A long-term climatology of boundary-layer height and stability at Hyytiälä in southern Finland.

The boundary-layer height and stability are two key parameters that control the exchange of energy between the surface and the atmosphere and strongly influence air quality and pollution dispersion. In this study, we investigate the boundary layer (BL) height and stability at the long-term measurement station of Hyytiälä, located in the Boreal forest of southern Finland at a latitude of 61.85N. A 41-year climatology of the annual and diurnal cycle of BL height was created based on ERA5 reanalysis data. The ability of ERA5 to correctly capture the BL height was determined by comparing ERA5 to BL heights derived from 847 radiosondes that were released from Hyytiälä as part of the 7.5 month “Biogenic Aerosols - Effects on Clouds and Climate” (BAECC) campaign in 2014. Four different methods to estimate the BL height were applied to the radiosondes. A 25-year climatology of surface-layer stability was created based on eddy covariance measurements and was used to identified under which conditions ERA5 can best capture the BL height and to better understand the annual and diurnal cycle of the BL height. The climatology results show that the shallowest (353~m) monthly median BL height occurs in February and the deepest (576~m) in June. The largest variability in BL height, and the largest diurnal range, was found in April and May. Notably, the shallowest BLs were found to occur at night in spring and summer which is also when very stable conditions were most likely to occur. Between November and February, there was no diurnal cycle in BL height due to the limited solar radiation at this time of year. Unstable conditions were rare during the cold season but so were very stable BLs. The absence of very stable conditions in winter is related to the stronger winds, and hence more shear-driven turbulence, compared to either spring of summer. Very shallow and stable BLs are also prevented from developing in autumn and winter due to the increased amount of cloud compared to spring and summer. Good agreement was found between the BL height in ERA5 and the BL height diagnosed from the radiosondes for almost all stability classes but ERA5 does overestimates the BL height in very stable conditions. In addition, ERA5 BL height differs most from the radiosondes at 18 UTC. These results suggest that ERA5 has adequate vertical resolution to correctly resolve the BL height in most conditions, that ERA5 struggles to correctly simulated stable BLs, and lastly that ERA5 does not resolve the evening transition correctly.