Regional Climate Dynamical Downscaling Over Tibetan Plateau: From Quarter-Degree To Kilometer-Scale

Famous as “Roof of the World”, the Tibetan Plateau is tough to be conquered by the meteorological scientists for its high altitude, steep terrain and harsh natural environments. Limitations and uncertainties of general observation tools spawned model simulations to obtain more comprehensive meteorological information. However, numerous studies have shown large biases in global climate model simulations over the Tibetan Plateau. To acquire more refined and representative datasets, dynamical downscaling, an important way to obtain high-resolution climate information in the target region, entering the limelight.

Influenced by the surface complexity of the Tibetan Plateau, the dynamic downscaling research of the Tibetan Plateau has always been a difficulty in climate modeling. Professor Yanhong Gao is one of the earliest scholars in this field. Her team has been focusing on dynamical downscaling simulations (at about quarter-degree spatial resolution) for about 10 years. They carried out a continuous simulation long-term climate simulations for a period of 33 years from 1979 to 2011, aiming at simulating the changes to the climate and environment. The study showed that, compared to the simulations at over 100-km resolution, the quarter-degree simulated the observed elevation-dependent warming characteristics more accurately, and better reproduced the different precipitation minus evaporation variability over the western hinterland and southeastern margin of the Tibetan Plateau. The simulations of evapotranspiration and wind speed were also improved to some extent.

Meanwhile, there are other research teams also have carried out dynamical downscaling simulations over the Tibetan Plateau. For instance, German scientists focused on the precipitation type and the seasonal and inter-annual variability of precipitation in the High Asia glacial region. Results agree with Gao’s viewpoint that orographic precipitation can be simulated more accurately using dynamical downscaling rather than 100-km simulations.

Thanks to supercomputing improvements, the era of kilometer-scale simulation has arrived. Compared with quarter-degree, kilometer-scale dynamical downscaling is able to resolve certain sub-grid convective processes and more accurately describe the influence of the lower boundary (e.g. resolve deep convection processes explicitly, eliminating the need for a deep-convection parameterization scheme and thus avoiding the associated uncertainties).

A series of works have been conducted by Gao’s group in kilometer-scale dynamical downscaling research field. (1) They found that kilometer-scale simulations differ little from quarter-degree simulations at altitudinal ranges where observations are available. However, in high-altitude areas over 5000 meters, there are significant differences between the two simulation scales. (2) Based on multiple sources of datasets, they evaluate the snow cover simulated at kilometer-scale and quarter-degree grid spacing in high-altitudes areas: kilometer-scale simulation outperforming the quarter-degree simulation, and both of them reproduced the distribution characteristics of snow cover with altitude more accurately, and resulted in a probability density distribution function that was closer to the reference snow product. (3) Addressed improvement mechanism of kilometer -scale simulation on daily variation of precipitation. the positive bias of wet convective instability in kilometer-scale simulations is less than that in quarter-degree simulations, which makes the timing of the triggering of convection, and its intensity, more consistent with observations, thus are able to simulate the diurnal cycle characteristics of precipitation more accurately than previous quarter-degree dynamical downscaling studies.

To sum up, dynamical downscaling is an effective way to obtain high-resolution gridded climate information. It can provide a solid support for the study of climate and environmental changes over the Tibetan Plateau and its response to global changes. Along with improvement in the grid spacing and the explicit resolving of deep convective processes, kilometer-scale simulations show certain value-added effects in the triggering time of convection and water vapor transport, heralding a new stage in the evolution of this approach.