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1.
An objective methodology is applied to ERA-40 (European Centre for Medium-Range Weather Forecasts 40-year Reanalysis) and NCEP/NCAR (National Centers for Environmental Prediction/National Center for Atmospheric Research) reanalyses, to build two storm-track databases for the Euro-Atlantic sector (85°W–70°E; 20°N–75°N), spanning the period December 1958–March 2000. The technique uses the full temporal (6-hourly) and spatial resolutions (1.125° and 2.5° regular grids, for ERA-40 and NCEP/NCAR, respectively) available. It is shown that the strong discrepancies in the number of storms in each dataset (higher for ERA-40) result from differences in the resolution of the fields subject to the storm detecting/tracking algorithm, and also from the characteristics of the integration models and assimilation schemes used for each reanalysis. An intercomparison of ERA-40 and NCEP/NCAR storm-tracks is performed for spatial distribution, and main characteristics, of the overall cyclone population and of a class of severe storms—explosive cyclones. Despite the discrepancies in storm numbers, both reanalyses agree on the main cyclone activity areas (formation, minimum central pressure, and lysis). The most pronounced differences occur where subsynoptic systems are frequent, as these are better resolved by ERA-40 data. The interannual variability of cyclone counts, analysed per intensity classes and for different regions of the domain, reveals reasonable agreement between the two datasets on the sign of trends (generally positive in northern latitudes, and negative in the Azores-Mediterranean band), but discrepancies regarding their strength in the most southern areas, where the mismatches between ERA-40 and NCEP/NCAR detected lows are greatest. Submitted to Climate Dynamics in December 2004  相似文献   

2.
The South Asian Highs (SAHs) at 100 hPa over China in the three reanalysis datasets NCEP1, NCEP2, and ERA-40 are evaluated by using station observation data. The results demonstrate a substantial discrepancy even between the reanalyses. First, the data of the three reanalyses generally underestimate the intensity of the SAH in the China domain. Second, there are interdecadal changes in the SAH, with highs in the 1960s and 1980s and lows in the 1970s, 1990s, and 2000s. This interdecadal variation of the SAH can be well depicted with NCEP1 data, but the high in the 1980s is missed by ERA-40. The NCEP2 corresponds well with NCEP 1 and captures the decreasing trend after 1979. Furthermore, the NCEP1 reanalysis overestimates the interdecadal changes of SAH, while ERA-40 underestimates the interdecadal changes. This work suggests that much caution should be exerted when the reanalysis datasets are adopted to study the interdecadal variability of SAH.  相似文献   

3.
In Colombia, the access to climate related observational data is restricted and their quantity is limited. But information about the current climate is fundamental for studies on present and future climate changes and their impacts. In this respect, this information is especially important over the Colombian Caribbean Catchment Basin (CCCB) that comprises over 80 % of the population of Colombia and produces about 85 % of its GDP. Consequently, an ensemble of several datasets has been evaluated and compared with respect to their capability to represent the climate over the CCCB. The comparison includes observations, reconstructed data (CPC, Delaware), reanalyses (ERA-40, NCEP/NCAR), and simulated data produced with the regional climate model REMO. The capabilities to represent the average annual state, the seasonal cycle, and the interannual variability are investigated. The analyses focus on surface air temperature and precipitation as well as on surface water and energy balances. On one hand the CCCB characteristics poses some difficulties to the datasets as the CCCB includes a mountainous region with three mountain ranges, where the dynamical core of models and model parameterizations can fail. On the other hand, it has the most dense network of stations, with the longest records, in the country. The results can be summarised as follows: all of the datasets demonstrate a cold bias in the average temperature of CCCB. However, the variability of the average temperature of CCCB is most poorly represented by the NCEP/NCAR dataset. The average precipitation in CCCB is overestimated by all datasets. For the ERA-40, NCEP/NCAR, and REMO datasets, the amplitude of the annual cycle is extremely high. The variability of the average precipitation in CCCB is better represented by the reconstructed data of CPC and Delaware, as well as by NCEP/NCAR. Regarding the capability to represent the spatial behaviour of CCCB, temperature is better represented by Delaware and REMO, while precipitation is better represented by Delaware. Among the three datasets that permit an analysis of surface water and energy balances (REMO, ERA-40, and NCEP/NCAR), REMO best demonstrates the closure property of the surface water balance within the basin, while NCEP/NCAR does not demonstrate this property well. The three datasets represent the energy balance fairly well, although some inconsistencies were found in the individual balance components for NCEP/NCAR.  相似文献   

4.
A multivariate analysis of the upper ocean thermal structure is used to examine the recent long-term changes and decadal variability in the upper ocean heat content as represented by model-based ocean reanalyses and a model-independent objective analysis. The three variables used are the mean temperature above the 14°C isotherm, its depth and a fixed depth mean temperature (250?m mean temperature). The mean temperature above the 14°C isotherm is a convenient, albeit simple, way to isolate thermodynamical changes by filtering out dynamical changes related to thermocline vertical displacements. The global upper ocean observations and reanalyses exhibit very similar warming trends (0.045°C per decade) over the period 1965–2005, superimposed with marked decadal variability in the 1970s and 1980s. The spatial patterns of the regression between indices (representative of anthropogenic changes and known modes of internal decadal variability), and the three variables associated with the ocean heat content are used as fingerprint to separate out the different contributions. The choice of variables provides information about the local heat absorption, vertical distribution and horizontal redistribution of heat, this latter being suggestive of changes in ocean circulation. The discrepancy between the objective analysis and the reanalyses, as well as the spread among the different reanalyses, are used as a simple estimate of ocean state uncertainties. Two robust findings result from this analysis: (1) the signature of anthropogenic changes is qualitatively different from those of the internal decadal variability associated to the Pacific Interdecadal Oscillation and the Atlantic Meridional Oscillation, and (2) the anthropogenic changes in ocean heat content do not only consist of local heat absorption, but are likely related with changes in the ocean circulation, with a clear shallowing of the tropical thermocline in the Pacific and Indian oceans.  相似文献   

5.
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.  相似文献   

6.
The spatial and temporal consistency of seasonal air temperature and precipitation in eight widely used gridded observation-based climate datasets (CANGRD, CRU-TS3.1, CRUTEM4.1, GISTEMP, GPCC, GPCP, HadCRUT3, and UDEL) and eight reanalyses (20CR, CFSR, ERA-40, ERA-Interim, JRA25, MERRA, NARR, and NCEP2) was evaluated over the Canadian Arctic for the 1950–2010 period. The evaluation used the CANGRD dataset, which is based on homogenized temperature and adjusted precipitation from climate stations, as a reference. Dataset agreement and bias were observed to exhibit important spatial, seasonal, and temporal variability over the Canadian Arctic with the largest spread occurring between datasets over mountain and coastal regions and over the Canadian Arctic Archipelago. Reanalysis datasets were typically warmer and wetter than surface observation-based datasets, with CFSR and 20CR exhibiting biases in total annual precipitation on the order of 300?mm. Warm bias in 20CR exceeded 12°C in winter over the western Arctic. Analysis of the temporal consistency of datasets over the 1950–2010 period showed evidence of discontinuities in several datasets as well as a noticeable increase in dataset spread in the period after approximately 2000. Declining station networks, increased automation, and the inclusion of new satellite data streams in reanalyses are potential contributing factors to this phenomenon. Evaluation of trends over the 1950–2010 period showed a relatively consistent picture of warming and increased precipitation over the Canadian Arctic from all datasets, with CANGRD giving moistening trends two times larger than the multi-dataset average related to the adjustment of the station precipitation data. The study results indicate that considerable care is needed when using gridded climate datasets in local or regional scale applications in the Canadian Arctic.  相似文献   

7.
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.  相似文献   

8.
The Atlantic Warm Pool (AWP) region, which is comprised of the Gulf of Mexico, Caribbean Sea and parts of the northwestern tropical Atlantic Ocean, is one of the most poorly observed parts of the global oceans. This study compares three ocean reanalyses, namely the Global Ocean Data Assimilation System of National Centers for Environmental Prediction (NCEP), the Climate Forecast System Reanalysis (CFSR) of NCEP, and the Simple Ocean Data Assimilation (SODA) for its AWP variation. The surface temperature in these ocean reanalyses is also compared with that from the Extended Range SST version 3 and Optimally Interpolated SST version 2 SST analyses. In addition we also compare three atmospheric reanalyses: NCEP-NCAR (R1), NCEP-DOE (R2), and CFSR for the associated atmospheric variability with the AWP. The comparison shows that there are important differences in the climatology of the AWP and its interannual variations. There are considerable differences in the subsurface ocean manifestation of the AWP with SODA (CFSR) showing the least (largest) modulation of the subsurface ocean temperatures. The remote teleconnections with the tropical Indian Ocean are also different across the reanalyses. However, all three oceanic reanalyses consistently show the absence of any teleconnection with the eastern equatorial Pacific Ocean. The influence of the AWP on the tropospheric temperature anomalies last for up to a one season lead and it is found to be relatively weak in R1 reanalyses. A simplified SST anomaly equation initially derived for diagnosing El Niño Southern Oscillation variability is adapted for the AWP variations in this study. The analysis of this equation reveals that the main contribution of the SST variation in the AWP region is from the variability of the net heat flux. All three reanalyses consistently show that the role of the ocean advective terms, including that associated with upwelling in the AWP region, is comparatively much smaller. The covariance of the SST tendency in the AWP with the net heat flux is large, with significant contributions from the variations of the surface shortwave and longwave fluxes.  相似文献   

9.
Tropical intraseasonal rainfall variability in the CFSR   总被引：2，自引：1，他引：1  
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.  相似文献   

10.
An intercomparison of summertime (JJA) subtropical geopotential heights from the ERA-40 and NCEP/NCAR reanalysis is specifically conducted over East Eurasia and the western North Pacific. The NCEP/NCAR is obviously lower than the ERA-40 in the mid-to-lower troposphere in most regions of East Eurasia before the mid-1970s, but becomes higher than the ERA-40 after the mid-1970s and thus demonstrates stronger increased trends during the period of 1958--2001. Both reanalyses are lower than the observations in most regions of China. The NCEP/NCAR especially shows tremendously systematic lower values before the mid-1960s and displays abrupt changes before the 1970s. Several indices of the western North Pacific subtropical high (WNPSH), calculated from both reanalyzed summer geopotential heights, also reveal that the variation trend of the NCEP/NCAR is stronger than that of the ERA-40 in the mid-to-lower troposphere from 1958 to 2001. Through singular value decomposition (SVD) analysis, the summer geopotential heights at 500 hPa from the ERA-40 are better than the NCEP/NCAR counterparts at interacting with the precipitation over the East Asian monsoon region. The results indicate that the NCEP/NCAR in the mid-and-lower troposphere may overestimate interdecadal changes and should be used cautiously to study the relationship between the WNPSH and precipitation ove ther East Asia Monsoon region before the mid-1970s.  相似文献   

11.
An intercomparison of summertime (JJA)subtropical geopotential heights from the ERA-40 and NCEP/NCAR reanalysis is specifically conducted over East Eurasia and the western North Pacific. The NCEP/NCAR is obviously lower than the ERA-40 in the mid-to-lower troposphere in most regions of East Eurasia before the mid-1970s, but becomes higher than the ERA-40 after the mid-1970s and thus demonstrates stronger increased trends during the period of 1958-2001. Both reanalyses are lower than the observations in most regions of China. The NCEP/NCAR especially shows tremendously systematic lower values before the mid-1960s and displays abrupt changes before the 1970s. Several indices of the western North Pacific subtropical high (WNPSH), calculated from both reanalyzed summer geopotential heights, also reveal that the variation trend of the NCEP/NCAR is stronger than that of the ERA-40 in the mid-to-lower troposphere from 1958 to 2001. Through singular value decomposition (SVD) analysis, the summer geopotential heights at 500 hPa from the ERA-40 are better than the NCEP/NCAR counterparts at interacting with the precipitation over the East Asian monsoon region. The results indicate that the NCEP/NCAR in the mid-and-lower troposphere may overestimate interdecadal changes and should be used cautiously to study the relationship between the WNPSH and precipitation ove ther East Asia Monsoon region before the mid-1970s.  相似文献   

12.
An evaluation is carried out of the moisture fields, the precipitation P and evaporation E, and the moisture transport and divergence in the atmosphere from the global atmospheric National Centers for Environmental Prediction (NCEP)–NCAR reanalyses produced with four-dimensional-data assimilation. The moisture fields are summarized by the precipitable water which is compared with analyzed fields from NVAP based primarily on Special Sensor Microwave Imager (SSM/I) over the oceans and rawinsonde measurements over land, plus TIROS Operational Vertical Sounder (TOVS). The moisture budgets are evaluated through computation of the freshwater flux at the surface E?P from the divergence of the total moisture transport, and this is compared with the reanalysis E?P that is based upon a 6-hour integration of the assimilating model and thus depends on the model parametrizations. The P field is evaluated using Xie– Arkin global precipitation estimates which, although containing considerable uncertainties, are believed to be reliable and good enough to show that there are substantial biases in the NCEP P. There are many fields of interest and which are improved over previous information available. On an annual mean basis the largest evaporation of over 6?mm/day is in the subtropical Indian Ocean. However, the NCEP moisture fields are shown to contain large and significant biases in the tropics. The tropical structures are less well defined and values are generally smaller where they should be high and higher where they should be low. In addition, the NCEP moisture fields contain less variability from year to year. The NCEP model P generally reveals a double intertropical convergence zone in the central Pacific and the location of the South Pacific Convergence Zone is not well captured. Rainfall amounts are lower than observed in the oceanic tropical convergence zones. The variability in the central tropical Pacific of P associated with El Niño-Southern Oscillation (ENSO) is underestimated in the NCEP reanalyses and, moreover, is not very well correlated with the Xie–Arkin product. A bias for too much rainfall in the model over the southeastern USA and southeast Asia is also present in northern summer. The comparison of E?P from the moisture budget with the model results reveal some strong systematic differences. In particular, remarkably, many island stations show up as bull’s-eyes in the difference field. These are identified as originating from small but systematic differences in vertical moisture profiles from those in the surrounding oceans, raising questions about the influence radius of rawinsonde moisture observations. Biases in E are inferred from the E?P differences in some places implying some spurious land moisture sources. While usually better, the residual method E?P estimates are inferior to those from the model parametrizations in some places. Both estimates are affected by biases in moisture, as analyzed, and the moisture divergence depends critically on the velocity divergence field. The model estimates also depend upon the parametrizations of subgrid scale processes, such as convection, that influence E and P. A discussion is given of sources of errors contributing to the moisture budgets.  相似文献   

13.
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。  相似文献   

14.
This study compared precipitation, mean air temperature (MAT) and mean sea level pressure (MSLP) from two widely used reanalysis datasets (ERA-40 and NCEP) with those from observed stations across eastern China. The evaluation was based on a comparison of both temporal and spatial variability and included several assessment criteria such as the mean values, normalized root mean square error, Mann–Kendall test, empirical orthogonal functions (EOFs) and probability density functions. The results showed that both the ERA-40 and NCEP datasets could capture temporal and spatial variability of the observed precipitation, MAT and MSLP over eastern China. The results showed that the two reanalysis datasets performed better for MAT and MSLP than for precipitation. Overall, the two reanalysis datasets revealed reasonable agreement with observations according to the evaluation. ERA-40 was better at capturing the temporal and spatial distributions for these three variables than NCEP, especially for MAT and MSLP. NCEP tended to overestimate the annual precipitation for both mean and extreme values, while ERA-40 tended to underestimate it, particularly for extreme values. The two reanalysis datasets performed better in the east and northeast regions of the study area than in other regions for capturing the temporal variability of MAT and MSLP. ERA-40 was poor at capturing the temporal variability of precipitation in northeastern China. According to the trend analysis, the two reanalysis datasets showed lower trends for MAT and precipitation and higher trends for MSLP. Both ERA-40 and NCEP had larger explained variances for the first two EOFs than the observed precipitation. This implies that both reanalysis datasets tend to simulate a more uniform spatial distribution for precipitation in the study area.  相似文献   

15.
The Geophysical Fluid Dynamics Laboratory has developed an ensemble coupled data assimilation (ECDA) system based on the fully coupled climate model, CM2.1, in order to provide reanalyzed coupled initial conditions that are balanced with the climate prediction model. Here, we conduct a comprehensive assessment for the oceanic variability from the latest version of the ECDA analyzed for 51 years, 1960–2010. Meridional oceanic heat transport, net ocean surface heat flux, wind stress, sea surface height, top 300 m heat content, tropical temperature, salinity and currents are compared with various in situ observations and reanalyses by employing similar configurations with the assessment of the NCEP’s climate forecast system reanalysis (Xue et al. in Clim Dyn 37(11):2511–2539, 2011). Results show that the ECDA agrees well with observations in both climatology and variability for 51 years. For the simulation of the Tropical Atlantic Ocean and global salinity variability, the ECDA shows a good performance compared to existing reanalyses. The ECDA also shows no significant drift in the deep ocean temperature and salinity. While systematic model biases are mostly corrected with the coupled data assimilation, some biases (e.g., strong trade winds, weak westerly winds and warm SST in the southern oceans, subsurface temperature and salinity biases along the equatorial western Pacific boundary, overestimating the mixed layer depth around the subpolar Atlantic and high-latitude southern oceans in the winter seasons) are not completely eliminated. Mean biases such as strong South Equatorial Current, weak Equatorial Under Current, and weak Atlantic overturning transport are generated during the assimilation procedure, but their variabilities are well simulated. In terms of climate variability, the ECDA provides good simulations of the dominant oceanic signals associated with El Nino and Southern Oscillation, Indian Ocean Dipole, Pacific Decadal Oscillation, and Atlantic Meridional Overturning Circulation during the whole analyzed period, 1960–2010.  相似文献   

16.
This study examines the Indian summer monsoon hydroclimate in the National Centers for Environmental Prediction (NCEP)-Department of Energy (DOE) Reanalysis (R2), the Climate Forecast System Reanalysis (CFSR), and the Modern Era Retrospective-Analysis for Research and Applications (MERRA). The three reanalyses show significant differences in the climatology of evaporation, low-level winds, and precipitable water fields over India. For example, the continental evaporation is significantly less in CFSR compared to R2 and MERRA. Likewise the mean boreal summer 925?hPa westerly winds in the northern Indian Ocean are stronger in R2. Similarly the continental precipitable water in R2 is much less while it is higher and comparable in MERRA and CFSR. Despite these climatological differences between the reanalyses, the climatological evaporative sources for rain events over central India show some qualitative similarities. Major differences however appear when interannual variations of the Indian summer monsoon are analyzed. The anomalous oceanic sources of moisture from the adjacent Bay of Bengal and Arabian Sea play a significant role in determining the wet or dry year of the Indian monsoon in CFSR. However in R2 the local evaporative sources from the continental region play a more significant role. We also find that the interannual variability of the evaporative sources in the break spells of the intraseasonal variations of the Indian monsoon is stronger than in the wet spells. We therefore claim that instead of rainfall, evaporative sources may be a more appropriate metric to observe the relationship between the seasonal monsoon strength and intraseasonal activity. These findings are consistent across the reanalyses and provide a basis to improve the predictability of intraseasonal variability of the Indian monsoon. This study also has a bearing on improving weather prediction for tropical cyclones in that we suggest targeting enhanced observations in the Bay of Bengal (where it is drawing the most moisture from) for improved analysis during active spells of the intraseasonal variability of the Indian monsoon. The analysis suggests that the land–atmosphere interactions contribute significant uncertainty to the Indian monsoon in the reanalyses, which is consistent with the fact that most of the global reanalyses do not assimilate any land-surface data because the data are not available. Therefore, the land–atmosphere interaction in the reanalyses is highly dependent on the land-surface model and it’s coupling with the atmospheric model.  相似文献   

17.
Based on the long-term data of satellite microwave radiometers, the estimates are obtained of interannual and intraannual variations in monthly mean values of total precipitable water in the atmosphere over the North Atlantic from 1988 to 2011, in particular, in the regions characterized by the maximum rate of heat and moisture exchange between the ocean and atmosphere such as the Gulf Stream, Newfoundland, and Norway-Greenland energy-active zones. Long-term trends in total precipitable water in the atmosphere over these regions are estimated. The variations in total precipitable water in the atmosphere in 2010 are noted which were caused by oil spills in the Gulf of Mexico (the spring of 2010) and by the severe drought in the European part of Russia (the summer of 2010).  相似文献   

18.
Summer precipitation products from the 45-Year European Centre for Medium-Range Weather Forecast (ECMWF) Reanalysis (ERA-40), and NCEP-Department of Energy (DOE) Atmospheric Model Intercomparison Project (AMIP-II) Reanalysis (NCEP-2), and Climatic Research Unit (CRU) TS 2.1 dataset are compared with the corresponding observations over China in order to understand the quality and utility of the reanalysis datasets for the period 1979–2001. The results reveal that although the magnitude and location of the rainfall belts differ among the reanalysis, CRU, and station data over South and West China, the spatial distributions show good agreement over most areas of China. In comparison with the observations in most areas of China, CRU best matches the observed summer precipitation, while ERA-40 reports less precipitation and NCEP-2 reports more precipitation than the observations. With regard to the amplitude of the interannual variations, CRU is better than either of the reanalyses in representing the corresponding observations. The amplitude in NCEP-2 is stronger but that of ERA-40 is weaker than the observations in most study domains. NCEP-2 has a more obvious interannual variability than ERA-40 or CRU in most areas of East China. Through an Empirical orthogonal function (EOF) analysis, the main features of the rainfall belts produced by CRU agree better with the observations than with those produced by the reanalyses in the Yangtze-Huaihe River valley. In East of China, particularly in the Yangtze-Huaihe River valley, CRU can reveal the quasi-biennial oscillation of summer precipitation represented by the observations, but the signal of ERA-40 is comparatively weak and not very obvious, whereas that of NCEP-2 is also weak before 1990 but very strong after 1990. The results also suggest that the magnitude of the precipitation difference between ERA-40 and the observations is smaller than that between NCEP-2 and the observations, but the variations represented by NCEP-2 are more reasonable than those given by ERA-40 in most areas of East China to some extent.  相似文献   

19.
We investigate the effects of realistic oceanic initial conditions on a set of decadal climate predictions performed with a state-of-the-art coupled ocean-atmosphere general circulation model. The decadal predictions are performed in both retrospective (hindcast) and forecast modes. Specifically, the full set of prediction experiments consists of 3-member ensembles of 30-year simulations, starting at 5-year intervals from 1960 to 2005, using historical radiative forcing conditions for the 1960–2005 period, followed by RCP4.5 scenario settings for the 2006–2035 period. The ocean initial states are provided by ocean reanalyses differing by assimilation methods and assimilated data, but obtained with the same ocean model. The use of alternative ocean reanalyses yields the required perturbation of the full three-dimensional ocean state aimed at generating the ensemble members spread. A full-value initialization technique is adopted. The predictive skill of the system appears to be driven to large extent by trends in the radiative forcing. However, after detrending, a residual skill over specific regions of the ocean emerges in the near-term. Specifically, natural fluctuations in the North Atlantic sea-surface temperature (SST) associated with large-scale multi-decadal variability modes are predictable in the 2–5 year range. This is consistent with significant predictive skill found in the Atlantic meridional overturning circulation over a similar timescale. The dependency of forecast skill on ocean initialization is analysed, revealing a strong impact of details of ocean data assimilation products on the system predictive skill. This points to the need of reducing the large uncertainties that currently affect global ocean reanalyses, in the perspective of providing reliable near-term climate predictions.  相似文献   

20.
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.  相似文献   

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