The climate of Peninsular Malaysia is dominated by the annual precipitation cycle, which includes two monsoon seasons. As is typical for the wider western Maritime Continent region, during boreal winter (November-February, NDJF), a warm, moist atmosphere near the surface combines with intense horizontal moisture transport associated with the Northeast monsoon to produce strong convective precipitation (Chang et al. 2005). Peninsular Malaysia is significantly influenced by precipitation extremes (e.g. Tangang et al. 2017) and associated flooding (e.g. Diya et al. 2014; Chan et al. 2015). These climate extremes can cause considerable economic losses, casualties and a deterioration of the environment (Mohd et al. 2006; Muqtada et al. 2014), posing a great threat to livelihoods and thus limiting economic growth. During the past 40 years, the observed rainfall record indicates a considerable increasing trend in the annual total precipitation over Peninsular Malaysia (Mayowa et al. 2015). An increasing trend in the frequency of extreme rainfall events has also been observed in Peninsular Malaysia (Syafrina et al. 2014; Chan 1997; 2015; Mayowa et al. 2015).
Given these trends, it is important to understand likely future changes in precipitation in the region given the impact further increases in total and extreme precipitation could have on society and the environment. Climate models provide a key tool to provide a physically-based assessment of prospective impacts and risks from changing hydrological extremes, which in turn help to inform strategies of disaster mitigation and adaptation in a changing climate. Many studies have examined the simulation of precipitation over Peninsular Malaysia (e.g. Syafrina et al. 2014; Tan et al. 2014; Noor et al. 2019; Salman et al. 2020) and the adjacent areas in Southeast Asia (Siew et al. 2014; Raghavan et al. 2018; Tan et al. 2017; Kamworapan and Surussavadee 2019) using global climate models (GCMs). However, these studies document considerable biases in the modelled regional precipitation patterns compared to observed precipitation. One possible reason for this is the limited horizontal resolution (with grid spacing > 100 km) of typical GCMs that makes it difficult to properly resolve the complex topography over the Maritime Continent and its interactions with the monsoon circulation (Chang et al. 2005). Coarse horizontal resolutions also lead to poorly resolved synoptic/mesoscale processes that are strongly associated with precipitation extremes in Peninsular Malaysia, such as Borneo Vortices (Tangang et al. 2008; Koseki et al. 2014, Liang et al. 2021) and easterly wave disturbances (Chen et al. 2013). Moreover, poor performance in simulating the eastward Kelvin wave activity coupled with equatorial convection has been demonstrated by GCMs with a horizontal resolution coarser than 50-km (Yang et al. 2009), which can limit the realism of simulated precipitation in the tropics.
The role of model horizontal resolution in the simulation of global precipitation is summarized by the study of Rauscher et al. (2016). They argued that fine horizontal resolutions are important for a model to capture intense precipitation because finer grid spacings can allow the precipitation-related vertical ascent to be further strengthened by the interaction between the constraint of fluid continuity and the emergent scaling properties of winds. In addition, a sufficiently high vertical resolution is important for modelling the vertical profile of cumulus latent heating rates, which are closely associated with tropical deep convection and the associated precipitation (Druyan et al. 2008). Inness et al. (2001) suggested that a relatively coarse vertical resolution can lead to underestimates of sub-seasonal precipitation variability associated with the Madden Julian Oscillation due to the poorly resolved vertical distribution of convective cloud-top height. In the Asian monsoon region, Richter et al. (2014) reported that a GCM with relatively coarse vertical resolution tends to simulate a stronger summer monsoon and associated precipitation. Models with finer vertical resolutions also exhibit a better representation of the vertical profiles of water vapor and temperature (Tompkins and Emanuel 2000), which are closely associated with precipitation intensity (Fujita and Sato 2017).
Previous studies have attempted to improve model performance by downscaling GCM simulations over Peninsular Malaysia using statistical (Juneng et al. 2010; Nadrah and Tukimat 2011; Hassan et al. 2015) and dynamical (Tangang et al. 2013; 2020; Kwan et al. 2014; Amin et al. 2017; Chin and Tan 2018; Jamaluddin et al. 2018; Ngai et al. 2020) downscaling. Although statistical downscaling is computationally cheap, it is dependent on the observational data at the target resolution used within the statistical model to be statistically correlated with the large-scale predictors. This process can be affected by uncertainties in both the observational data (e.g. Kotsuki and Tanaka 2013; Tan et al. 2015; Qi et al. 2016) and GCM performance (San-Martín et al. 2017). There are also known issues in dynamical downscaling using regional climate models, including the dependence on the driving boundary conditions from the GCM (e.g. Wu et al. 2005; Tangang et al. 2020) and the lack of two-way interaction between the downscaled fields and the external large-scale environment in the one-way nested simulations that are typically used (e.g. Harris and Durran 2010; Bowden et al. 2012). Regional climate models (RCMs) also exhibit considerable uncertainties in simulating precipitation over Southeast Asia (e.g. Nguyen-Thuy et al. 2020; Tangang et al. 2020) as the performance of RCMs is dependent on both the RCM configuration and the lateral boundary conditions from GCMs.
In recent years, high-resolution GCM simulations (with grid spacings < 50 km) have been used in precipitation studies and have exhibited an improved ability in representing the observed regional precipitation patterns compared to coarse-resolution GCMs (e.g. Iorio et al. 2004; Mo et al. 2005; Sato et al. 2009; Feng et al. 2011; Kopparla et al. 2013; Yashiro et al. 2016; Kim et al. 2019; Bador et al. 2020; Kong et al. 2020). For instance, Arakawa and Kitoh (2005) and Sato et al. (2009) showed the improved ability of GCMs to represent the detailed topography and simulations of land-sea breezes and the associated diurnal cycle of precipitation when finer horizontal resolutions are used. GCMs with finer horizontal resolution have also been shown to be capable of producing improved simulations of the interaction between the sea surface and the troposphere, which are important for realistic simulations of deep convection and associated precipitation (Scher et al. 2017). Comparing high-resolution GCMs to those with relatively coarse resolutions, many studies have also shown improved simulation of precipitation-related synoptic systems including tropical cyclones (e.g. Strachan et al. 2013; Roberts et al. 2020; Vannière et al. 2020) and extratropical cyclones (Colle et al. 2013; Zappa et al. 2013; Priestley et al. 2020). In Peninsular Malaysia, the precipitation patterns are associated with multiple physical factors that may be sensitive to changes in model resolution, such as complex synoptic systems (Chen et al. 2013; Koseki et al. 2014) and atmosphere-topographic interactions during the different monsoon seasons (Chang et al. 2005; Varikoden et al. 2010; 2011). However, there has been little research on the impact of increased horizontal and vertical resolution on GCM-simulated precipitation over the region.
The availability of high-resolution GCM data from the High Resolution Model Intercomparison Project (HighResMIP, Haarsma et al. 2016) of Coupled Model Intercomparison Project Phase 6 (CMIP6) provides a good opportunity to study the impact of model resolution on GCM simulations of precipitation. For instance, a recent study by Xin et al. (2021) evaluated the ability of the HighResMIP models to simulate precipitation in China and found improved simulation of precipitation in simulations at finer horizontal resolution due to the better resolved topographical rainfall. Molteni et al. (2020) has demonstrated the ability of the HighResMIP models to simulate the tropical Indo‑Pacific rainfall and its modulations by the El Niño-Southern Oscillation (ENSO) and the North Atlantic Oscillation (NAO). Their study found that most of the HighResMIP GCMs simulate the late-winter ENSO teleconnection reasonably well; however, it is difficult to detect a consistent change in the realism of the simulated atmospheric teleconnections when model resolution is increased. Ajibola et al. (2020) assessed the ability of the HighResMIP GCMs to simulate West African summer monsoon rainfall and suggested limited improvements in simulated mean precipitation when GCM simulations at finer horizontal resolution are compared with those at coarser resolution. For these studies of precipitation simulations based on HighResMIP, three main limitations are noted in the model evaluation process. First, the role of vertical resolution in precipitation simulation is usually ignored. Second, the validation data used to assess model performance is usually based on a single dataset, which may bring considerable uncertainties to model evaluation. Third, though the HighResMIP GCMs has presented a better performance in simulating large-scale patterns of precipitation in Asia than other CMIP6 GCMs (Dong and Dong 2021), it still remains unknown whether HighResMIP can tackle the challenge of simulating regional-scale (less than 500-km) precipitation patterns.
In this paper, an assessment of the ability of atmosphere/land-only (AMIP style) GCM simulations of HighResMIP, with a spectrum of both horizontal and vertical resolutions of the GCMs, to simulate the precipitation climatology and variability in Peninsular Malaysia will be performed. The assessment will provide model validations based on ensembles of multiple precipitation observations and climate reanalysis datasets, which helps to achieve more robust processes of model validation. The assessment aims to provide useful information on how to interpret the latest GCM simulation in Malaysia and the adjacent Western Maritime Continent for the further use of the models in hydrological impact studies. The specific objectives of this study are:
a) To assess the realism of precipitation in different monsoon seasons over Peninsular Malaysia as simulated by the GCM simulations of HighResMIP.
b) To investigate how the simulated precipitation in Peninsular Malaysia is affected by horizontal and vertical resolutions in those GCMs.
This paper is structured as follows. Section 2 briefly describes the study area, observational precipitation data and the HighResMIP experiments together with the methodology used for assessing the performance of the HighResMIP GCMs in simulating precipitation in Peninsular Malaysia. Section 3 presents the results of the assessment of the performance of the HighResMIP GCMs with different horizontal and vertical resolutions in simulating precipitation. Section 4 summarizes and discusses the main findings of the research.