A long-term Climatology of Planetary Boundary Layer Height over the Tibetan Plateau revealed by ERA5

The Planetary Boundary Layer Height (PBLH) is important for the exchange of energy, water, and momentum between the surface and the free atmosphere, making it a significant factor in studies of surface climate and atmospheric circulation. Over the Tibetan Plateau (TP) - a vast elevated heat source exerting significant influence on the Asian monsoon systems - the climate is changing rapidly. Among the many climate variables expected to change as global temperatures rise is the PBLH which, in addition to temperature profile, mechanical turbulence production, vertical velocity, and horizontal advection, is highly dependent on the surface sensible heat  fluxes. Our understanding of PBLH over the TP is very limited, although scattered estimates has indicated that it sometimes reach unusual heights – up to the vicinity of the tropopause. Long-term assessment of PBLH covering the whole TP is hampered by the fact that observations are scarce in time and space. This study takes advantage of a recently available high-resolution reanalysis (ERA5) for 1979-2018 to create a multi-decadal climatology of PBLH over the TP, and assess the seasonality, interannual variation and long-term trend of PBLH in relation to other climate variables such as tropopause height and surface sensible heat flux as well as large-scale atmospheric circulation. 

The results show that the most prominent feature of the PBLH trend is a large region of decline in the central TP during the monsoon season. Notably, this is a region where the temperature increase is smaller than in the rest of the region, and the precipitation shows a statistically significant increasing trend. Increased cloudiness may therefore have decreased the surface heating and thus the sensible heat flux and PBLH. Assessing the spatially averaged trends for the first and second halves of the period separately reveals that the monsoon season PBLH does in fact increase during the first half of the period. In the dry season in contrast, the spatially averaged PBLH decreases by almost 30 meter per decade during the first half of the period and increases slightly in the second. Although none of the spatially averaged PBLH trends are statistically significant at the 95% level, it can be noted that the shift from decreasing to increasing PBLH for the dry season is in accordance with a recent study of spring sensible heat flux over the TP. The aforementioned study found that although the sensible heat flux has been declining because of wind speed decreases, it has recently started to recover in response to an increased difference between the ground surface temperature and the air temperature. Given that the PBLH is highly dependent on the surface sensible heat flux, this decline and recovery may very well have produced the PBLH trends for the dry season. In the monsoon season, with cloudy conditions and less solar radiation reaching the ground, other factors are likely of greater importance for the PBLH.