Towards Ensemble-Based Kilometer-Scale Climate Simulations over the Third Pole Region

Abstract

The Tibetan Plateau and its surrounding mountains have an average elevation of 4,400 m and a glaciated area of ∼100,000 km 2 giving it the name "Third Pole (TP) region". The TP is the headwater of many major rivers in Asia that provide fresh water to hundreds of millions of people. Climate change is altering the energy and water cycle of the TP at a record pace but the future of this region is highly uncertain due to major challenges in simulating weather and climate processes in this complex area. The Convection-Permitting Third Pole (CPTP) project is a Coordinated Regional Downscaling Experiment (CORDEX) Flagship Pilot Study (FPS) that aims to revolutionize our understanding of climate change impacts on the TP through ensemble-based, kilometerscale climate modeling. Here we present the experimental design and first results from multi-model, multi-physics ensemble simulations of three case studies. The five participating modeling systems show high performance across a range of meteorological situations and are close to having "observational quality" in simulating precipitation and nearsurface temperature. This is partly due to the large differences between observational datasets in this region, which are the leading source of uncertainty in model evaluations. However, a systematic cold bias above 2,000 m exists in most modeling systems. Model physics sensitivity tests performed with the Weather Research and Forecasting (WRF) model show that planetary boundary layer (PBL) physics and microphysics contribute equally to model uncertainties. Additionally, larger domains result in better model performance. We conclude by describing high-priority research needs and the next steps in the CPTP project.

Wet bias of summer precipitation in the northwestern Tibetan Plateau in ERA5 is linked to overestimated lower-level southerly wind over the plateau

“…Besides the impact of horizontal resolution on RCM simulations of precipitation over the TP, the domain size, especially the locations of boundaries, is another important influencing factor (Prein et al 2022). The boundaries of the domain for a RCM should cover the major dynamic systems that affect the local circulation and precipitation.…”

“…We observed that the northern boundaries of the RCMs used in the work of Ou et al (2020) and Ma et al (2021a) were at approximately 40° N, which may affect the ability of their models to capture the interactions between the Westerlies, Asian summer monsoons, and high plateau. A recent study by Prein et al (2022), based on a set of cross-model CPM case study simulations, also indicated that the domain boundaries of RCMs are very important in enabling the realistic simulation of precipitation over the TP; their simulations with larger domains performed better than those with relatively smaller domains, with the northern boundary located at approximately 40° N. Therefore, we here propose a large domain RCM simulation, with the northern boundary covering the location of the core of the summer upper-tropospheric subtropical Westerlies over the TP; we use this to examine the ability of a RCM to simulate summer precipitation over the NWTP. We hypothesize that a RCM with such a large domain, which may develop its own climatology and potentially modify some large-scale climate structures more realistically, can successfully capture the interactions between the Westerlies, Asian summer monsoons, and high plateau, and in turn, realistically simulate the precipitation climatology over the NWTP.…”

Mesoscale convective systems in the third pole region: Characteristics, mechanisms and impact on precipitation

“…In numerical weather models, summer precipitation over the TP has been shown more sensitive to the cloud microphysics schemes than other parameterization schemes, such as planetary boundary layer and radiation schemes (Orr et al, 2017;Lv et al, 2020). Prein et al (2022) also showed that the uncertainties of microphysics schemes are larger for cases of summer precipitation including an MCS case over the TP than that in winter, which points to the importance of further exploring the microphysics processes within MCSs.…”

“…These include, for instance, the High Asia Refined analysis (HAR; Maussion et al, 2014;Wang et al, 2021c) with a horizontal grid spacing of 10 km and a WRFbased downscaling product with a horizontal grid spacing of 9 km (Ou et al, 2020;2023; Table 4). In addition, a CORDEX Flagship Pilot study named "Convection-Permitting Third Pole" (Prein et al, 2022) has been launched and endorsed by WCRP-CORDEX to coordinate the high-resolution modeling work focusing on MCSs and precipitation over the TP and its downstream regions. One goal of this project is to systematically investigate the sensitivities related to different modeling systems, model dynamics, and model physics, in order to define an optimal model configuration for multi-decadal past and future simulations over the TP.…”

“…In particular, small-scale processes that influence the initiation and organization of convection such as entrainment and vertical mass fluxes are still parametrized (e.g., microphysical and planetary boundary layer parameterizations, land-atmosphere interactions). Because parameterization schemes often require tuning to specific cases and regions, the choice of parameterization scheme affects the representation of convective processes and results in different precipitation characteristics (Lv et al, 2020;Prein et al, 2022). In addition to that, the representation of convective processes is also influenced by the dynamic core of the model system and the general model configuration.…”