The ABL testbed is exploiting observations from compact ground-based remote sensing instruments (Kotthaus et al. 2023) for the detection of ABL heights. Automatic detection is implemented using aerosol-based approaches based on observations from automatic lidars and ceilometers (ALC). Thoese are compared to turbulence-based retrievals that use Doppler wind lidar (DWL) and thermodynamic approaches based on observations from micro-wave radiometers (MWR).
Automatic lidars and ceilometers (ALC)
Lidar systems are active remote-sensing instruments. For aerosol backscatter lidars, it can be generally differentiated between high-power lidar systems and the comparatively low-power, compact automatic lidars and ceilometers (ALC). ALC are simple lidars which operate at wavelengths mostly in the infrared or visible spectral region. Main output is the cloud base height and the profile of of a range-corrected signal that can be converted into the attenuated backscatter variable through absolute calibration. ALC characteristics that make these sensors particularly useful for monitoring the atmospheric boundary layer (Kotthaus et al. 2023) include:
- ALC can be operated continuously and autonomously under all weather conditions with very low maintenance so that their data have a much greater temporal coverage than those collected by research lidars which often focus on specific periods of interest.
- ALC have a smaller region of incomplete optical overlap compared to high-power lidar systems. This means they can provide quality observations even at very low altitudes above the surfaces which makes them more suitable for the detection of shallow layer heights.
- ALC have an unprecedented spatial distribution (e.g. more than 450 sensors across Europe in the E-PROFILE network).
The capabilities and limitations vary greatly between ALC makes and models that are available from various manufacturers. Important characteristics include the signal-to-noise ratio and the blind zone in the near range that stems from an incomplete optical overlap. Further details on ALC characteristics and operations are summarised in Wagner et al. (2024). The ABL testbed network includes a variety of ALC models with different signal strengths:
| High SNR | Medium SNR | Low SNR | |
| Good optical overlap < 200 m | Vaisala CL61 | Vaisala CL51 | Vaisala CL31 |
| Blind zone < 200 m | Ott Hydromet/Lufft CHM15k |
Doppler wind lidar (DWL)
Doppler wind lidars are active remote sensing systems similar to aerosol backscatter lidars. Heterodyne DWL that are mostly used to probe the atmospheric boundary layer operate at wavelengths between 1.5-2.0 µm. Based on the detected Doppler shift between the emitted and backscattered signal the radial velocity along the laser beam direction can be derived. By combining beams in multiple directions, the three wind components can be derievd along the profile. Using high temporal resolution data, indicators of atmospheric turbulence, such as the vertical velocity variance or the eddy dissipation rate can be calculated as advanced products. Further details on DWL capabilities and limitations are summarised in Kotthaus et al. (2023) and Preissler et al. (2024).
Microwave radiometers (MWR)
Microwave radiometers (MWR) are passive ground-based remote sensing instruments that capture the downwelling radiance naturally emitted by the atmosphere at selected band channels.The measured radiance is internally converted to the atmospheric brightness temperature (Tb). This holds information on atmospheric thermodynamic conditions so that further atmospheric variables (e.g., temperature, humidity, liquid water path) can be derived using retrieval methods aided by some a priori knowledge. Further details on MWR capabilities and limitations are summarised in Kotthaus et al. (2023) and Knist et al. (2024).