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Observations are required to understand monsoon changes over time. Reanalyses are useful supplements to observations; however, their ability to reveal long-term monsoon changes remains unknown. Here, we evaluate against observations the performance of two reanalysis datasets covering the whole twentieth century, the ECMWF reanalysis of the twentieth century (ERA20C) and the Twentieth Century Reanalysis Project (20CR), in terms of reproducing the climatological averages, the centennial co-variation, and long-term trends of Northern Hemisphere land monsoon rainfall (NHLMR). Their performance is compared with three other widely used reanalyses, the NCEP–NCAR reanalysis (NCEP1), the ECMWF 40-year reanalysis (ERA40), and the Japanese 55-year Reanalysis Project (JRA55), for the period after 1955. Results show that the five reanalysis datasets reasonably reproduce the climatological NHLMR and NHLM domains. The leading co-variation mode of NHLMR, with an overall increasing tendency before 1955 and a decreasing tendency afterwards, is captured by the corresponding principal components of the two long-term datasets, with some spatial biases. For the long-term precipitation trends, both the ERA20C and 20CR reproduce the increasing trend during 1901–1955, except for parts of the North African (NAF) monsoon region. However, neither dataset captures the decreasing trend of NHLMR after 1955 because of limitations in the NAF and North American (NAM) monsoon region. The NCEP1, ERA40, and JRA55 datasets also have shortcomings in the NAM region. An overall decreasing NHLMR is only shown in the NCEP1 dataset. A moisture budget analysis reveals that the long-term trends of NHLMR are dominated by the dynamic component of moisture convergence, suggesting a prominent role for atmospheric circulation changes. The influence of residual terms related to observations assimilated in the reanalyses is also discussed.

A limited area convection permitting model (CPM) based on the Met Office Unified Model, with a 0.04° (4.4 km) horizontal grid spacing, is used to simulate an entire warm-season of the East Asian monsoon (from April to September 2009). The simulations are compared to rain gauge observations, reanalysis and to a lower resolution regional model with a 0.12° (13.2 km) grid spacing that has a parametrization of subgrid-scale convective clouds and precipitation. The 13.2 km simulation underestimates precipitation intensity, produces rainfall too frequently, and shows evident biases in reproducing the diurnal cycle of precipitation and low-level wind fields. In comparison, the CPM shows significant improvements in the spatial distribution of precipitation intensity, although it overestimates the intensity magnitude and has a wet bias over central eastern China. The diurnal cycle of precipitation over Mei-yu region, southern China and the eastern periphery of the Tibetan Plateau, as well as the diurnal cycle of low-level winds over both the Mei-yu region and southern China are better simulated by the CPM. Over the Mei-yu region, in both simulations and observations, the local atmospheric instability in the afternoon is favorable for upward motion and rainfall. The CPM receives more sensible heat flux from the surface, has a stronger upward motion, and overestimates water vapor convergence based on moisture budget diagnosis. All these processes help explain the excessive late afternoon rainfall over the Mei-yu region in the CPM simulation.

In this study, atmospheric water vapor transport was analyzed to study the changes in spring persistent rainfall (SPR) over southeastern China from 1980 to 2012. The performances of five sets of reanalysis data in reproducing the climatology, the long-term trend and interannual variability of the SPR were synthetically evaluated. To understand the mechanisms dominating SPR variation, the major components of moisture budget, including vertical moisture advection, horizontal moisture advection and evaporation, were examined based on the five reanalysis datasets. The results show that all five reanalysis datasets reproduce the climatology of the SPR reasonably well. Strong westerly wind flow over the southern Tibetan Plateau and southwest wind flow over the western North Pacific are the two main channels remotely supplying water vapor for the SPR. Locally, moisture budget diagnosis shows that the SPR is primarily contributed by the evaporation and vertical moisture advection terms. The SPR shows a decreasing tendency [−0.38 mm day−1 (10 year)−1] during 1980–2012 along with strong interannual variation, which is reasonably captured by all five reanalysis datasets. Vertical moisture advection dominates the SPR variation, and the decreasing trend of the SPR is mainly due to the weakening of ascending motion during this period. In an El Nino decaying spring, the anomalous lower-tropospheric anticyclone over the western North Pacific intensifies vertical upward motion and leads to more precipitation. Both the Japanese 55-year reanalysis (JRA55) and the European Centre for Medium-Range Weather Forecast interim reanalysis (ERAIM) show higher skill in reproducing the climatology and changes of the SPR.