Climatic Effects of Frozen-Thawing and Snow-Melting over the Tibetan Plateau and Application in Seasonal Predictability

The thermodynamic effects of Tibetan Plateau (TP) significant affect the regional and global climate. One of the remarkable features of TP land surface is the extensive distribution of frozen ground and snow, and these significantly influence the surface diabatic heating over TP. The information of snow-melting and frozen-thawing during spring are used for operational summer climate prediction. However, how does the surface diabatic heating anomalies link with snow-melting and frozen-thawing processes? What is the underlying mechanisms behind the diabatic heating anomalies induced by snow-melting and frozen-thawing influencing the general circulation over East Asia and summer precipitation in East China (EC)? Moreover, how to improve the model capability in calculation of surface diabatic heating?

A remarkable behavior is the water storage effect of frozen ground, soil moisture anomalies from the preceding autumn can persist until the subsequent spring via soil freeze-thaw (FT) process. Furthermore, soil moisture in spring can influence diabatic heating exchange between land surface and atmosphere, which further affects TP thermal forcing and general circulation of its surrounding atmosphere due to its long memory. Model simulations have confirmed that soil moisture anomalies over the TP during the preceding autumn and winter had similar climatic effects as the subsequent spring. The TP thermal forcing anomalies caused by soil moisture anomalies would change the subtropical westerlies and affect stationary Rossby wave train propagation in middle latitudes, especially on the northwest and northeast sides of the TP.

Snow cover is another factor that strongly influences surface adiabatic heating over TP, and it is also an important signal used to operational climate prediction. The uncertainties caused by the spatial-temporal heterogeneity of snow cover have been disturbing the operational practice. Observation and simulation results show that snow cover in different regions of TP (southern, northern and western TP) leads to different patterns of summer rainfall anomalies in the Yangtze River basin, northeastern China, and southern China. The key linkage can be attributed to the position change of the exit region of westerly jet, anomalous anticyclone over the East China Sea, and the corresponding 850-hPa water vapor convergence anomalies.

Improving the simulation skill of model is always a hard goal, one side is to reduce the biases of surface diabetic heating. Based on in-situ measurements, meteorological observation data, we have calibrated the hydrothermal parameters, modified FT scheme in Community Land Model (CLM), developed the fully coupled water-heat transport model (FCS) which considers water vapor effects on soil water and heat transport, these improvements of model parameterizations have reduced the uncertainties in simulating surface sensible heat and latent heat fluxes during FT process, improved description of climatic effects of frozen-thawing and snow-melting over the TP in model.