On the mesoscale distribution of precipitation deficits

Summary Minor shifts in the pattern of the general circulation frequently result in the development of a mosaic of localized wet and dry patches, creating regions of mesoscale drought. A method of identifying the spatial pattern of such droughts is developed, using the conditional probability of adjacent stations experiencing precipitation deficits simultaneously. The method was tested using seasonal data for the eastern portion of North Carolina, U.S.A. The resulting patterns can be related to the synoptic climatology of the area. Summer reveals numerous small areas of precipitation deficit, while the other seasons have larger deficit areas which reflect changing depression tracks, coastal influences, and rainshadow effects. Comparison of the results with the commonly used Palmer Drought Severity Index indicates that the latter is an appropriate measure only when drought is widespread, and cannot be used to identify mesoscale droughts. The present method allows identification of areas prone to mesoscale drought, suggests causal mechanisms, and provides a basis for monitoring the development of such droughts.

---

Zusammenfassung Geringfügige Verschiebungen der allgemeinen Zirkulation führen oft zur Entstehung quasi eines Mosaiks feuchter und trockener Flecken, wobei sich im Mesoscale Dürregebiete bilden.Mit Hilfe der bedingten Wahrscheinlichkeit, an Nachbarstationen gleichzeitig Niederschlagsdefizite festzustellen, wurde eine Methode entwickelt, das räumliche Muster dieser Dürre zu beschreiben. Diese Methode wurde anhand jahreszeitlicher Daten für den Osten North Carolinas (USA) erprobt. Die daraus resultierenden Verteilungsmuster können mit der synoptischen Klimatologie des Gebietes in Beziehung gebracht werden. Im Sommer verzeichnet man zahlreiche kleine Gebiete mit Niederschlagsdefiziten, während in den anderen Jahreszeiten größere Trockengebiete auszumachen sind, die die unterschiedlichen Zugbahnen der Tiefdruckgebiete, Küsteneinflüsse und Regenschatteneffekte widerspiegeln. Ein Vergleich mit dem üblicherweise verwendeten Palmer Drought Severity Index zeigt, daß letzterer nur ausgedehnte Trockenheiten im geeigneten Maß darstellt, die Identifikation von Dürre im Mesoscale jedoch nicht zuläßt. Die vorliegende Methode erlaubt eine Identifikation jener Gebiete, die für mesoskale Dürre predästiniert sind, gibt ein Bild der kausalen Zusammenhänge und schafft damit die Grundlage zur Beobachtung der Entstehung solcher Trokkenheiten.

---

---

With 6 Figures     

On the tropical origin of uncertainties in the global land precipitation response to global warming

Understanding the response of the global hydrological cycle to recent and future anthropogenic emissions of greenhouse gases and aerosols is a major challenge for the climate modelling community. Recent climate scenarios produced for the fourth assessment report of the Intergovernmental Panel on Climate Change are analysed here to explore the geographical origin of, and the possible reasons for, uncertainties in the hydrological model response to global warming. Using the twentieth century simulations and the SRES-A2 scenarios from eight different coupled ocean–atmosphere models, it is shown that the main uncertainties originate from the tropics, where even the sign of the zonal mean precipitation change remains uncertain over land. Given the large interannual fluctuations of tropical precipitation, it is then suggested that the El Niño Southern Ocillation (ENSO) variability can be used as a surrogate of climate change to better constrain the model reponse. While the simulated sensitivity of global land precipitation to global mean surface temperature indeed shows a remarkable similarity between the interannual and climate change timescales respectively, the model ability to capture the ENSO-precipitation relationship is not a major constraint on the global hydrological projections. Only the model that exhibits the highest precipitation sensitivity clearly appears as an outlier. Besides deficiencies in the simulation of the ENSO-tropical rainfall teleconnections, the study indicates that uncertainties in the twenty-first century evolution of these teleconnections represent an important contribution to the model spread, thus emphasizing the need for improving the simulation of the tropical Pacific variability to provide more reliable scenarios of the global hydrological cycle. It also suggests that validating the mean present-day climate is not sufficient to assess the reliability of climate projections, and that interannual variability is another suitable and possibly more useful candidate for constraining the model response. Finally, it is shown that uncertainties in precipitation change are, like precipitation itself, very unevenly distributed over the globe, the most vulnerable countries sometimes being those where the anticipated precipitation changes are the most uncertain.     

全球地面降水月值历史数据集研制

全球降水历史数据是开展气候、水循环等研究的基础。收集整理全球12个数据源降水历史月值资料,通过站号、站名甄别不同数据源中相同台站,对344个通过相关系数、一致率、均值t检验、方差F检验的台站多源资料进行拼接,尽可能多地融合各套数据产品优势,最终形成全球降水历史月值数据集(CMA Global PrecipitationV1.0, CGP)。数据集重点解决当前国际数据产品在东亚地区站点稀少、同时应用多套数据应用门槛较高等问题。数据集收录3.1万个台站共计1.87×107组月降水记录, 4152个台站序列长度达百年。与美国大气海洋局(NOAA)的全球降水数据集(GHCN-M V2.0)对比,CGP新增1万个站点、0.5×107组有效观测记录和1030条百年序列,其中141条百年序列通过多源整合技术获取。CGP的站点和数据量优势主要体现在东亚、东欧、西伯利亚等站点稀疏地区。基于CGP分析的全球降水时空特征与国际同类产品的结果较一致。新增的数据虽然没有改变全球降水分布的总体特征,但对区域性的百年降水变化检测有一定影响。基于CGP的全球降水百年序列结果显示,20世纪前半叶全球降水量偏小,近20年是1900年以来全球降水量最大的时期,各纬度带、各个国家或地区的降水长期变化趋势呈现显著的差异。      

降雨过程中雨水pH值连续观测资料分析

1985年我们在上海对每次降水过程进行了雨水连续短时间分段采样观测。从雨水pH值和降雨量的观测资料分析得到:在降水过程中雨水pH值是随时间变化的;雨水酸度是与降水的天气背景和降水云系有关。     

The global distribution of methane in the troposphere

Methane has been measured in air samples collected at approximately weekly intervals at 23 globally distributed sites in the NOAA/GMCC cooperative flask sampling network. Sites range in latitude from 90° S to 76° N, and at most of these we report 2 years of data beginning in early 1983. All measurements have been made by gas chromatography with a flame ionization detector at the NOAA/GMCC laboratory in Boulder, Colorado. All air samples have been referenced to a single secondary standard of methane-in-air, ensuring a high degree of internal consistency in the data. The precision of measurements is estimated from replicate determinations on each sample as 0.2%. The latitudinal distribution of methane and the seasonal variation of this distribution in the marine boundary layer has been defined in great detail, including a remarkable uniformity in background levels of methane in the Southern Hemisphere. We report for the first time the observation of a complete seasonal cycle of methane at the South Pole. A significant vertical gradient is observed between a sea level and a high altitude site in Hawaii. Globally averaged background concentrations in the marine boundary layer have been calculated for the 2 year-period May 1983–April 1985 inclusive, from which we find an average increase of 12.8 ppb per year, or 0.78% per year when referenced to the globally averaged concentration (1625 ppb) at the mid-point of this period. We present evidence that there has been a slowing down in the methane growth rate.Presented at the Conference on the Scientific Application of Baseline Observations of Atmospheric Composition (SABOAC), Aspendale, Australia, 7–9 November 1984.     

Analyses of summer lightning activity and precipitation in the Central and Eastern Mediterranean

The lightning activity and precipitation in two 3-hour time intervals in the grid boxes of 0.25 × 0.25° over East and Central Mediterranean during the summer of 2005 and 2006 are analysed. The results show that the frequency distribution of the precipitation amount is shifted towards larger values for the cases with lightning as compared with the cases without lightning. It was found that the number of cases with 3-hour accumulated rainfall greater than 10 mm was bigger when lightning occurred (65%) than when it was absent (35%). Investigation of diurnal and spatial distributions of lightning shows that the afternoon flash density peak is associated mainly with lightning over the land, which is in accordance with the results of earlier works. The early morning flash density peak is associated mainly with flashes over the sea. High correlation coefficients (0.89 during the morning hours and 0.98 during afternoon) were found between rain rate (mm/h) and average flash density (fl/km²) when flash density is averaged in logarithmic intervals of rain rate.     

A statistical downscaling scheme to improve global precipitation forecasting

Based on hindcasts obtained from the “Development of a European Multimodel Ensemble system for seasonal to inTERannual prediction” (DEMETER) project, this study proposes a statistical downscaling (SD) scheme suitable for global precipitation forecasting. The key idea of this SD scheme is to select the optimal predictors that are best forecast by coupled general circulation models (CGCMs) and that have the most stable relationships with observed precipitation. Developing the prediction model and further making predictions using these predictors can extract useful information from the CGCMs. Cross-validation and independent sample tests indicate that this SD scheme can significantly improve the prediction capability of CGCMs during the boreal summer (June–August), even over polar regions. The predicted and observed precipitations are significantly correlated, and the root-mean-square-error of the SD scheme-predicted precipitation is largely decreased compared with the raw CGCM predictions. An inter-model comparison shows that the multi-model ensemble provides the best prediction performance. This study suggests that combining a multi-model ensemble with the SD scheme can improve the prediction skill for precipitation globally, which is valuable for current operational precipitation prediction.     

On the effect of ENSO precipitation anomalies in a global ocean GCM

An ocean general circulation model is used to study the influence of positive precipitation anomalies associated with El Nino and La Nina events. In this idealized model, the precipitation over the appropriate part of the equatorial Indo-Pacific region is doubled for one year. At the surface, salinity anomalies of up to –0.9 parts per thousand result from this anomalous precipitation. Perturbation surface currents ranging from 10–100% of the climatological values are induced in the tropical Indian and Pacific Oceans. A return flow is found beneath the thermocline with upwelling (downwelling) in (outside) the region of enhanced precipitation. The net effect of the precipitation anomalies is to generate a zonal overturning cell which transports fresher surface water away from the forcing region and replaces it with cooler, more saline water from below.     

APHRODITE高分辨率逐日降水资料在中国大陆地区的适用性

通过收集、整理中国历史上各类逐小时降水数据源,建立了中国国家级地面气象台站逐小时降水数据集。质量评估结果显示,绝大部分台站实有率在90%以上,中国东部沿海和贵州、湖南等省份数据实有率偏低。各台站数据正确率普遍较高,正确率低于98%的台站数占总站数的0.7%,正确率在98%~99%区间的台站占总站数的1.8%,其余97.5%的站数据正确率都高于99%。逐小时降水量的日合计量和人工观测日降水量差值在1 mm以内的占92.1%,在0.5~5 mm间的占16.37%,超过5 mm的不到9‰。

     

Representation of tropical subseasonal variability of precipitation in global reanalyses

Tropical subseasonal variability of precipitation from five global reanalyses (RAs) is evaluated against Global Precipitation Climatology Project (GPCP) and Tropical Rainfall Measuring Mission (TRMM) observations. The RAs include the three generations of global RAs from the National Center for Environmental Prediction (NCEP), and two other RAs from the European Centre for Medium-Range Weather Forecasts (ECMWF) and the National Aeronautics and Space Administration/Goddard Space Flight Center (NASA/GSFC). The analysis includes comparisons of the seasonal means and subseasonal variances of precipitation, and probability densities of rain intensity in selected areas. In addition, the space–time power spectrum was computed to examine the tropical Madden-Julian Oscillation (MJO) and convectively coupled equatorial waves (CCEWs). The modern RAs show significant improvement in their representation of the mean state and subseasonal variability of precipitation when compared to the two older NCEP RAs: patterns of the seasonal mean state and the amplitude of subseasonal variability are more realistic in the modern RAs. However, the probability density of rain intensity in the modern RAs show discrepancies from observations that are similar to what the old RAs have. The modern RAs show higher coherence of CCEWs with observed variability and more realistic eastward propagation of the MJO precipitation. The modern RAs, however, exhibit common systematic deficiencies including: (1) variability of the CCEWs that tends to be either too weak or too strong, (2) limited coherence with observations for waves other than the MJO, and (3) a systematic phase lead or lag for the higher-frequency waves.     

多普勒天气雷达速度场特征及在人工增雨中的应用

对大连2003-2005年降水天气过程的多普勒雷达速度场资料进行统计和分析,得到几种雷达回波速度场特征对降水变化及人工增雨作业的影响,总结了雷达回波速度场的零速度线呈"S"型、"弓"型和速度不对称型几种常见的分布特征对人工增雨潜力分析和作业条件判别的辅助判据指标.并结合EVAD技术和变分法定量计算大气平均散度和垂直速度方法,对大连2006年夏季一次人工增雨作业过程进行了分析,得到了回波速度场特征与计算的平均散度和垂直速度的较好的配合关系.     

Numerical simulation of the probability distribution function of precipitation over Morocco

A variable resolution version of the global GCM ARPEGE is constructed, so that Morocco has maximum resolution. A 30-year simulation, driven by observed sea surface temperature 1971–2000, is carried out. This paper examines the precipitation over northern Morocco during the extended winter season (from October to March), comparing model simulations with daily values at 14 stations. An approach utilising weather regimes has been adopted. The model is successful in representing the frequency and the interannual variability of the regimes. The precipitation over Morocco differs from one regime to another, but the model is not enough rainy along the Atlantic coast in general. The model is too persistent with too long dry spells, but is able to produce heavy rainfall as well as long dry periods in the centre of the area.     

GRAPES_GFS模式全球降水预报的主要偏差特征

利用2017年1、4、7、10月"全球降水观测(global precipitation measurement,GPM)计划"每日08时(北京时)的24 h累计降水量和逐30 min降水量观测产品,从降水量和频率等角度,对同期GRAPES全球模式(GRAPES_GFS)第1(D1)、3(D3)、5天(D5)的全球降水...     

山地降水的空间分布特征研究综述

降水是影响自然环境基础的因子，是影响气候变化的重要指标，关于降水尤其是山地降水的空间特征研究对防灾减灾和开发利用水资源具有重要意义，本文对山地降水中地形的影响、降水空间分布研究方法以及国内外研究动态进行了综述，认为：对山地降水空间特征的研究必须结合数据本身的特点和空间特性选择合适的插值方法，必须把随机插值方法和确定性方法结合起来，要想进一步减小误差，需要更多的降水数据和更为精准的地图数据。