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 automatic lidars and ceilometers (ALC). ALC are compact, simple lidars which operate at wavelengths mostly in the infrared or visible spectral region. Main output is the cloud base altitude and the profile of attenuated backscatter signal. The signal-to-noise ratio capabilities vary greatly between ALC due to the wide range models available from various manufacturers. ALC performance may be especially limited in pristine environments where aerosol load is low or at elevated heights above the sensor (e.g., deep ABL development). Further details on ALC operations are summarised in Kotthaus et al. (2022). This review lists the following ALC capabilities which make them particularly useful for monitoring the atmospheric boundary layer:
- 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 400 sites across Europe in the E-PROFILE network).
The ABL testbed network includes a variety of ALC models with different signal strengths:
- Relatively high signal-to-noise ratio: Lufft CHM15k, Vaisala CL61
- Relatively low signal-to-noise ratio: Vaisala CL31, CL51.
Doppler wind lidar (DL)
Doppler wind lidars are active remote sensing systems similar to aerosol backscatter lidars. Heterodyne DL 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 radiation the radial velocity along the laser beam direction can be derived. By combining beams in multiple directions, the three wind components are observed along the profile. Using high temporal resolution data, indicators of atmospheric turbulence, such as the vertical velocity variance or the eddy dissipation rate are calculated as advanced products. Further details on DL capabilities and limitations are summarised in Kotthaus et al. (2022).
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, water liquid path and integrated water vapour content) 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. (2022).