3.2 Sensitivity to different treatment of convection in simulating MCS precipitation
3.2.1 Summer MCS tracks and the spatial distribution of summer mean MCS precipitation
We further check the sensitivity to different treatment of convection (explicit .vs. hybrid .vs. fully-parametrized) in simulating MCS precipitation at the O(10 km) grid spacing. All MCS tracks among observations and simulation are shown in Fig. 2. A total of 3370, 3493, 2499 and 2630 intense MCS precipitation systems over the EASM region were tracked in CMORPH, N1280-EC, N1280-HC and N1280-PC, respectively. It should be noted that the N1280-EC (Fig. 2b) has slightly more MCSs than observed (+ 3.6%), but both the N1280-HC (-25.8%; Fig. 2c) and the N1280-PC (-22.0%; Fig. 2d) have considerably fewer MCS tracks compared with those observed in CMORPH (Fig. 2a).
The spatial distributions of MCS precipitation among observations and simulation are shown in Fig. 3. There are three main MCS precipitation centers over the EASM region, in accordance with the summer total precipitation: one rainfall center is located over southeastern China, one center is the southwest-northeast-elongated rain belt, which is called “Mei-yu” in China and “Chang-ma” in South Korea, and the third one is located in southern Japan (Fig. 3a).
N1280-HC and N1280-PC fail to properly reproduce the spatial distributions of the summer MCS precipitation over the EASM domain (Fig. 3c and 3d), with a relatively lower pattern correlation coefficients (PCCs) of 0.65 and 0.61, and a higher root-mean-square errors (RMSEs) of 1.45 mm day− 1 and 1.48 mm day− 1, respectively. Specifically, they cannot simulate the precipitation over southeastern China (Fig. 3c and 3d), and underestimate the summer MCS precipitation within the “Mei-yu” and “Chang-ma” rain-belt (Fig. 3c and 3d). However, they better simulate the MCS precipitation over southern Japan (Fig. 3c and 3d). Another interesting feature is that the MCS precipitation in the N1280-HC and N1280-PC is anchored by the topography along the so-called “second-step” to “third-step” terrain region (the region where the 300, 600, 900m topography contours become denser around 32\(^\circ\)~40\(^\circ\)N) over eastern China (Fig. 3c and 3d), which indicates that the precipitation enhancement -effects of the topography are magnified in these two model configurations; this will be specifically discussed in the later section 3.2.2. The overestimation of precipitation along the steep terrain region is smaller in N1280-HC (Fig. 3c), compared with those in N1280-PC (Fig. 3d).
Among three different model configurations, N1280-EC best reproduces the observed pattern of MCS precipitation (Fig. 3b), with a higher PCC of 0.72 and a lower RMSE of 1.37 mm day− 1, compared with N1280-HC and N1280-PC. It well simulates the MCS precipitation center over the southeastern China, as well as better reproduces the “Mei-yu” and “Chang-ma” rain-belt (Fig. 3b). However, it still has some obvious deficiencies: it overestimates the MCS precipitation over southeastern China but underestimates the magnitude of “Mei-yu” and “Chang-ma” rain-belt, as well as the MCS precipitation over southern Japan (Fig. 3b).
3.2.2 The diurnal cycle of MCS precipitation
The diurnal cycle of convective precipitation exhibits some interesting behavior over the EASM region, such as a phase delay running east-west along the Yangtze River (Yu et al. 2007, 2015; Chen et al. 2010). As such, the diurnal cycle represents a rigorous test-bed for validating convection parametrizations and other physical schemes in numerical models (Yuan et al. 2013; Zhou et al. 2018; Li et al. 2020a). In this study, one of the most remarkable differences among the three model configurations is the different diurnal variations of the MCS precipitation over the EASM region (Fig. 4 and Fig. 5). In observations, the diurnal variations of MCS precipitation over the eastern periphery of the Tibetan Plateau (ETP) are dominated by the nocturnal precipitation (Fig. 4a and Fig. 5a), which is due to the low-level moisture transport increasing after sunset and reaching its maximum before dawn, similar with the diurnal variations of the total precipitation which have been documented in previous studies (Chen et al. 2010; Chen et al. 2014; Zhang et al. 2019; Muetzelfeldt et al. 2021). It should be noted that there is a marked regional difference over this steep terrain region. To the west of the ETP, it is the late-afternoon precipitation that dominates the diurnal variations at higher altitudes (regions where the topography exceeds 2700m) over the TP (Fig. 4a), which is quite different from the nocturnal/morning peak at lower altitudes (i.e., the Sichuan Basin) within the ETP region (Li J et al. 2021).
All HadGEM3-GC3.1 simulations well simulate the nighttime MCS precipitation over the ETP (Fig. 4b-d), although with a stronger magnitude (Fig. 5a). However, N1280-PC cannot reproduce the late-afternoon MCS precipitation at higher altitude regions over the TP, it produces more nocturnal rainfall at higher altitudes over the TP than the observed, and therefore incorrectly reproduces the distinct regional features of the diurnal variations over this complex terrain region (Fig. 4d). In contrast, N1280-EC and N1280-HC better reproduce the late-afternoon MCS precipitation at higher altitudes over the TP.
In CMORPH, the diurnal variations of MCS precipitation in the monsoonal “Mei-yu” and “Chang-ma” rain-belt are dominated by the early morning diurnal peaks (Fig. 4a), because of the early-morning acceleration of the low-level monsoon flow and the subsequent strengthening of its convergence (Chen et al. 2013, 2017; Guan et al. 2020). The MCS precipitation in the middle-to-lower reaches of the Yangtze River basin (YRB-ML) in eastern China exhibits two diurnal peaks: one is the early-morning peak; the other one is the late-afternoon rainfall peak (Fig. 4a and Fig. 5b), which is associated with more “surface-driven” intense MCS precipitation that occurs beyond the large-scale monsoonal rain-belt and become predominantly active during the “break” monsoon periods, based on previous studies (Yuan et al. 2010; Yu et al. 2014; Yu and Li 2015). The MCS precipitation over the southeastern China (SEC) shows two weak diurnal peaks in observations (Fig. 4a and Fig. 5c): one primary peak during late-afternoon (1600 ~ 1800 LST) and a secondary peak in the nighttime to early-morning (0200 ~ 0800 LST). The MCS precipitation over the lower reaches of the Yellow River basin (LYB) exhibits two comparable diurnal peaks (Fig. 4a and Fig. 5d).
In the simulations, N1280-HC and N1280-PC both overestimate the magnitude of nighttime MCS precipitation over the YRB-ML (Fig. 5b) and LYB (Fig. 5d). A consistent model bias of N1280-HC and N1280-PC (especially N1280-PC) is that there is a 3 ~ 5 hours delayed phase in simulating the late-afternoon MCS precipitation peak over mainland eastern China (observed at 1600 ~ 1800LST), including the YRB-ML, the SEC, as well as the LYB (Fig. 5b-5d). In addition, N1280-HC and N1280-PC cannot simulate the diurnal variations of the MCS precipitation over the central north China. Specifically, both N1280-HC and N1280-PC (Figure. 4c and 4d) cannot reproduce the northwest-to-southeast delayed phase from mountain to plain along around the 40\(^\circ\)N (Figure. 4a), which will be discussed in more detail in section 3.2.3.
In contrast, N1280-EC better simulates the diurnal variations of MCS precipitation over the YRB-ML (Fig. 5b), SEC (Fig. 5c), as well as LYB (Fig. 5d), particularly for the phase of the peaks. Furthermore, N1280-EC reproduces the diurnal variations of the MCS precipitation over central north China (Fig. 4a and 4b), which indicates that the explicit convection version can reproduce the initiation as well as the propagating features of the MCSs, which generally form over the mountain regions in the afternoon, then propagate downstream at night, inducing heavy rainfall.
The results here are consistent with and complementary to those of Muetzelfeldt et al. (2021), who analysed the same simulations and found that the summertime diurnal cycle of total precipitation over Asia was best simulated in model configurations without parameterized convection. Furthermore, they found that the diurnal variations only improved at finer resolution in simulations without parameterized convection. As MCSs are relatively fine-scale phenomena, it is therefore consistent that the diurnal cycle of their precipitation should be improved in models with explicit convection. Here, we have looked at a dynamical phenomenon, and so our results show that the general findings of Muetzelfeldt et al. (2021) apply to specific features, MCSs, over Asia. This could be interpreted in two ways – these are not necessarily mutually exclusive. First, the improved diurnal cycle at large (synoptic) scales leads to a corresponding improvement in the diurnal cycle at smaller scales, including that of MCSs. Second, the improved representation of MCSs, as shown above in Sect. 3.2.1 showing the number of simulated MCSs, leads to an upscale improvement in the diurnal cycle. In our view, the first of these is most likely, although further work would be required to disentangle these two effects.
3.2.3 Properties of observed and simulated MCSs over the EASM
After evaluating the model differences in simulating the diurnal variations of the MCS precipitation, the MCS statistical properties, including MCS lifetime, rainfall area, average/ maximum hourly precipitation over four sub-regions among CMORPH and simulation are shown in Fig. 6. In general, all three different configurations agree well with the CMORPH in terms of the MCS duration (Fig. 6a), but the MCS in the N1280-HC simulation has relatively longer duration, compared with other two configurations and CMORPH (Fig. 6a).
The most distinct differences between the CMORPH and HadGEM3-GC3.1 simulation are in the rainfall area (Fig. 6b) and intensity (Fig. 6c and 6d) of the MCSs. The MCSs in the all the three different configurations of HadGEM3-GC3.1 simulations have a notably smaller rainfall area (Fig. 6b), but with a much stronger rainfall intensity (both the average and maximum hourly precipitation; Fig. 6c and 6d), compared with CMORPH. This indicates that the model behaviors relating to the spatial morphological characteristics (such as the rainfall area and intensity) of MCS precipitation systems might not be sensitive to the different treatment of convection, and should be attributed to other key elements or parameterizations in the model, for instance the evolution of density currents in the boundary-layer parameterization as in Jucker et al. (2020).
We proceed to investigate the performance of the three model configurations in simulating the dynamical evolution of long-lived MCS precipitation characteristics (MCS rainfall area: Fig. 7; maximum hourly precipitation: Fig. 8) over eastern China from a statistical standpoint. It should be noted that the solid line in Fig. 7 and Fig. 8 indicates the mean of MCSs to represent the general features of the dynamic evolution of MCS precipitation properties, and the shadings indicates the interquartile range across all MCSs.
In general, the three different configurations can partly reproduce the dynamical evolution of the MCS rainfall area, but with a systematic underestimation during almost the whole lifetime, among all four sub-regions (Fig. 7a-7d). The MCS rainfall area in the HadGEM3-GC3.1 simulations increases more slowly in the developing stage and does not grow to a large-enough size at mature stage, compared with that in CMORPH (Fig. 7). Over the YRB-ML, the MCS rainfall area in all three HadGEM3-GC3.1 simulations persists for longer than in CMORPH (Fig. 7b), consistent with the longer MCS duration over the YRB-ML in the simulation (Fig. 6a). Among all three model configurations, the MCS rainfall area in N1280-HC persists relatively longer over ETP (Fig. 7a), YRB-ML (Fig. 7b), as well as LYB (Fig. 7d), compared with those in the CMORPH and other two configurations.
There exists an obvious asymmetry in the dynamical evolution of the MCS maximum hourly precipitation (Fig. 8). The MCS rainfall intensity increases quickly, reaching its peak intensity during the developing stage (Fig. 8). This kind of intense convective precipitation is related to the strong updrafts within an MCS when the precipitating cumulonimbus clouds aggregate and develop. Then the MCS rainfall intensity gradually weakens with a slower rate for the remainder of the MCS’s lifetime (Figure. 8). This kind of relatively moderate stratiform precipitation is induced by large-scale condensation when the updrafts become weaker and cannot support vertical advection of precipitation particles. All three model configurations of the HadGEM3-GC3.1 can generally reproduce the asymmetry in the development of MCS rainfall intensity, but the maximum hourly precipitation intensity in the simulations is about three times higher than in CMORPH.
3.2.4 Different model behaviors in simulating summer MCS precipitation over complex terrain
There is a distinct difference among the three configurations in simulating summer MCS precipitation over complex terrain. Here this difference is illustrated for a region in central north China, and the underlying physical mechanisms are investigated.
The spatial distributions of summer MCS precipitation over central north China, as well as their diurnal variations are shown in Fig. 9 and Fig. 10. A consistent model bias in N1280-PC is that the excessive MCS precipitation is anchored by steep terrain: too much MCS precipitation is concentrated at the mountain slope, the areas where the topography is between the 300m and 600m (Fig. 9d). Additionally, the magnitude of the “Mei-yu” and “Chang-ma” rain-belt is significantly underestimated in N1280-PC, especially the MCS precipitation over the lower-to-middle reaches of the Yangtze River basin in China, as well as the MCS precipitation over South Korea. Therefore, N1280-PC has relatively lower skill over in simulating the spatial pattern of the MCS precipitation over central north China and its surroundings, with a low PCC value of 0.41 and a high RMSE value of 1.59 mm day− 1 (Fig. 9d). The overestimation of the summer MCS precipitation along steep terrain is partly reduced in N1280-HC (Fig. 9c), which might indicate a benefit from using a hybrid convection scheme, but similar kinds of model bias were also found in this model configuration. Thus N1280-HC has moderate skill in simulating the MCS precipitation pattern, with a PCC value of 0.57 and a RMSE value of 1.33 mm day− 1.
A step-change improvement was found between N1280-HC to N1280-EC (from Fig. 9c to 9b), when the convection parametrization scheme is completely disabled. The excessive MCS precipitation over the steep terrain region has been mostly eliminated, and the “Mei-yu” and “Chang-ma” rain-belt are much better depicted (Fig. 9b). As a result, the N1280-EC has higher skill in reproducing the spatial distributions of the MCS precipitation, with a higher PCC value of 0.76 and a lower RMSE value of 0.98 mm day− 1.
We further investigated the diurnal variations and propagating features of the summer MCS precipitation over central north China in CMORPH observations and three simulations (Fig. 10 and Fig. 11). In CMORPH, the MCS precipitation initializes and enhances over the northwestern mountainous region in the late-afternoon (Fig. 10c), thereafter the organized convection propagates to the lower altitudes, i.e., the southeastern plains (Fig. 10d and 10e). The diurnal variations of MCS precipitation are related to the MCS propagating features, as shown by the averaged moving direction and speed of MCSs in Fig. 11. The observed MCSs move eastward from northwestern mountainous region to the southeastern plain with velocities of 35 ~ 40 km h− 1 (Fig. 11a), resulting in intriguingly pronounced spatial distributions of the MCS diurnal variations associated with the topography, where an obvious delayed phase can be seen from northwestern mountains to southeastern plains (Fig. 9e). Following sunrise, the MCS precipitation over the mountains rapidly decreases (Fig. 10f), and reaches a minimum around local noon (Fig. 10a).
N1280-PC cannot accurately reproduce the late-afternoon MCS precipitation over the northwestern mountain (Fig. 10u), nor can it simulate the propagating features of the MCS precipitation (Fig. 10v). In contrast, too much MCS precipitation is seen at lower altitudes following the steep terrain (Fig. 10u), whereas too little MCS precipitation is seen over the northwestern mountainous region where it should occur (Fig. 10c). The excessive MCS precipitation center at lower altitudes in the steep terrain persists throughout the night (Fig. 10v-10x), and continues to exist even after sunrise (Fig. 10s and 10t). These phenomena are also reflected in the averaged MCS propagation direction and speed (Fig. 11). In N1280PC, there are much more MCSs at lower altitudes alongside the steep terrain, and the MCSs remain quasi-stationary with a much lower propagation speed (Fig. 11d), compared to those in CMORPH observations (Fig. 11a). Therefore, the summer MCS precipitation in N1280-PC exhibits inaccurate diurnal variations (Fig. 9h) and distinct wet biases related to the complex terrain over central north China (Fig. 9d).
In N1280-HC, the excessive night MCS precipitation at lower altitudes along the steep terrain is not as pronounced as that in N1280-PC (Fig. 10o-10r), but the MCS in N1280-HC still preferentially occurs and exhibits a quasi-stationary propagating feature at lower altitudes along the steep terrain (Fig. 11c), and the summer MCS precipitation remains active throughout the day (Fig. 10m-10r), thus leading to a similar wet bias of the summer MCS precipitation in N1280-HC (Fig. 9c).
In contrast to N1280-PC and N1280-HC, N1280-EC can better reproduce the late-afternoon MCS precipitation over mountainous region (Fig. 10h and 10i), as well as represent the propagating features of the organized convection systems from late-afternoon to night (Fig. 10j and 10k). In addition, N1280-EC reasonably simulates the direction and speed of the summer MCS propagating features (Fig. 11b), and better depicts the underlying diurnal variations of MCS precipitation over central north China (Fig. 10g-l and Fig. 9f). As a result, N1280-EC better simulates the spatial distribution of the summer MCS precipitation over this complex terrain region (Fig. 9b) and closely resembles CMORPH (Fig. 9a).