November 26, 2021
Impacts of complex terrain on water vapor transfer and precipitation over the Tibetan Plateau
#### Prof. Kun Yang (Department of Earth System Science, Tsinghua University) #### 2021년 11월 30일 (화) 16:00 #### Zoom
#### Abstract The Southern Tibetan Plateau (STP) is the region in which water vapor passes from South Asia into the Tibetan Plateau (TP). The accuracy of precipitable water vapor (PWV) modeling for this region depends strongly on the quality of the available estimates of water vapor advection and the parameterization of land evaporation models. In this study, PWV data from four (MERRA, ERA-interim, JRA-55 and NCEP-final) reanalyses are evaluated against ground-based GPS measurements at nine stations over the STP, which covers the summer monsoon season from 2007 to 2013. The key finding is that all the reanalyses have positive biases along the PWV seasonal cycle, which is linked to the well-known wet bias over the TP of current climate models. The PWV diurnal cycle in the reanalysis models are also stronger than observed one. To understand the cause of this bias, WRF simulations with 30 km, 10 km and 2 km resolutons were conducted in the Himalayan Range. The result shows that the higher solution modeling yields less water vapor transport from South Asia to STP. Accordingly, the precipitation bias in the 2km-resolution simulation is much less than the 30km-resolution simulation. The larger magnitude of water vapor flux in the coarser resolution simulation is mainly due to the impacts of small-scale (~ km) complex terrain and land cover on the water vapor not being reflected in the simulation. These impacts include the turbulent ogrographic form drag (TOFD), small-scale-terrain triggered precipitation and vertical mixing of water vapor, and snow/glacier cooling. All these impacts retard or reduce water vapor transfer from South Asia into Tibetan Plateau. We introduced the TOFD scheme developed by Beljaars et al. (2004) in WRF3.7 to simulate the climate over the complex terrain of the TP from May to October 2010, with a resolution of 30 km. The new TOFD scheme alleviates the mean bias in wind speed and lowers water vapor flux over the STP. Accoridngly, the simulated precipitation with the new scheme is improved. We futher applied the WRF to simulating precipition in South central Himalaya. In order to validate the climate modeling accurcy in this region, a network consisting of 14 rain gauges was set up from elevations of 2800 m to 4500m asl along a valley. The results show that the simulations with high horizontal resolution (dx=3km) can not only increase the spatial consistency between the observed and simulated precipitation, but also considerably reduce the wet bias by more than 250%. Adding the TOFD scheme further reduces the precipitation bias by 50% or so at almost all stations. Both high horizontal resolution and TOFD enhance the orographic drag to slow down wind; as a result, less water vapor is transported from lowland to the high altitudes of Himalaya, causing more precipitation at lowland area and less at high altitudes. Therefore, in this highly terrain-complex region, it is crucial to use a high horizontal resolution to depict mesoscale complex terrain and a TOFD scheme to parameterize the drag caused by microscale complex terrain. Finally, we applied the revised WRF to the whole Plateau, which is very time-consuming. The simulation obviously improves the simulation of wind and precipitation amount, compared to ERA5 and HAR data sets.