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To assess the model performance in simulating the observed rainfall, a daily gridded gauge precipitation dataset, CN05.1, with a resolution of 0.25° × 0.25°, is employed (Wu and Gao, 2013; Wu et al., 2017). CN05.1 was produced based on 2400 observation stations across China and has been widely used (Sui et al., 2015). Considering the lack of hourly rainfall measurements in CN05.1, we use the China Meteorological Forcing Dataset (CMFD) to evaluate the diurnal cycle. CMFD was derived from a combination of station data, reanalysis datasets and remote sensing products, and has a 3-h temporal resolution and a 0.1° × 0.1° spatial resolution (He et al., 2020). Due to its continuous temporal coverage and consistent quality, CMFD is one of the most widely used climate datasets for China (Peng et al., 2022; Wang et al., 2022). In addition, the fifth major global reanalysis produced by ECMWF (ERA5) is utilized to assess the simulation of atmospheric circulation (Hersbach et al., 2020).
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In this study, the RCM used is WRF version 4.0, which was developed at the National Center for Atmospheric Research. It is a mesoscale numerical model consisting of a numerical solver of the fully compressible Eulerian and non-hydrostatic equations (Skamarock et al., 2019). WRF has been widely used in regional climate studies over China, and the results prove it can reproduce the main features of climate and extremes well. Moreover, the performance of the interannual variation provided by WRF is obviously better compared to its driving data (Sato and Xue, 2013; Yu et al., 2015).
Although the biases in GCMs can be mitigated to some extent through the downscaling process, they still potentially impact the downscaling results. Thus, regional downscaling is best driven by GCMs that reasonably simulate the region’s climate. Considering WRF is driven by the GCM’s prognostic variables that are related to rainfall but not by rainfall itself, a realistic simulation of rainfall by WRF is dependent on the accuracy of these prognostic boundary conditions (Sato and Xue, 2013; Rastogi et al., 2022). Hence, we first employ 10 GCMs (at the time, the only 10 models providing 6-h temporal-resolution output) and evaluate the simulated spring low-level winds (850 and 925 hPa), which are highly correlated with spring rainfall in China (Wu et al., 2012; Li et al., 2019b). The overall performance of each model can be judged using a comprehensive rating index (MR), which provides a statistical summary between simulations and ERA5, in terms of their spatial correlation coefficient, root-mean-square difference, and standard deviation difference. The details of MR can be found at Li et al. (2016a). From Table 1, MIROC6 (0.59) exhibits the best MR performance among all 10 models, followed by NorESM2-MM (0.57) and then GFDL-ESM4 (0.56). In pervious works (Kataoka et al., 2020; Tian et al., 2021), MIROC6 performed better in reproducing the seasonal progression of the East Asian monsoon, which is consistent with our results. Therefore, MIROC6 is used for the boundary and initial conditions of WRF. MIROC6 is composed of three sub-models: atmosphere, sea ice–ocean and land. The atmosphere model is based on the Center for Climate System Research, University of Tokyo/National Institute for Environmental Studies (Numaguti et al., 1997) atmospheric general circulation model; the sea ice–ocean model is based on the Center for Climate System Research Ocean Component model (Hasumi, 2006); and the land surface model is based on the Minimal Advanced Treatments of Surface Interaction and Runoff model (Takata et al., 2003). The atmospheric component of MIROC6 has a horizontal resolution of 1.4° × 1.4° (Tatebe et al., 2019).
Model ID MR Rank ACCESS-CM2 0.51 4 CMCC-CM2-SR5 0.43 7 CMCC-ESM2 0.41 8 GFDL-CM4 0.47 6 GFDL-ESM4 0.56 3 IPSL-CM6A-LR 0.21 10 MIROC6 0.59 1 MPI-ESM1-2-HR 0.28 9 MRI-ESM2-0 0.49 5 NorESM2-MM 0.57 2 Table 1. The 10 CMIP6 models used in this study and their MR scores.
To further illustrate the accuracy of the driving MIROC6, we provide a basic comparison between the observations and MIROC6 (Fig. 2). In general, the spring rainfall climatology of MIROC6 matches that of CN05.1 with a correlation of 0.64. Additionally, MIROC6 can capture the strength and pattern of the winds well, with a correlation coefficient of 0.74 at 925 hPa and 0.84 at 850 hPa between ERA5 and MIROC6. The strength and pattern of water vapor flux in MIROC6 are consistent with those in ERA5, with a pattern correlation coefficient of 0.88. Generally, the spring rainfall and associated atmospheric conditions in MIROC6 are consistent with those in observations, which lay the basis for achieving a reliable rainfall simulation by using MIROC6 to drive WRF.
Figure 2. The spatial distribution of spring (a–c) total rainfall amount (units: mm), (d–f) 925-hPa wind speed (units: m s−1), (g–i) 850-hPa wind speed (units: m s−1), and (j–l) water vapor flux in the whole layer below 200 hPa [arrows; units: kg (m−1 s−1)] and its divergence [shaded; units: 10−5 kg (m−2 s−1)] in 1995–2014 from observations, MIROC6 and WRF.
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A triple-nested downscaling simulation based on WRF is applied in the present study. Figure 1b shows the boundary and model topography of each domain. The outermost-domain (Domain1) grid is centered at (29°N, 110°E) and has 36 km horizontal grid spacing (with 91 × 91 horizontal grid points). To ensure smooth solutions, Domain1 grid cells of WRF closer than five cells from an outer boundary are relaxed towards MIROC6. The innermost-domain (Domain3) grid has 4 km grid spacing (with 127 × 166 horizontal grid points). Regarding the vertical coordinates, we configured 33 terrain-following eta levels from the surface to 50 hPa in each domain. The initial and lateral boundary conditions for Domain1 are derived from the historical simulation and future projection of MIROC6 at 6-h intervals for meridional–zonal wind, specific humidity, air temperature, calculated geopotential height, 2-m air temperature, skin temperature, surface pressure and sea level pressure. Besides, daily sea surface temperature, monthly soil moisture and temperature are also provided by MIROC6.
For WRF downscaling, we use the following configuration: the New Thompson microphysics scheme (Thompson et al., 2008); the Kain–Fritsch convection scheme (Kain, 2004); the Noah Land Surface Model (Niu et al., 2011); the Rapid Radiative Transfer Model longwave radiation scheme; the Dudhia shortwave radiation scheme (Dudhia, 1989); and the Yonsei University boundary layer scheme (Hong et al., 2006). Among them, the microphysics scheme constitutes a key configuration in rainfall modelling. The New Thompson microphysics scheme includes ice, snow, graupel, and their associated processes, which performs well in high-resolution rainfall simulations over the complex terrain of East Asia (Strong et al., 2017; Tiwari et al., 2018; Huang et al., 2020b). Considering the spatial resolutions are sufficiently high for the non-hydrostatic dynamical core of the model to partially resolve sub-mesoscale convective motions (Li et al., 2019a), we therefore negate the convection parameterization of Domain3.
To save computational resource, two time-slice integrations are used to represent the typical present and future climate. Here, we use the end of the 21st century as a typical future climate to emphasize the signals of change in precipitation due to warming (Feng et al., 2011). The two integrations cover the period from 24/25 February to 1 June during 1995–2014 and 2075–2094 (last 20 years of available SSP output of MIROC6), respectively. The first five days (24/25 February to 28/29 February) is considered as a spin-up and is therefore not included in the subsequent analysis. The projection outputs of MIROC6 forced with the SSP2-4.5 and SSP5-8.5 scenarios are employed. SSP2-4.5 is a combined scenario of a medium energy-intensive, socioeconomic developmental path with a rising radiative forcing peaking at 4.5 W m−2 in 2100. It is comparable to several planned emission pathways required by the mitigation policies of the Paris climate agreement, and is therefore considered a more preferable scenario compared with other SSP scenarios (O'Neill et al., 2016). SSP5-8.5 represents a combined scenario of a high energy-intensive, socioeconomic developmental pathway with strong radiative forcing, which peaks at 8.5 W m−2 by 2100. Although SSP5-8.5 is the highest emission scenario available in CMIP6, it still assumes emissions well below what the current energy mix would produce in the future (Birkinshaw et al., 2017). Based on the SSP5-8.5 scenario, we can discuss the rainfall changes under extreme warming conditions (Wang and Kotamarthi, 2015).
A non-parametric bootstrapping approach is used to estimate confidence intervals based on the following procedure. First, 500 non-parametric bootstrap samples are generated based on time series obtained by resampling of time steps from the original 20-year dataset of regional averages. Then, quantiles are applied to the 500 estimates of change to obtain 90% confidence intervals (Shao et al., 2012; Hailegeorgis et al., 2013).
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To understand the differences in changes in CREs between WRF and MIROC6, we employ a moisture budget analysis (Chou et al., 2013; Ma et al., 2015b; Li et al., 2020). The moisture budget equation is
where P is precipitation;
$ \mathbf{V} $ is wind vector; q is specific humidity; E is evaporation, which is calculated from the latent heat fluxes; δ is a residual term including transient eddies, contributions from model interpolation bias, and surface processes;$\left\langle{\;}\right\rangle $ represents a vertical mass integration through the troposphere (below 200 hPa);$ {-}{\partial}_{\text{t}}\left\langle{{q}}\right\rangle $ is the time derivative of q, which implies the change in local water vapor storage; and$ {-}\nabla\cdot \left\langle{{q}\mathbf{V}}\right\rangle $ is the convergence of integrated moisture flux. This can further be divided into two terms according to the mass conservation equation: a horizontal moisture advection term and a vertical moisture advection term. Therefore, Eq. (1) can be reformulated asTo understand the mechanisms responsible for the trend of change, according to Eq. (2), the precipitation changes can be decomposed into
Here, the prime symbol
$ {}^{'} $ represents the departure from the historical simulation. In Eq. (3), the changes of vertical moisture advection$ -{\left\langle{\mathrm{\omega }{\partial }_{\mathrm{p}}\mathrm{q}}\right\rangle}^{{{'}}} $ can be further divided as follows:where
$ \stackrel{-}{\omega } $ denotes the climatology in the historical simulation. The first term on the right-hand side of Eq. (4) denotes the changes in q with$ \omega $ unchanged, commonly called the thermodynamic component of the vertical moisture advection term, contributed by the changes in water vapor; the second term denotes the changes in$ \omega $ with q remaining constant, associated with changes in vertical velocity, which is induced by atmospheric circulation changes, and often called the dynamic component; the third term is a nonlinear term that is induced by changes in q and$ \omega $ , and is much smaller than the other terms and can be ignored (Seager et al., 2010; Li et al., 2020). Therefore, Eq. (3) can be written asTo analyse the effect of synoptic processes on precipitation, each term of the moisture budget is calculated using the daily mean data.
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When the accumulated rainfall amount of one day is equal to or more than 1.0 mm within 24 h, the day is defined as one rainy day. A CRE is set to begin if any of the consecutive conditions occur, as described in Zheng et al. (2020). To quantify the CREs, we use four indices related to geological disasters: the occurrence frequency (OCF), rainy days (TRD), rainfall amount (ACR), and rainfall intensity (INT) (Corominas and Moya, 1999). The OCF describes the frequency of CREs, while the TRD and ACR are the sum of CRE rainy days and accumulated rainfall amounts, respectively. INT describes the mean daily rainfall intensity during CREs, which is not independent of ACR and TRD but equal to ACR divided by TRD (Zheng et al., 2020).
Model ID | MR | Rank |
ACCESS-CM2 | 0.51 | 4 |
CMCC-CM2-SR5 | 0.43 | 7 |
CMCC-ESM2 | 0.41 | 8 |
GFDL-CM4 | 0.47 | 6 |
GFDL-ESM4 | 0.56 | 3 |
IPSL-CM6A-LR | 0.21 | 10 |
MIROC6 | 0.59 | 1 |
MPI-ESM1-2-HR | 0.28 | 9 |
MRI-ESM2-0 | 0.49 | 5 |
NorESM2-MM | 0.57 | 2 |