Abstract
Abstract. The Tibetan Plateau (TP) region, often referred to as the Third Pole, is the world's highest plateau and exerts a considerable influence on regional and global climate. The state of the snowpack over the TP is a major research focus due to its great impact on the headwaters of a dozen major Asian rivers. While many studies have attempted to validate atmospheric reanalyses over the TP area in terms of temperature or precipitation, there have been – remarkably – no studies aimed at systematically comparing the snow depth or snow cover in global reanalyses with satellite and in situ data. Yet, snow in reanalyses provides critical surface information for forecast systems from the medium to sub-seasonal timescales. Here, snow depth and snow cover from four recent global reanalysis products, namely the European Centre for Medium-Range Weather Forecasts (ECMWF) ERA5 and ERA-Interim reanalyses, the Japanese 55-year Reanalysis (JRA-55) and the NASA Modern-Era Retrospective analysis for Research and Applications (MERRA-2), are inter-compared over the TP region. The reanalyses are evaluated against a set of 33 in situ station observations, as well as against the Interactive Multisensor Snow and Ice Mapping System (IMS) snow cover and a satellite microwave snow depth dataset. The high temporal correlation coefficient (0.78) between the IMS snow cover and the in situ observations provides confidence in the station data despite the relative paucity of in situ measurement sites and the harsh operating conditions. While several reanalyses show a systematic overestimation of the snow depth or snow cover, the reanalyses that assimilate local in situ observations or IMS snow cover are better capable of representing the shallow, transient snowpack over the TP region. The latter point is clearly demonstrated by examining the family of reanalyses from the ECMWF, of which only the older ERA-Interim assimilated IMS snow cover at high altitudes, while ERA5 did not consider IMS snow cover for high altitudes. We further tested the sensitivity of the ERA5-Land model in offline experiments, assessing the impact of blown snow sublimation, snow cover to snow depth conversion and, more importantly, excessive snowfall. These results suggest that excessive snowfall might be the primary factor for the large overestimation of snow depth and cover in ERA5 reanalysis. Pending a solution for this common model precipitation bias over the Himalayas and the TP, future snow reanalyses that optimally combine the use of satellite snow cover and in situ snow depth observations in the assimilation and analysis cycles have the potential to improve medium-range to sub-seasonal forecasts for water resources applications.
Highlights
Often referred to as the Third Pole, the Tibetan Plateau (TP) region is the world’s highest plateau, with an average elevation of 4000 m above sea level
We have shown that several recent, state-of-the-art reanalyses produce an over-extensive snowpack in autumn, winter and spring over parts of the TP
This is at odds with observational studies revealing that snowfall events are very transient, that the snow cover vanishes rapidly on timescales of days and that large parts of the TP can remain snow-free in winter (e.g. Basang et al, 2017; Li et al, 2018)
Summary
Often referred to as the Third Pole, the Tibetan Plateau (TP) region is the world’s highest plateau, with an average elevation of 4000 m above sea level. The formation and variability of the Asian summer monsoon, in particular, is affected by the TP through thermal and mechanical effects (Wu et al, 2012, 2015; Xiao and Duan, 2016), with remote impacts both downstream The extent and variability of the snowpack over the TP has been a major focus of investigation because of the role of snow in the surface energy balance and in the hydrological cycle, as well as its potential impact on the large-scale circulation through radiative or thermodynamical feedbacks (Xiao and Duan, 2016; Lin et al, 2016; Henderson et al, 2018). The TP is a challenging region for snow-related research due to the complex terrain and the relative paucity of in situ observation stations in this vast, sparsely populated expanse, especially in its western part
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