Surface mean temperature from the observational stations and multiple reanalyses over the Tibetan Plateau

The Tibetan Plateau (TP), also called the “Third pole”, is sensitive to climate change due to extensive areas at high elevation presently dominated by snow and ice. In this study, observed surface temperature trends at 150 stations over the TP during 1979–2018 are analyzed and compared with surface temperatures from multiple reanalyses (NCEP1, NCEP2, ERA-Interim, MERRA, JRA55). Observed warming at the stations has a mean annual rate of 0.46 °C/decade during 1979–2018. Although all reanalyses underestimate observed temperatures (cold bias), most reproduce much of the inter-decadal variations of surface temperature shown in the observations. Absolute errors of mean surface temperature (reanalysis minus observation) are closely correlated with elevation errors, suggesting that parts of the cold bias can be interpreted by elevation errors of reanalysis. After elevation-temperature correction, about half of the cold bias is typically eliminated, more for both ERA-Interim and JRA55. Compared with the observations, corrected NCEP2 surface temperatures still have larger cold biases, and fail to capture the overall warming over the TP. Since the elevation-temperature correction fails to improve trend magnitudes even when a significant proportion of the bias has been removed, this suggests that a more sophisticated modeling of the lapse rate in each reanalysis is required to realistically model warming trends across complex topography.

     

A comparison of reanalyses in the tropical stratosphere. Part 1: thermal structure and the annual cycle

 An intercomparison of the thermal structure and the annual cycle in the tropical lower stratosphere of two reanalysis datasets is presented. These are from the National Centers for Environmental Prediction (NCEP)/National Center for Atmospheric Research (NCAR) and the European Centre for Medium Range Weather Forecasts (ECMWF Re-Analysis: ERA). Generally, the ERA data are coldest and in better agreement with radiosonde observations; this is particularly apparent at 100 hPa where there is also a strong geographic bias, the maximum differences (more than 4 K) occurring over the southern Pacific and the Indian Ocean, with much smaller (sometimes reversed) differences over land. The NCEP temperatures are biased towards satellite-derived values, while the ERA data resolve the low tropopause temperatures much better. The lower ERA temperatures have important implications for the cross-tropopause exchange of water vapor. The meridional-height structure of the annual cycles agree quite well, but the amplitude in the ERA data is about 50% stronger than in NCEP at 70 hPa (in better agreement with previous studies) and weaker at lower pressures. As in previous studies, an anticorrelation is found between the tropical and extratropical temperatures of the reanalyses. The mean meridional flow at the equator is northward all year at all stratospheric levels in the NCEP data, implying a mass transport from the Southern to the Northern Hemisphere; in the ERA data the expected annual cycle (flow from summer to winter) is reproduced with very small annual mean exchange. Received: 17 June 1997/Accepted: 17 December 1997     

多套大气再分析资料的南亚高压强度变化特征及其与海表温度异常关系的比较分析

以往的研究中多采用NCE/NCAR再分析资料来讨论南亚高压的变化特征及其与海表温度的关系,鉴于其分析结果具有一定的片面性,本文采用ERA40、ERA—Interim、NCEWNCAR、NCEP—DOE和JRA．25五套再分析资料,以及应用全球、热带印度洋和热带大西洋1978--2008年逐月观测海表温度分别驱动NCARCAM5．1全球大气环流模式的数值模拟结果,比较了它们的夏季南亚高压强度变化特征及其与海表温度的关系。再分析资料问的比较结果表明,NCEWNCAR、NCEP—DOE两套再分析资料与ERA40、ERA—Interim、JRA-25三套再分析资料的南亚高压强度变化在20世纪70年代末至90年代初存在非常明显的差异,前两套再分析资料揭示的该时段南亚高压强度显著偏高,可能是不真实的,进而导致南亚高压强度与海表温度异常的关系与后三套再分析资料的结果差异明显。ERA40、ERA—Interim和JRA-25三套再分析资料和数值试验结果均表明,20世纪70年代末以后,夏季南亚高压强度异常与前期冬季、春季及同期夏季的热带印度洋海表温度异常关系持续密切,表明热带印度洋是影响夏季南亚高压强度变化的关键海区。当热带印度洋偏暖时,热带地区对流层温度增暖,南亚高压强度增强、面积增大、南扩、东伸西展,反之亦然。     

Trends and variability in column-integrated atmospheric water vapor

An analysis and evaluation has been performed of global datasets on column-integrated water vapor (precipitable water). For years before 1996, the Ross and Elliott radiosonde dataset is used for validation of European Centre for Medium-range Weather Forecasts (ECMWF) reanalyses ERA-40. Only the special sensor microwave imager (SSM/I) dataset from remote sensing systems (RSS) has credible means, variability and trends for the oceans, but it is available only for the post-1988 period. Major problems are found in the means, variability and trends from 1988 to 2001 for both reanalyses from National Centers for Environmental Prediction (NCEP) and the ERA-40 reanalysis over the oceans, and for the NASA water vapor project (NVAP) dataset more generally. NCEP and ERA-40 values are reasonable over land where constrained by radiosondes. Accordingly, users of these data should take great care in accepting results as real. The problems highlight the need for reprocessing of data, as has been done by RSS, and reanalyses that adequately take account of the changing observing system. Precipitable water variability for 1988–2001 is dominated by the evolution of ENSO and especially the structures that occurred during and following the 1997–98 El Niño event. The evidence from SSM/I for the global ocean suggests that recent trends in precipitable water are generally positive and, for 1988 through 2003, average 0.40±0.09 mm per decade or 1.3±0.3% per decade for the ocean as a whole, where the error bars are 95% confidence intervals. Over the oceans, the precipitable water variability relates very strongly to changes in SSTs, both in terms of spatial structure of trends and temporal variability (with a regression coefficient for 30°N–30°S of 7.8% K^?1) and is consistent with the assumption of fairly constant relative humidity. In the tropics, the trends are also influenced by changes in rainfall which, in turn, are closely associated with the mean flow and convergence of moisture by the trade winds. The main region where positive trends are not very evident is over Europe, in spite of large and positive trends over the North Atlantic since 1988. A much longer time series is probably required to obtain stable patterns of trends over the oceans, although the main variability could probably be deduced from past SST and associated precipitation variations.     

Spurious shifts in the pattern of diurnal variation of sea level pressure of reanalysis datasets

The main purpose of this work is to report the presence of spurious discontinuities in the pattern of diurnal variation of sea level pressure of the three reanalysis datasets from: the National Centers for Environmental Prediction (NCEP) and National Center for Atmospheric Science (R1), the NCEP and Department of Energy (R2), and the European Centre for Medium Range Weather Forecasting (ERA-40). Such discontinuities can be connected to the major changes in the global observing system that have occurred throughout reanalyses years. In the R1, the richest period in discontinuities is 1956–1958, coinciding with the start of modern radiosonde observation network. Rapid increase in the density of surface-based observations from 1967 also had an important impact on both R1 and ERA-40, with larger impact on R1. The reanalyses show discontinuities in the 1970s related to the assimilation of radiances measured by the Vertical Temperature Profile Radiometer and TIROS-N Operational Vertical Sounders onboard satellites. In the ERA-40, which additionally assimilated Special Sensor Microwave/Imager data, there are discontinuities in 1987–1989. The R1 also presents further discontinuities, in 1988–1993 likely connected to replacement/introduction of NOAA-series satellites with different biases, and to the volcanic eruption of Mount Pinatubo in June 1991, which is known to have severely affected measurements of infrared radiances for several years. The discontinuities in 1996–1998 might be partially connected to change in the type of radiosonde, from VIZ-B to VIZ-B2. The R2, which covers only satellite era (1979-on), shows discontinuities mainly in 1992, 1996–1997, and 2001. The discontinuities in 1992 and 2001 might have been caused by change in the satellite measurements and those in 1996–1997 by some changes in land-based observations network.     

对流层和平流层温度中ENSO信号的多种资料比较

本文利用1980～1999年卫星资料Microwave Sounding Unit (MSU)和5种再分析资料(ERA40、JRA25、NCEP1、NCEP2、MERRA),分析了北半球冬季全球对流层中层和平流层低层温度变化中的ENSO信号,讨论了ENSO暖冷位相时温度异常的对称性和非对称性,并以MSU卫星资料为标准,...     

An assessment of the surface climate in the NCEP climate forecast system reanalysis

This paper analyzes surface climate variability in the climate forecast system reanalysis (CFSR) recently completed at the National Centers for Environmental Prediction (NCEP). The CFSR represents a new generation of reanalysis effort with first guess from a coupled atmosphere?Cocean?Csea ice?Cland forecast system. This study focuses on the analysis of climate variability for a set of surface variables including precipitation, surface air 2-m temperature (T2m), and surface heat fluxes. None of these quantities are assimilated directly and thus an assessment of their variability provides an independent measure of the accuracy. The CFSR is compared with observational estimates and three previous reanalyses (the NCEP/NCAR reanalysis or R1, the NCEP/DOE reanalysis or R2, and the ERA40 produced by the European Centre for Medium-Range Weather Forecasts). The CFSR has improved time-mean precipitation distribution over various regions compared to the three previous reanalyses, leading to a better representation of freshwater flux (evaporation minus precipitation). For interannual variability, the CFSR shows improved precipitation correlation with observations over the Indian Ocean, Maritime Continent, and western Pacific. The T2m of the CFSR is superior to R1 and R2 with more realistic interannual variability and long-term trend. On the other hand, the CFSR overestimates downward solar radiation flux over the tropical Western Hemisphere warm pool, consistent with a negative cloudiness bias and a positive sea surface temperature bias. Meanwhile, the evaporative latent heat flux in CFSR appears to be larger than other observational estimates over most of the globe. A few deficiencies in the long-term variations are identified in the CFSR. Firstly, dramatic changes are found around 1998?C2001 in the global average of a number of variables, possibly related to the changes in the assimilated satellite observations. Secondly, the use of multiple streams for the CFSR induces spurious jumps in soil moisture between adjacent streams. Thirdly, there is an inconsistency in long-term sea ice extent variations over the Arctic regions between the CFSR and other observations with the CFSR showing smaller sea ice extent before 1997 and larger extent starting in 1997. These deficiencies may have impacts on the application of the CFSR for climate diagnoses and predictions. Relationships between surface heat fluxes and SST tendency and between SST and precipitation are analyzed and compared with observational estimates and other reanalyses. Global mean fields of surface heat and water fluxes together with radiation fluxes at the top of the atmosphere are documented and presented over the entire globe, and for the ocean and land separately.     

A comparison of reanalyses in the tropical stratosphere. Part 3: inclusion of the pre-satellite data era

 In this study, previous evaluations of the monthly mean structure of the tropical lower stratosphere in reanalyzed datasets are extended to include the period 1958–1978, when no satellite-based observations were available. It is shown that a large discontinuity, in temperatures near the tropical tropopause, in the NCEP data occurred when the Tiros Operational Vertical Sounder (TOVS) became operational. When only rawinsonde data were available, the tropopause temperatures in the NCEP dataset are in better agreement with ERA data for TOVS period. Both NCEP and NASA reanalyses show similar deviations from the ERA data in the TOVS renalyses show similar deviations from the ERA data in the TOVS period. There is also a stepwise change in the lower stratospheric meridional velocity when the TOVS data were introduced into the NCEP reanalyses. This discontinuity is such that in the 1958–1978 period, the annual cycle in zonal mean meridional velocity in the NCEP data resembles that of the ERA data in the 1979–1993 period. The differences are shown to result from large changes in the local meridional flow in the Indonesian region. The temporal consistency of the QBO is examined; it is shown that the NCEP assimilation system is sensitive to the data available. There is a change in the zonally asymmetric structure of the zonal wind over time, presumably related to the changes in input data and the inability of the model to represent the three-dimensional structure of the tropical lower stratosphere. These results provide further evidence of the value of rawinsonde data in data assimilation systems as well as the need to use satellite radiance data in an appropriate manner. Received: 7 April 1997 / Accepted: 4 September 1998     

A comparison of reanalyses in the tropical stratosphere. Part 2: the quasi-biennial oscillation

 Reanalysis datasets potentially offer the opportunity to examine the tropical quasi-biennial oscillation (QBO) in greater detail than in the past, including the associated meridional circulation and the links with other parts of the atmosphere. For such studies to be useful, the QBO represented by the reanalyses should be realistic. In this work, the QBO in the ERA and NCEP reanalyses is validated against rawinsonde observations from Singapore. Monthly mean data are used. In the lower stratosphere (at 50 hPa and 30 hPa) the ERA QBO is reasonable, although the wind extrema in both phases are too weak and the vertical shear and the temperature anomalies are too small. The NCEP QBO is weaker still. At 10 hPa neither reanalysis system performs well, both systems failing to reproduce the westerlies, possibly because of the proximity of the upper boundary. The Singapore wind is representative of the zonal means in the reanalyses. The weak wind extrema in the reanalyses would not support a wave-mean flow interaction theory of the QBO, because a large portion of the gravity wave spectrum which would be absorbed in reality would be transmitted beyond 10 hPa. The stronger shear zones captured in the ERA data are associated with larger, more realistic temperature perturbations near 30 hPa. The northward velocities in the NCEP data show a more realistic structure than in the ERA reanalysis, where they are dominated by a vertical “gridpoint wave” structure in the lowermost stratosphere. Despite the shortcomings of the reanalyses, the high correlations of the wind at 30 hPa and 50 hPa with the observations at Singapore mean that the reanalyses could potentially be used to examine the effects of the QBO away from the tropical stratosphere. Future reanalyses need to take full account of the wind shears evident in the rawinsonde observations and use models with an adequate resolution to capture these vertical scales. Received: 23 June 1997/Accepted 17 December 1998     

Hydrological issues in lateral boundary conditions for regional climate modeling: simulation of east asian summer monsoon in 1998

The atmospheric branch of the hydrological cycle associated with the East Asian summer monsoon is intricate due to its distinct land-sea configurations: the highest mountains are to its west, the oceans are to its south and east, and mid-latitude influences come from its north. Here we use the weather research and forecast (WRF) model to demonstrate that using two different large-scale driving fields, derived from the NCEP/DOE R2 and ERA40 reanalysis data and the same model configuration yielded remarkable differences. We found that the differences are primarily caused by uncertainties in the water vapor influx across the lateral boundaries in the reanalyses. The summer-mean water vapor convergence into the model domain computed from the ERA40 reanalysis is 47% higher than that from the R2 reanalysis. The largest uncertainties in moisture transport are found in the regions of the Philippine Sea and the Bay of Bengal, where the moisture transport has the most significant impacts on the East Asian summer monsoon rainfall distribution. The sensitivity test results suggest that the biases in the seasonal mean, seasonal march of the rain band, and individual rainfall events may be reduced by using an “ensemble” average of R2 and ERA40 as lateral boundary forcing. While the large-scale forcing field does not conserve water vapor, the WRF simulation conserves water vapor in the inner model domain. The regional model simulation has corrected the biases in the total amount and the month-to-month distribution of precipitation in the large-scale driving field. However, RCM’s daily precipitation is poorer than that in the reanalysis filed. Since the RCM solutions may sensitively depend on the reanalysis forcing, intercomparison of models’ performance based on a single set of the reanalysis may not be reliable. This calls for attention to reshape our strategy for validation of RCMs.     

On the evaluation of temperature trends in the tropical troposphere

A series of model experiments with the coupled Max-Planck-Institute ECHAM5/OM climate model have been investigated and compared with microwave measurements from the Microwave Sounding Unit (MSU) and re-analysis data for the period 1979?C2008. The evaluation is carried out by computing the Temperature in the Lower Troposphere (TLT) and Temperature in the Middle Troposphere (TMT) using the MSU weights from both University of Alabama (UAH) and Remote Sensing Systems (RSS) and restricting the study to primarily the tropical oceans. When forced by analysed sea surface temperature the model reproduces accurately the time-evolution of the mean outgoing tropospheric microwave radiation especially over tropical oceans but with a minor bias towards higher temperatures in the upper troposphere. The latest reanalyses data from the 25?year Japanese re-analysis (JRA25) and European Center for Medium Range Weather Forecasts Interim Reanalysis are in very close agreement with the time-evolution of the MSU data with a correlation of 0.98 and 0.96, respectively. The re-analysis trends are similar to the trends obtained from UAH but smaller than the trends from RSS. Comparison of TLT, computed from observations from UAH and RSS, with Sea Surface Temperature indicates that RSS has a warm bias after 1993. In order to identify the significance of the tropospheric linear temperature trends we determined the natural variability of 30-year trends from a 500?year control integration of the coupled ECHAM5 model. The model exhibits natural unforced variations of the 30?year tropospheric trend that vary within ±0.2?K/decade for the tropical oceans. This general result is supported by similar results from the Geophysical Fluid Dynamics Laboratory (GFDL) coupled climate model. Present MSU observations from UAH for the period 1979?C2008 are well within this range but RSS is close to the upper positive limit of this variability. We have also compared the trend of the vertical lapse rate over the tropical oceans assuming that the difference between TLT and TMT is an approximate measure of the lapse rate. The TLT?CTMT trend is larger in both the measurements and in the JRA25 than in the model runs by 0.04?C0.06?K/decade. Furthermore, a calculation of all 30?year TLT?CTMT trends of the unforced 500-year integration vary between ±0.03?K/decade suggesting that the models have a minor systematic warm bias in the upper troposphere.     

热带大气ISO在几种再分析资料中的对比分析

利用NCEP/NCAR, NCEP/DOE和ERA40 3套再分析资料的逐日200 hPa纬向风数据,选取1961—1990年、1971—2000年和1981—2010年3种不同气候态,对比分析了3种气候态下热带大气季节内振荡 (ISO) 的基本气候特征及其在不同再分析资料中的异同。研究表明:1981—2010年气候态下,热带大气ISO冬春强、夏秋弱的年循环特征更加明显,东传短波能量增强,起始北传时间偏晚。NCEP/NCAR与NCEP/DOE资料所表征的热带大气ISO在空间分布、强度和能量传播方面的一致性较好。NCEP/NCAR资料反映的热带大气ISO强度在热带印度洋和热带西太平洋地区较ERA40资料偏弱,在赤道东太平洋地区较ERA40资料偏强;ERA40资料反映的热带大气ISO强度在12月—次年3月中旬较NCEP/NCAR资料偏强,而在3月中旬—11月偏弱;ERA40资料反映的热带大气ISO振荡位相较NCEP/NCAR资料超前10 d左右;NCEP/NCAR资料反映的东传谱能量弱于ERA40资料,西传能量强于ERA40资料;7月中旬,NCEP/NCAR资料反映的东亚地区大气ISO经向北传较ERA40资料偏晚。     

长时间序列多源土壤湿度产品在中国地区的比较分析

研究利用实测数据对遥感产品(ECV CCI)、全球陆面同化系统产品(GLDAS/CLM、NOAH2.7、NOAH3.3、VIC、MOSAIC)、再分析资料(ERA-Interim、NCEP/DOE)等不同来源的8套长时间序列土壤湿度产品进行时空比较。结果表明:上述产品均可以模拟出中国不同区域土壤湿度的空间分布;从各产品平均态与实测数据的偏差、均方根误差、相关系数统计结果来看,NOAH模式和ERA-Interim表现最好,MOSAIC和NCEP/DOE与实测相差较大;从区域尺度上来看,所有产品在东北区域表现最好,在华南、西南南区和西北西区较差;ERA-Interim有效的模拟了土壤湿度的年际变化。     

3种再分析资料的高空温度与中国探空温度资料的对比:年平均特征

利用中国105站的探空资料以及NCEP/NCAR、ERA和JRA 3种再分析资料,采用线性趋势、标准差、相关系数、EOF分析等多种统计分析方法,对再分析资料年平均的高空温度在中国区域的可信度进行了分析.研究表明:在数值上,3种再分析资料的高空温度均小于探空资料的高空温度,NCEP/NCAR资料在对流层上层更接近于探空资料,ERA和JRA资料则在对流层中下层与探空资料更为接近;在描述年际变化和长期变化趋势方面,ERA资料在我国北方的对流层上层的再现能力较好,NCEP/NCAR资料在我国南方的对流层上层的再现能力较好,而3种再分析资料在对流层中下层的再现能力相当;在时空变化特征方面,NCEP/NCAR和ERA资料能较好地表现高空温度的年代际变化特征,而ERA和JRA资料则能较好地表现年际变化特征.     

Representation of the Caribbean mean diurnal cycle in observation, reanalysis, and CMIP3 model datasets

This study explores how the Caribbean mean diurnal cycle is represented in observed, reanalyzed, and general circulation model data of the CMIP3 generation, with a focus on the central Antilles Islands (18–21° N, 80–66° W). Their trade wind climate, high solar forcing, and eastward decrease in size challenge dataset ability to represent a diurnal cycle of sufficient amplitude and correct phase. The mean difference between maximum and minimum temperature (ΔT) provides a useful metric, and values of 5–12°C are observed at stations around the Caribbean basin. The ΔT in various reanalyses and CMIP3 models generally yield correct values over South America, but few exhibit elevated ΔT over the central Antilles Islands. NCEP2 and ECMWF reanalyses differ in their representation despite similar resolution, the latter reflecting elevated ΔT. Only one of the four CMIP3 models evaluated correctly simulates Antilles ΔT, but does so with a dry bias. An intercomparison of mean diurnal rainfall in the Antilles is conducted. According to satellite and high-resolution reanalysis, the rain rate doubles in the afternoon. However NCEP2/ECMWF reanalysis and CMIP3 models yield an amplitude about half the observed and exhibit a nocturnal peak typical of marine climates. The reason for the variety of outcomes is related to model parameterizations that translate surface fluxes to boundary layer responses, and to horizontal resolution that affects the representation of sea breeze confluence over large islands.     

Tropical intraseasonal rainfall variability in the CFSR

While large-scale circulation fields from atmospheric reanalyses have been widely used to study the tropical intraseasonal variability, rainfall variations from the reanalyses are less focused. Because of the sparseness of in situ observations available in the tropics and strong coupling between convection and large-scale circulation, the accuracy of tropical rainfall from the reanalyses not only measures the quality of reanalysis rainfall but is also to some extent indicative of the accuracy of the circulations fields. This study analyzes tropical intraseasonal rainfall variability in the recently completed NCEP Climate Forecast System Reanalysis (CFSR) and its comparison with the widely used NCEP/NCAR reanalysis (R1) and NCEP/DOE reanalysis (R2). The R1 produces too weak rainfall variability while the R2 generates too strong westward propagation. Compared with the R1 and R2, the CFSR produces greatly improved tropical intraseasonal rainfall variability with the dominance of eastward propagation and more realistic amplitude. An analysis of the relationship between rainfall and large-scale fields using composites based on Madden-Julian Oscillation (MJO) events shows that, in all three NCEP reanalyses, the moisture convergence leading the rainfall maximum is near the surface in the western Pacific but is above 925?hPa in the eastern Indian Ocean. However, the CFSR produces the strongest large-scale convergence and the rainfall from CFSR lags the column integrated precipitable water by 1 or 2?days while R1 and R2 rainfall tends to lead the respective precipitable water. Diabatic heating related to the MJO variability in the CFSR is analyzed and compared with that derived from large-scale fields. It is found that the amplitude of CFSR-produced total heating anomalies is smaller than that of the derived. Rainfall variability from the other two recently produced reanalyses, the ECMWF Re-Analysis Interim (ERAI), and the Modern Era Retrospective-analysis for Research and Applications (MERRA), is also analyzed. It is shown that both the ERAI and MERRA generate stronger rainfall spectra than the R1 and more realistic dominance of eastward propagating variance than R2. The intraseasonal variability in the MERRA is stronger than that in the ERAI but weaker than that in the CFSR and CMORPH.     

Inconsistencies of precipitation in the eastern and central Tibetan Plateau between surface adjusted data and reanalysis

The Tibetan Plateau (TP) is the source of many Asian river systems and serves as “the Asian water tower”. Precipitation variability is a strong component of both hydrological processes and energy cycles, and the study of precipitation in the TP is of great importance in the content of global warming. In this study, the annual and seasonal (spring: MAM; summer: JJA; autumn: SON; and winter: DJF) variations in precipitation are investigated in the eastern and central TP during 1961–2007, based on surface raw and adjusted observations as well as both NCEP/NCAR (1961–2007) and ERA-40 (1961–2001) reanalyses. The adjusted precipitation in the TP is higher than raw values on both the annual and seasonal basis due to adjustments of solid precipitation by a bias experiential model. At the annual spring and winter scales, the adjusted precipitation shows a significant increase calculated by the Mann–Kendall trend test. Compared with adjusted precipitation; both NCEP/NCAR and ERA-40 reanalyses capture the broad spatial distributions of mean annual and seasonal precipitation, but are less good at repeating the decadal variability. Both reanalyses show the drying phenomena in most regions and fail to represent the change patterns of precipitation observed by the adjusted observations. Both NCEP/NCAR and ERA-40 have larger inconsistencies which may be caused by the differences between actual and model topography. This suggests that it is crucial to use the adjusted precipitation in the climate research and reanalysis products should be paid more attention in the TP.     

Large-scale variability modes of freshwater flux and precipitation over the Atlantic

 Precipitation (P) and freshwater (E-P) fluxes at the air-sea interface are investigated in the Atlantic Ocean sector using the reanalyses of the European Centre for Medium Range Weather Forecasts (ERA) and of the National Centers for Environmental Prediction (NCEP). A canonical correlation analysis method between these fields and sea level pressure (SLP) is used to identify patterns. We also test whether precipitation and freshwater fluxes can be reconstructed from SLP data. In the winter months, patterns associated with both the North Atlantic Oscillation (NAO) and the East Atlantic (EA) mode are identified. The signals are strong enough to be reconstructed from the reanalysis fields, and they correspond to a significant part of the variability. The NAO signal is more robust than the EA one. The NAO-related variability mode is also present when the monthly precipitation rate is averaged for the winter season and even for annual averages. However, in the later case, other variability of natural origin (for instance, ENSO variability) or noise from the model and assimilation system prevents the reconstruction of E-P associated with NAO from SLP variability. Difficulties are identified in the tropical Atlantic with a different behaviour of NCEP and ERA precipitation variability, especially near the Inter Tropical Convergence Zone (ITCZ). The ERA patterns suggest a NAO signature in the tropical Atlantic which has clear monthly patterns and indicates a link between the phase of NAO and changes in the position and intensity of ITCZ. However, the analysis of winter rainfall based on satellite and in situ data does not support the monthly tropical pattern of ERA precipitation although it suggests a relation between convection near 15°S and NAO during northern winter. Received: 10 February 2000 / Accepted: 7 May 2001     

Parallel comparison of the northern winter stratospheric circulation in reanalysis and in CMIP5 models

A parallel comparison is made of the circulation climatology and the leading oscillation mode of the northern winter stratosphere among six reanalysis products and 24 CMIP5(Coupled Model Intercomparison Project Phase 5) models. The results reveal that the NCEP/NCAR, NECP/DOE, ERA40, ERA-Interim and JRA25 reanalyses are quite consistent in describing the climatology and annual cycle of the stratospheric circulation. The 20 CR reanalysis, however, exhibits a remarkable"cold pole" bias accompanied by a much stronger stratospheric polar jet, similar as in some CMIP5 models. Compared to the1–2 month seasonal drift in most coupled general circulation models(GCMs), the seasonal cycle of the stratospheric zonal wind in most earth system models(ESMs) agrees very well with reanalysis. Similar to the climatology, the amplitude of Polar Vortex Oscillation(PVO) events also varies among CMIP5 models. The PVO amplitude in most GCMs is relatively weaker than in reanalysis, while that in most of the ESMs is more realistic. In relation to the "cold pole" bias and the weaker oscillation in some CMIP5 GCMs, the frequency of PVO events is significantly underestimated by CMIP5 GCMs; while in most ESMs, it is comparable to that in reanalysis. The PVO events in reanalysis(except in 20CR) mainly occur from mid-winter to early spring(January–March); but in some of the CMIP5 models, a 1–2 month delay exists, especially in most of the CMIP5 GCMs. The long-term trend of the PVO time series does not correspond to long-term changes in the frequency of PVO events in most of the CMIP5 models.     

Evaluation of Precipitation in Multi-Generation Reanalyses with Land Observations over Zhejiang Province

Based on various statistical indices, the abilities of multi-generation reanalyses, namely the NCEP/NCAR Reanalysis 1 (R1), the NCEP-DOE Reanalysis 2 (R2) and the NCEP Climate Forecast System Reanalysis (CFSR), to reproduce the spatiotemporal characteristics of precipitation over Zhejiang Province are comprehensively compared. The mean absolute bias percentages for three reanalyses are 20% (R1), 10% (R2) and 37% (CFSR). R2 (R1) gives the best (worst) general depiction of the spatial characteristics of the observed precipitation climatology, whereas a significant wet bias is noticed in CFSR. All reanalyses reasonably reproduce the interannual variability with the correlation coefficients of 0.72 (R1), 0.72 (R2) and 0.84 (CFSR). All reanalyses well represent the first two modes of the observed precipitation through Empirical Orthogonal Function analysis, with CFSR giving the best capture of the principal components. The root-mean-square error (RMSE) is the largest (smallest) in CFSR (R2). The large RMSE of CFSR in summer (especially in June) contributes mostly to its systematic wet bias. After 2001, the wet-bias of CFSR substantially weakens, probably attributed to increasing observations assimilated in the CFSR. On a monthly basis, the percentage of neutral bias cases are similar for all reanalyses, while the ratio of positive (negative) bias cases for CFSR is distinctly larger (smaller) than that of R1 and R2. The proportions of negative bias cases for R1 and R2 begin to increase after 2001 while keeping stable for CFSR. On a daily basis, all reanalyses give good performances of reproducing light rain; however, the reflection of moderate rain and heavier rain by CFSR is better than R1 and R2. Overall, despite being a third-generation reanalysis product, CFSR does not exhibit comprehensive superiorities over R1 and R2 in all aspects on a regional scale.