Land-atmosphere interaction and cloud-precipitation characteristics in Tibetan plateau

Land surface processes and summer cloud-precipitation characteristics in the Tibetan Plateau (TP) can impact downstream weather and climate changes. They are also the key to understand Asian monsoon system and atmospheric circulation changes in the Northern Hemisphere.

A research team led by Prof. MA Yaoming from the Institute of Tibetan Plateau Research (ITP) of the Chinese Academy of Sciences and Prof. FU Yunfei from the University of Science and Technology of China systematically reviewed recent progresses in land-atmosphere interaction, cloud-precipitation characteristics and their impacts on downstream weather.

For the key characteristic parameters in land-atmosphere interaction, aerodynamic roughness length was one order of magnitude larger than thermodynamic roughness length in the TP. The excess resistance to heat transfer exhibited obvious diurnal variation.

Remote sensing parameterization schemes for multi-source satellites extended the "point" land-atmosphere flux observation to the entire TP. The temporal resolution of estimated land surface heat fluxes was also improved from days to hours. In the context of TP warming and wetting, the sensible heat flux decreased overall while the latent heat flux increased from 2001 to 2012

The precipitation wet biases modeled by WRF over the TP can be effectively reduced by taking turbulent orographic form drag of complex terrain into account. It was also revealed that soil frozen-thawing significantly affects the soil water and energy budget. It will further enhance TP's thermal forcing to the subtropical westerlies and affect stationary Rossby wave train propagation in middle latitudes.

Diurnal variations of cloud macroscopic and microphysical parameters, together with dynamical characteristics inside clouds were revealed. The vertical distributions of cloud phases and particle size in deep convective clouds were also identified.

The intensity and frequency of precipitation increased from the western TP towards the eastern and southeastern TP, while the storm top altitudes showed contrary trends. The weak deep convective precipitation was the dominant precipitation form in the TP. The thickness of the precipitation cloud was actually compressed by the TP terrain leading to the precipitation profiles difference between the TP and non-plateau regions.

The eastward propagation of convective systems caused by TP heating had profound impacts on the downstream rainstorms over the Yangtze River Basin. The mechanisms were mainly attributed to the interactions among the TP heating, South Asian High, and the western Pacific subtropical high.

The researchers also discussed some aspects that deserve further systematic investigations, such as how to use cloud models and weather models to correctly simulate the physical processes of cloud and precipitation, and how to get an accurate latent heat profile of cloud precipitation in the TP from the observed data in order to evaluate the latent heat structure of the model.