Boundary-layer height and surface stability at Hyytiälä, Finland, in ERA5 and observations

Abstract

Abstract. We investigate the boundary-layer (BL) height at Hyytiälä in southern Finland diagnosed from radiosonde observations, a microwave radiometer (MWR) and ERA5 reanalysis. Four different, pre-existing algorithms are used to diagnose the BL height from the radiosondes. The diagnosed BL height is sensitive to the method used. The level of agreement, and the sign of systematic bias between the four different methods, depends on the surface-layer stability. For very unstable situations, the median BL height diagnosed from the radiosondes varies from 600 to 1500 m depending on which method is applied. Good agreement between the BL height in ERA5 and diagnosed from the radiosondes using Richardson-number-based methods is found for almost all stability classes, suggesting that ERA5 has adequate vertical resolution near the surface to resolve the BL structure. However, ERA5 overestimates the BL height in very stable conditions, highlighting the ongoing challenge for numerical models to correctly resolve the stable BL. Furthermore, ERA5 BL height differs most from the radiosondes at 18:00 UTC, suggesting ERA5 does not resolve the evening transition correctly. BL height estimates from the MWR are also found to be reliable in unstable situations but often are inaccurate under stable conditions when, in comparison to ERA5 BL heights, they are much deeper. The errors in the MWR BL height estimates originate from the limitations of the manufacturer's algorithm for stable conditions and also the misidentification of the type of BL. A climatology of the annual and diurnal cycle of BL height, based on ERA5 data, and surface-layer stability, based on eddy covariance observations, was created. The shallowest (353 m) monthly median BL height occurs in February and the deepest (576 m) in June. In winter there is no diurnal cycle in BL height; unstable BLs are rare, yet so are very stable BLs. The shallowest BLs occur at night in spring and summer, and very stable conditions are most common at night in the warm season. Finally, using ERA5 gridded data, we determined that the BL height observed at Hyytiälä is representative of most land areas in southern and central Finland. However, the spatial variability of the BL height is largest during daytime in summer, reducing the area over which BL height observations from Hyytiälä would be representative.

On the formation of biogenic secondary organic aerosol in chemical transport models: an evaluation of the WRF-CHIMERE (v2020r2) model with a focus over the Finnish boreal forest

“…total number of particles up to 150 nanometers and clwc, occurred below about 1000 meters a.s.l. which is roughly the estimated average PBL height during summer period at SMEAR-II (Sinclair et al, 2022), with larger increases, in both particles number and clwc, slight below the 1000 m a.s.l altitude. Climatic feedbacks from biogenic particles over boreal area were very recently reported in Yli-Juuti et al, 2021 by the means of remote sensing and ACSM observations available at the SMEAR-II stations.…”

Vertical profiles of volatile organic compounds and fine particles in atmospheric air by using aerial drone with miniaturized samplers and portable devices

“…The boundary layer (BL) is the lowest part of troposphere and connects the ground and the free atmosphere. The average boundary layer height at Hyytiälä SMEAR II Station in autumn (October) was around 500 m (Sinclair et al, 2022), explaining why we found higher BC concentration at high altitudes. For comparison, Table 2 shows the BC mass concentrations measured at high altitudes in different areas.…”

Atmospheric boundary layer height from ground-based remote sensing: a review of capabilities and limitations

“…The Heffter method often overestimates MBLH, and the definition of thresholds was found challenging (see discussion in Caicedo et al, 2020, and references therein). Sinclair et al (2022) find that agreement and sign of systematic biases depend on atmospheric stability. The T b regression method (Cimini et al, 2013) provides height estimates that are mostly consistent with the bulk Richardson approach.…”