The Influence of Summer Deep Soil Temperature on Early Winter Snow Conditions in Eurasia in the NCEP CFSv2 Simulation

Abstract:

Analyzing a long-term simulation of the National Centers for Environmental Prediction (NCEP) Climate Forecast System version 2 (CFSv2), this study explores the potential triggering effect of a major cold bias in the model’s deep soil temperature during summer on the excessive Eurasian snow cover in early winter. We have established that CFSv2 produces excessive snow cover and snow amount in Eurasia throughout the winter-spring season, partly due to an overactive snow-albedo feedback. Compared with observations, the model snow cover also appears earlier during the fall, especially in the western Eurasia. The early seasonal transition is possibly because the model land surface temperature (LST) reaches its freezing point there earlier than the observed so that new snow falling on the frozen ground cannot melt away. This process initiates the snow-albedo feedback earlier in the model. Further evidence shows that the early seasonal cooling of LST is partially influenced by a seasonal re-surfacing of the cold bias in a deep layer 100 to 200 centimeters (cm) beneath the surface. From winter to early spring, a cold model bias prevails in LST and soil temperature in the upper 0-10 cm as the snow cover sustains. During this season, the deep soil (100-200 cm) temperature is generally warmer than the upper one and shows little model bias.

From spring to summer, the cold bias in the upper soil temperature becomes smaller as the upper layer soil warms up in response to solar heating. On the other hand, the model deep soil temperature has a noticeably smaller seasonal change than the observed one, resulting in a severe cold bias during summer. As the solar radiation reduces quickly in the early fall, the cold deep soil temperature causes additional cooling in the upper soil layer and helps to bring LST to the freezing point early in western Eurasia, which lead to enhanced bias in the snow properties over there in early winter.

This study may provide further insight into the mechanisms for influence of the land thermal condition on Eurasian climate, especially the Asian monsoon. The results of this paper provide direction for further development of the coupled model to improve the representation of the mean state in the Asian monsoon region.

References:

Shukla RP., B. Huang, PA. Dirmeyer and JL. Kinter (2019) The influence of summer deep soil temperature on early winter snow conditions in Eurasia in the NCEP CFSv2 simulation. (under minor revision in 2nd time: Journal of Geophysical Research: Atmospheres)