model, we obtain the following results: (a) ice-sheet expansion in North America at the end of the Eemian can be reduced or even completely suppressed when pre-industrial or Eemian ocean/vegetation is prescribed. (b) A warmer surrounding ocean and, in particular, a large Laurentide ice sheet reduce the size of the Greenland ice sheet before and during the LGI. (c) A changing ocean contributes much stronger to the expansion of the Laurentide ice sheet when we apply the CO₂ reconstruction according to Barnola et al. (Nature 329:408–414, 1987) instead of Petit et al. (Nature 399:429–436, 1999). (d) In the fully coupled model, the CO₂ reconstruction used has only a small impact on the simulated ice sheets but it does impact the course of the climatic variables. (e) For the Greenland ice sheet, two equilibrium states exist under the insolation and CO₂ forcing at 128,000 years before present (128 kyear BP); the one with an ice sheet reduced by about one quarter as compared to its simulated pre-industrial size and the other with nearly no inland ice in Greenland. (f) Even the extreme assumption of no ice sheet in Greenland at the beginning of our transient simulations does not alter the simulated expansion of northern hemispheric ice sheets at the LGI.     

Sensitivity of the Asian summer monsoon to the horizontal resolution: differences between AMIP-type and coupled model experiments

A set of experiments forced with observed SST has been performed with the Echam4 atmospheric GCM at three different horizontal resolutions (T30, T42 and T106). These experiments have been used to study the sensitivity of the simulated Asian summer monsoon (ASM) to the horizontal resolution. The ASM is reasonably well simulated by the Echam4 model at all resolutions. In particular, the low-level westerly flow, that is the dominant manifestation of the Asian summer monsoon, is well captured by the model, and the precipitation is reasonably simulated in intensity and space appearance. The main improvements due to an higher resolution model are associated to regional aspects of the precipitation, for example the Western Ghats precipitation is better reproduced. The interannual variability of precipitation and wind fields in the Asian monsoon region appears to be less affected by an increase in the horizontal resolution than the mean climatology is. A possible reason is that the former is mainly SST-forced. Besides, the availability of experiments at different horizontal resolution realized with the Echam4 model coupled to a global oceanic model allows the possibility to compare these simulations with the experiments previously described. This analysis showed that the coupled model is able to reproduce a realistic monsoon, as the basic dynamics of the phenomenon is captured. The increase of the horizontal resolution of the atmospheric component influences the simulated monsoon with the same characteristics of the forced experiments. Some basic features of the Asian summer monsoon, as the interannual variability and the connection with ENSO, are further investigated.     

IAP AGCM 4.1对淮河流域夏季降水的预报技巧评估

基于中国科学院大气物理研究所新一代大气环流模式IAP AGCM 4.1共30 a(1981—2010年)的集合回报试验结果,评估了模式对淮河流域夏季降水的预报技巧。分析结果表明,模式总体上可以较好地再现出淮河流域夏季平均降水南多北少的空间分布特征,其中模式模拟的6月降水量与观测值的空间相关可达0.93。但降水强度与观测相比具有系统性的偏差,且模式模拟的降水年际变率显著偏弱。基于降水距平相关系数的确定性预报技巧分析表明,模式对流域西南部夏季降水的预测技巧较高,达到0.2以上,且模式对6月降水异常的预测能力相对最好,7月次之。针对淮河不同子流域的预报技巧分析表明,IAP AGCM 4. 1对蚌埠、鲁台子、王家坝水文控制站以上集水面积的夏季面雨量异常具有一定的预报技巧,30 a集合回报的时间相关系数分别为0. 11、0. 13、0. 16。基于降水等级的概率预报技巧评估表明,模式对7月淮河流域南部少雨事件具有很好的预报能力,同时对6月流域中部多雨事件的预报技巧也较高。     

Quantitative forecast experiment of a heavy rainfall event over Korea in a global model: horizontal resolution versus lead time issues

In this study, we examine the deterministic predictability of heavy rainfall over the Korean peninsula using a global model, the Global/Regional Integrated Model system, by focusing on the effects of horizontal resolution and lead time prior to the onset of the target event. The control run reproduces locally concentrated heavy rainfall over the mid-western part of the Korean peninsula on 27–29 July 2011, with a model setup of about 25 km grid spacing and 24 h in advance of the onset of heavy rainfall. We found that small-scale features are represented well at higher resolution, but without significant change in the distribution of rainfall. Increase of lead time reduces the predictability of large-scale features, accompanying a northward shift of major rainfall. At lower resolution, the accuracy of the heavy rainfall prediction decreases more rapidly as lead time increases. We concluded that the increase in predictability of heavy rainfall achieved by enhancing horizontal resolution is promising, but an additional set of experiments also indicates that efforts should be made to improve the physics packages in models.     

Evaluation of All-Sky Assimilation of FY-3C/MWHS-2 on Mei-yuPrecipitation Forecasts over the Yangtze-Huaihe River Basin

正1School of Atmospheric Sciences, Chengdu University of Information Technology, Chengdu 610025, China2International Center for Climate and Environment Sciences, Institute of Atmospheric Physics,Chinese Academy of Sciences, Beijing 100029, China3University of Chinese Academy of Sciences, Beijing 100049, China     

土地利用变化对长江流域气候及水文过程影响的敏感性研究

使用区域气候模式(RegCM3)和大尺度汇流模型(LRM), 研究土地利用/植被覆盖变化对长江流域气候及水文过程的影响。RegCM3嵌套于欧洲数值预报中心 (ECMWF) 再分析资料ERA40, 分别进行了中国区域在实际植被和理想植被分布情况下两个各15年 (1987～2001年) 时间长度的积分试验。随后, RegCM3 两个试验的输出径流结果分别用来驱动LRM, 研究土地利用/植被覆盖变化对长江流域河川径流的影响。研究结果指出, 中国当代土地利用变化对长江流域降水、蒸散发、径流深及河川径流等水文气候要素的改变较大, 对气温的改变并不明显。土地利用变化引起长江干流河川径流量在夏季(6～8月)有所增加, 并且越向下游增加幅度越大, 其中大通站径流量增加接近15%。总体而言, 土地利用改变加剧了长江流域夏季水循环过程, 使得夏季长江中下游地区降水增多, 径流增大。     

Performance evaluation and bias correction of gridded precipitation products over Arun River Basin in Nepal for hydrological applications

Theoretical and Applied Climatology - An accurate estimation of precipitation amount is crucial for various studies and planning related to water resource management, effective flood prediction and...     

海南岛海风雷暴结构的数值模拟

利用WRF(ARW)V3.6模式模拟了2010年10月5-6日发生在海南的一次秋季大暴雨过程,从降水、风场、反射率和云结构等方面分析WRF模式中3个积云参数化方案(KF,BMJ,TiedTke)和4个微物理参数化方案(Lin et al,WSM5,WSM6,Thompson)对海南岛秋季暴雨模拟的影响。结果表明:此次秋季暴雨过程模拟对不同的积云参数化方案和微物理参数化方案组合是比较敏感的,不同的积云参数化方案和微物理参数化方案组合通过调整温湿场结构,从而影响模拟降水的时间、强度和落区。对比发现,Thompson微物理方案的组合对于降水量级的模拟更为敏感,能较合理的描述暴雨发生发展过程中的水汽输送、热力和动力条件,并通过影响雨水混合比和云水混合比的高度和大小从而影响降水。其中Thompson微物理方案和TiedTke积云方案的组合能较好的模拟出本次暴雨过程的特征,与实测最为接近,该组合模拟的最大垂直速度和反射率区与最大云水混合比对应。另外,积云方案和微物理方案的选择不影响水汽混合比的模拟。

     

Experiments using new initial soil moisture conditions and soil map in the Eta model over La Plata Basin

An effort towards a more accurate representation of soil moisture and its impact on the modeling of weather systems is presented. Sensitivity tests of precipitation to soil type and soil moisture changes are carried out using the atmospheric Eta model for the numerical simulation of the development of a mesoscale convective system over northern Argentina. Modified initial soil moisture conditions were obtained from a hydrological balance model developed and running operationally at INPE. A new soil map was elaborated using the available soil profile information from Brazil, Paraguay, Uruguay, and Argentina and depicts 18 different soil types. Results indicate that more accurate initial soil moisture conditions and incorporating a new soil map with hydraulic parameters, more representative of South American soils, improve daily total precipitation forecasts both in quantitative and spatial representations.     

黄河流域未来气候-水文变化的模拟研究

将大尺度半分布式水文模型VIC应用到黄河上中游流域(花园口水文断面以上),并利用区域气候模式RegCM4.0单向嵌套全球气候模式BCC_CSM1.1,动力降尺度到黄河流域的模拟结果驱动VIC模型,开展在新的典型浓度路径下(RCP4.5和RCP8.5)黄河流域未来气候和水文变化的离线模拟。模拟结果显示,在RCP4.5和RCP8.5排放情景下,黄河流域21世纪平均地表气温相对于1971—2000年均呈显著上升趋势,2019—2048年上升1.2—1.5℃,2069—2098年上升2.19—3.9℃。未来年平均降水量有微弱的增大,2019—2048年增幅为6%左右,2069—2098年增幅为1.4%—5.6%。未来蒸发量增大明显,2069—2098年年平均蒸发量最大可增加9.6%。2019—2048年花园口水文站的年平均径流量增大3.4%—7.4%,2069—2098年年平均径流量转为减少,减幅为3.3%—5.3%。黄河上游地区未来气候和水文变化趋势与黄河流域基本一致,但未来年径流量变幅低于黄河流域,相对比较稳定。     

一次江淮暴雨高分辨率数值预报中云微物理方案敏感性分析

基于全可压非静力中尺度预报模式WRF,选取Lin、WSM3、WSM5、WSM6、Goddard五种云微物理方案和Kain-Fritsch积云对流参数化方案,对2017年6月10日的江淮暴雨过程开展高分辨率数值预报试验,重点研究了云微物理方案对强降水预报的敏感性。结果表明:Lin方案模拟的局地暴雨区降水量随时间的演变与实况较为吻合,但降雨量偏小,WSM5、WSM6和Goddard方案模拟的降水量级与实况更为吻合;不同云微物理方案对此次江淮暴雨的预报能力具有明显差异,小雨量区域的模拟效果基本一致,暴雨和大暴雨对云微物理方案更加敏感;云中水成物的三维结构特征差异明显,其水成物含量也显著不同。WSM5方案模拟的雨水和云水含量较高,其降水量和落区质量较好;不同微物理方案产生差异明显的垂直速度,导致云量、云高有所差异,进而影响降水预报的性能,说明选用更为敏感的云微物理方案对降水预报质量的改善具有重要作用。     

四川盆地暴雨对初值敏感性的模拟研究

本文基于区域暴雨数值预报模式AREM,针对2007年7月发生在四川地区的多次强降水过程进行数值试验,检验了NCEP和站点资料（STN）初始分析场资料预报结果,发现由于台站资料稀少,NCEP资料在四川地区的评分较高。讨论了四川盆地降水对初值的高度敏感性,揭示了四川盆地降水对初值中各个物理量场的不同敏感性,其中,降水对初值中湿度场的响应最为显著。初值不仅决定着降水的范围和强度,还对降水的发生时间产生明显影响。     

四川盆地暴雨对初值敏感性的模拟研究

本文基于区域暴雨数值预报模式AREM,针对2007年7月发生在四川地区的多次强降水过程进行数值试验,检验了NCEP和站点资料(STN)初始分析场资料预报结果,发现由于台站资料稀少,NCEP资料在四川地区的评分较高。讨论了四川盆地降水对初值的高度敏感性,揭示了四川盆地降水对初值中各个物理量场的不同敏感性,其中,降水对初值中湿度场的响应最为显著。初值不仅决定着降水的范围和强度,还对降水的发生时间产生明显影响。