Thermal structure and water-ice heat transfer in a shallow ice-covered thermokarst lake in central Qinghai-Tibet Plateau

A large number of lakes and ponds are unevenly distributed over the Qinghai-Tibet Plateau (QTP). Little is known about their ice processes and thermal regimes. Seasonal ice mass balance, thermal regime and stratification of under-ice water were investigated in a shallow thermokarst lake in central QTP using in situ observations and global reanalysis data. Congelation ice grew to 60-70 cm depth while continuous surface sublimation caused a total ice loss of over 30 cm. The bulk lake temperature below ice remained above 3.0 degrees C through the ice freezing period and rose gradually up to 7-9 degrees C during the melting period. The vertical thermal structure of under-ice water consisted of a stable strongly stratified interfacial layer (IL) and an underlying convective layer reaching the lake bottom. A warm layer existed just beneath the IL since the middle equilibrium period and increased in thickness and temperature. There was seasonal variation of IL depth and temperature gradient in response to the ice thermodynamics and atmospheric conditions. The calculated daily water-ice heat flux (F-w) was of 10 W m(-2) magnitude. Seasonal variation of F-w manifested both diffusive and convective heat transport to the ice-water interface. This study suggests that strong penetration of solar radiative flux is the dominant contributor to high F-w which results in a relatively thin ice compared with other equivalent high-latitude climate.