Evaluation of convective boundary layer height estimates using radars operating at different frequency bands

Abstract

Abstract. Knowledge of the atmospheric boundary layer state and evolution is important for understanding air pollution and low-level cloud development, among other things. There are a number of instruments and methods that are currently used to estimate boundary layer height (BLH). However, no single instrument is capable of providing BLH measurements in all weather conditions. We proposed a method to derive a daytime convective BLH using clear air echoes in radar observations and investigated the consistency of these retrievals between different radar frequencies. We utilized data from three vertically pointing radars that are available at the SMEAR II station in Finland, i.e. the C band (5 GHz), Ka band (35 GHz) and W band (94 GHz). The Ka- or W-band cloud radars are an integral part of cloud profiling stations of pan-European Aerosol, Clouds and Trace Gases Research Infrastructure (ACTRIS). Our method will be utilized at ACTRIS stations to serve as an additional estimate of the BLH during summer months. During this period, insects and Bragg scatter are often responsible for clear air echoes recorded by weather and cloud radars. To retrieve a BLH, we suggested a mechanism to separate passive and independently flying insects that works for all analysed frequency bands. At the lower frequency (the C band) insect scattering has been separated from Bragg scattering using a combination of the radar reflectivity factor and linear depolarization ratio. Retrieved values of the BLH from all radars are in a good agreement when compared to the BLH obtained with the co-located HALO Doppler lidar and ERA5 reanalysis data set. Our method showed some underestimation of the BLH after nighttime heavy precipitation yet demonstrated a potential to serve as a reliable method to obtain a BLH during clear-sky days. Additionally, the entrainment zone was observed by the C-band radar above the CBL in the form of a Bragg scatter layer. Aircraft observations of vertical profiles of potential temperature and water vapour concentration, collected in the vicinity of the radar, demonstrated some agreement with the Bragg scatter layer.

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

“…After 10:00 UTC (11:40 LST, 13:00 local legal time), the MWR BL height mostly follows the 10 • C isotherm at an altitude of 1 km a.g.l, while the BL height from ERA5 is higher, typically at 1.5 km a.g.l. The BL height in the cloud classification product is between 0.8-1.2 km a.g.l., indicated approximately by the top of the insects category (Chandra et al, 2010;Franck et al, 2021), and resembles the MWR BL height evolution, even though the MWR estimates the BL height slightly higher. This suggests that between 10:00-17:00 UTC, ERA5 overestimates the BL height.…”

Using cloud radar to investigate the effect of rainfall on migratory insect flight

“…The high sensitivity to low reflectivities means that millimetre‐wavelength radars (e.g. Ka‐ and W‐band) can see small insects that may not be detectable by radars operating at S‐band or X‐band (Franck et al, 2021).…”

Investigating atmospheric and convective boundary layer heights via active optical sensor signals and statistical techniques

“…3 To derive the Convective Boundary Layer Height (CBLH), the researchers proposed a method utilizing clear air echoes in radar observations and assessed the consistency of retrievals among different radar frequencies. 4 In a comparative study, JC (1982) evaluated three in-situ and five remote sensing techniques for measuring the height of the daytime CBL, and they found rawinsonde to be effective but with some uncertainty. For remote sensing, a combination of methods was recommended.…”