Evaluation of Asian summer precipitation in different configurations of a high-resolution GCM at a range of decision-relevant spatial scales

Abstract

Abstract. High-resolution general circulation models (GCMs) can provide new insights into the simulated distribution of global precipitation. We evaluate how summer precipitation is represented over Asia in global simulations with a grid length of 14 km. Three simulations were performed: one with a convection parametrization, one with convection represented explicitly by the model's dynamics, and a hybrid simulation with only shallow and mid-level convection parametrized. We evaluate the mean simulated precipitation and the diurnal cycle of the amount, frequency and intensity of the precipitation against satellite observations of precipitation from the Climate Prediction Center morphing method (CMORPH). We also compare the high-resolution simulations with coarser simulations that use parametrized convection. The simulated and observed precipitation is averaged over spatial scales defined by the hydrological catchment basins; these provide a natural spatial scale for performing decision-relevant analysis that is tied to the underlying regional physical geography. By selecting basins of different sizes, we evaluate the simulations as a function of the spatial scale. A new BAsin-Scale Model Assessment ToolkIt (BASMATI) is described, which facilitates this analysis. We find that there are strong wet biases (locally up to 72 mm day−1 at small spatial scales) in the mean precipitation over mountainous regions such as the Himalayas. The explicit convection simulation worsens existing wet and dry biases compared to the parametrized convection simulation. When the analysis is performed at different basin scales, the precipitation bias decreases as the spatial scales increase for all simulations; the lowest-resolution simulation has the smallest root mean squared error compared to CMORPH. In the simulations, a positive mean precipitation bias over China is primarily found to be due to too frequent precipitation for the parametrized convection simulation, and too intense precipitation for the explicit convection simulation. The simulated diurnal cycle of precipitation is strongly affected by the representation of convection: parametrized convection produces a peak in precipitation too close to midday over land, whereas explicit convection produces a peak that is closer to the late afternoon peak seen in observations. At increasing spatial scale, the representation of the diurnal cycle in the explicit and hybrid convection simulations improves when compared to CMORPH; this is not true for any of the parametrized simulations. Some of the strengths and weaknesses of simulated precipitation in a high-resolution GCM are found: the diurnal cycle is improved at all spatial scales with convection parametrization disabled; the interaction of the flow with orography exacerbates existing biases for mean precipitation in the high-resolution simulations; and parametrized simulations produce similar diurnal cycles regardless of their resolution. The need for tuning the high-resolution simulations is made clear. Our approach for evaluating simulated precipitation across a range of scales is widely applicable to other GCMs.

Sensitivity of simulated mesoscale convective systems over East Asia to the treatment of convection in a high-resolution GCM

“…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 con gurations without parameterized convection. Furthermore, they found that the diurnal variations only improved at ner resolution in simulations without parameterized convection.…”

“…This has an important consequence: if the representation of MCS and its remote drivers is important (e.g., to represent extremes in rainfall), it would be desirable to use an explicit-convection simulation and a global setup. The simulations investigated in this study are promising in this regard but currently have substantial mean-state biases in Asian precipitation, thus tuning of these simulations should be a priority (Muetzelfeldt et al 2021).…”

“…As MCSs are relatively ne-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 ndings of Muetzelfeldt et al (2021) apply to speci c features, MCSs, over Asia. This could be interpreted in two ways -these are not necessarily mutually exclusive.…”

“…By using global simulations, we perform our modelling with consistent physics at all scales and potentially capture phenomena such as mid-latitude and tropical interactions with the EASM in a physically consistent way. Our simulations are fully described in Muetzelfeldt et al (2021), here we note the important aspects for this study. We make use of free running high-resolution global Met O ce Uni ed Model simulations performed with three different con gurations: fully parametrized convection (N1280-PC), fully explicit convection (N1280-EC) and hybrid convection (N1280-HC).…”

“…N1280-HC has the deep convection scheme disabled but retains the shallow scheme. Both of these con gurations use broadly the same settings as N1280-PC but have several other minor modi cations for technical reasons and numerical stability (Muetzelfeldt et al 2021). All three simulations were run for at least 4 years covering the period 2005-2008.…”