Ruolan Xiang

The Hengduan Mountains region (HM), located in the Eastern Tibetan Plateau, is renowned for its rich biodiversity. High-resolution climate data from past periods are essential for gaining deeper insights into the ecological and evolutionary processes that have shaped this unique and diverse region. In this study, we applied the non-hydrostatic limited-area model COSMO, with a resolution of 12 km over East Asia, to simulate two distinct climatic periods: the mid-Pliocene (~3 Ma), representing a warmer period, and the Last Glacial Maximum (LGM; ~21 ka), a colder period, both compared to present-day conditions.Our results reveal that, despite contrasting changes in moisture supply, both warm and cold periods experienced a weakened Indian summer monsoon, attributed to the exposure of the Indochina continental shelf during these times—caused by sea-level drops during the LGM and dynamic topography during the mid-Pliocene. During the mid-Pliocene, an earlier northward migration of the Western Jet led to an earlier onset of the Indian summer monsoon and a wetter spring in the HM. In contrast, the HM experienced increased precipitation during the LGM in both summer and winter. Increased summer precipitation was driven by enhanced moisture supply from the south, while enhanced winter precipitation, primarily in the form of snowfall at high elevations, was associated with more unstable atmospheric stratification.The local precipitation characteristics of the HM are thus influenced by the interplay between large-scale atmospheric dynamics and regional topographical features such that, in contrast to most mid-latitude regions, the HM did not experience drying and wetting during glacial-interglacial cycles. The stability of mean precipitation across different climatic periods likely played a pivotal role in supporting the HM's high biodiversity, providing a stable and moist environment conducive to supporting diverse ecosystems.

To better understand the landscape dynamics and changes in habitat connectivity influenced by glacial and interglacial oscillations over the biodiversity-rich Hengduan Mountains (HM) region, located at the south-eastern edge of the Tibetan Plateau (TP), high-resolution climate data for past periods are essential. We apply the non-hydrostatic limited-area model COSMO with a resolution of 12 km over East Asia for the Last Glacial Maximum (LGM), a period characterized by a generally colder and dryer climate compared to present-day conditions. We perform the downscaling with a novel approach for paleoclimate modelling, the Pseudo-Global Warming (PGW) method. The PGW method minimizes Global Climate Models' (GCMs') inherent biases and reduces computational demands. Despite its widespread use in future climate downscaling, this study represents the first application of the PGW method for paleoclimate simulation to our knowledge. The COSMO PGW simulation for the LGM shows that the regional climate model (RCM) replicates the large-scale dynamics of the driving GCM simulation in the colder climate. Both models suggest weaker Asian summer monsoon systems for this period. Consequently, regions such as the Bay of Bengal, the South China Sea, and the coastal region of China, which typically receive substantial monsoon rainfall, experienced significantly reduced precipitation. However, besides these model similarities, the high-resolution COSMO simulation exhibits distinctive differences on a smaller scale for variables like near-surface wind and precipitation — particularly over land. For instance, COSMO suggests a more significant southward shift of the jet stream during the LGM winter, with more pronounced annual cooling and a prolonged Meiyu season in southern China. Moreover, the COSMO simulation features increased LGM precipitation amounts for the majority of the HM despite the overall weaker summer monsoon circulation. Regarding snowfall, which is a crucial factor for the glaciation extent of the TP during LGM, COSMO suggests increases for certain sub-areas (e.g., central TP) during all seasons, while in the GCM, the increase is primarily constrained to summer and limited to a smaller region near the Himalayas. Furthermore, evaluation with proxy data indicates an improved representation of local climate by COSMO for specific regions. Our study suggests that the resource-saving PGW approach is a suitable method to bridge the gap between coarser climate data and regional climate impacts, also for past periods like the LGM.

Aerosols can alter atmospheric stability through radiative forcing, thereby changing mean and daily extreme precipitation on regional scales. However, it is unclear how extreme sub-daily precipitation responds to aerosol radiative effects. In this study, we use the regional climate model (RCM) Consortium for Small-scale Modeling (COSMO) to perform convection-permitting climate simulations at a kilometer-scale (0.04°/~4.4 km) resolution for the period 2001–2010. By evaluating against the observed hourly precipitation-gauge data, the COSMO model with explicit deep convection can effectively reproduce sub-daily and daily extreme precipitation events, as well as diurnal cycles of summer mean precipitation and wet hour frequency. Moreover, aerosol sensitivity simulations are conducted with sulfate and black carbon aerosol perturbations to assess the direct and semi-direct aerosol effects on extreme sub-daily precipitation in the COSMO model. The destabilizing effects associated with decreased sulfate aerosols intensify extreme sub-daily precipitation, while increased sulfate aerosols tend to induce an opposite change. In contrast, the response of extreme sub-daily precipitation to black carbon aerosol perturbations exhibits a nonlinear behavior and potentially relies on geographical location. Overall, the scaling rates of extreme precipitation intensities decrease and approach the Clausius–Clapeyron rate from hourly to daily time scales, and the responses to sulfate and black carbon aerosols vary with precipitation durations. This study improves the understanding of aerosol radiative effects on sub-daily extreme precipitation events in RCMs.

To better understand the landscape dynamics and changes in habitat connectivity influenced by glacial and interglacial oscillations over the biodiversity-rich Hengduan Mountains (HM) region, high-resolution climate data for past periods are essential. We apply the non-hydrostatic limited-area model COSMO, with a resolution of 12 km over East Asia, to simulate the Last Glacial Maximum (LGM), a period characterized by a generally colder and drier climate compared to present-day conditions. We perform the downscaling with a novel approach for paleoclimate modelling, the Pseudo-Global Warming (PGW) method. The COSMO PGW simulation for the LGM shows that COSMO generally replicates the large-scale dynamics of the driving global climate model simulation in the colder climate. Both models suggest weaker Asian summer monsoon systems during this period. Consequently, regions such as the Bay of Bengal, and the South China Sea, which typically receive substantial monsoon rainfall, experience significantly reduced precipitation. However, despite these model similarities, the high-resolution COSMO simulation exhibits distinctive differences on a smaller scale—particularly over land. For instance, COSMO suggests a more pronounced southward shift of the jet stream during the LGM winter, with cooler conditions in southern China. Moreover, the COSMO simulation, despite the overall weaker summer monsoon circulation, features increased precipitation amounts for much of the HM. Additionally, COSMO suggests a more extensive increase in snowfall over the High Mountain Asia region. Our study suggests that the resource-saving PGW approach is a suitable method to bridge the gap between large-scale projections and regional climate impacts—also for past periods like the LGM.

The Hengduan Mountains (HM) are located on the southeastern edge of the Tibetan Plateau and feature high mountain ridges (>6,000 m MSL) separated by deep valleys. The HM region also features an exceptionally high biodiversity, believed to have emerged from the topography interacting with the climate. To investigate the role of the HM topography on regional climate, we conduct simulations with the regional climate model COSMO at high horizontal resolutions (at ∼12 km and a convection‐permitting scale of ∼4.4 km) for the period 2001–2005. We conduct one control simulation with modern topography and two idealized experiments with modified topography, inspired by past geological processes that shaped the mountain range. In the first experiment, we reduce the HM's elevation by applying a spatially non‐uniform scaling to the topography. The results show that, following the uplift of the HM, the local rainy season precipitation increases by ∼25%. Precipitation in Indochina and the Bay of Bengal (BoB) also intensifies. Additionally, the cyclonic circulation in the BoB extends eastward, indicating an intensification of the East Asian summer monsoon. In the second experiment, we remove deep valleys by applying an envelope topography to quantify the effects of terrain undulation with high amplitude and frequency on climate. On the western flanks of the HM, precipitation slightly increases, while the remaining fraction of the mountain range experiences ∼20% less precipitation. Simulations suggest an overall positive feedback between precipitation, erosion, and valley deepening for this region, which could have influenced the diversification of local organisms.

The Hengduan Mountains (HM) are located on the southeastern edge of the Tibetan Plateau (TP) and feature high mountain ridges (> 6000 m a.s.l.) separated by deep valleys. The HM region also features an exceptionally high biodiversity, believed to have emerged from the topography interacting with the climate. To investigate the role of the HM topography on regional climate, we conduct simulations with the regional climate model COSMO at high horizontal resolutions (at ~12 km and a convection-permitting scale of ~4.4 km) for the present-day climate. We conduct one control simulation with modern topography and two idealised experiments with modified topography, inspired by past geological processes that shaped the mountain range. In the first experiment, we reduce the HM’s elevation by applying a spatially non-uniform scaling to the topography. The results show that, following the uplift of the HM, the local rainy season precipitation increases by ~25%. Precipitation in Indochina and the Bay of Bengal (BoB) also intensifies. Additionally, the cyclonic circulation in the BoB extends eastward, indicating an intensification of the East Asian summer monsoon. In the second experiment, we remove the deep valley by applying an envelope topography to quantify the effects of terrain undulation with high amplitude and frequency on climate. On the western flanks of the HM, precipitation slightly increases, while the remaining fraction of the mountain range experiences ~20% less precipitation. Simulations suggest an overall positive feedback between precipitation, erosion, and valley deepening for this region, which could have influenced the diversification of local organisms.

The Hengudan Mountains, located at the south-eastern fringe of the Tibetan Plateau, reveal exceptionally high biodiversity. It is believed that this feature is linked to past complex interactions between climate, land surface dynamics and plate tectonics in this region. Contemporary topography was formed by plate tectonics, causing surface uplift, and spatially heterogeneous erosion, which shaped the deep river valleys. The non-hydrostatic regional climate model COSMO is applied to study the impact of surface uplift and river incision. Decades-long simulations at horizontal resolutions of 12 and 4.4 km grid spacing are performed. To study the impact of local topography on climate, we consider two idealized experiments with terrains deviating from the present-day topography: In the first experiment, we reduce the topography in a spatially non-uniform way. This altered topography reflects a past potential state of the Hengduan Mountains. In the second experiment, we remove the deep valleys by applying an envelope topography to quantify the effects of deep valleys on the local climate. Both experiments assume that that the large-scale (continental) climate did not change, i.e., the experiments are driven by large-scale reanalysis data. Preliminary results from the coarse-resolution 12 km COSMO simulation indicate that the uplift of the Hengduan Mountains has a strong impact on the summer monsoon over South Asia caused by circulation changes around the uplifted region. The uplift of the Hengduan Mountains strengthens the westerly wind anomalies from the ocean in South Asia and markedly intensifies the precipitation in Indochina and southwestern China. Besides, the cyclonic circulation in the Bay of Bengal extends eastward, indicating an intensification of the East Asian summer monsoon. The diabatic heating in the eastern Tibetan Plateau increases in response to the regional uplift and it is coupled with the increased precipitation in summer through moist processes. On the contrary, the uplift has little impact on the strengthening of the winter monsoon. In the next stage, we will conduct the same simulations at a higher horizontal resolution of 4.4 km, which captures local terrain more accurate. These simulations will use explicit rather than parameterized convection, thereby providing more realistic estimates of heavy precipitation and erosion. Subsequently, we will run the same experiments for the envelop topography. We expect to relate the changes in the frequency and intensity of extreme precipitation to the changes in the local moisture transport and vertical movement in the high-resolution perspective. In the future, the two different topographies along with the modern topography will be used for simulations of two time periods in the past (i.e., the Last Glacial Maximum (21,000 years ago) and a phase in the Late Miocene (∼7 Ma)) and the future climate (2070–2100).