Continuous Ceilometer-Derived Mixed Layer Height Observations at an Eddy Covariance Flux Tower Site in the Semi-Arid U.S. Southwest

Abstract

The planetary boundary layer (PBL) is the atmospheric layer directly influenced by the Earth’s surface. The height of the top of the PBL (PBLH) is a key parameter that determines the extent of vertical mixing, which plays a crucial role in micrometeorology, pollution dispersion, and the exchange of energy and matter between the land and the atmosphere. In the past, PBLH estimates have relied largely on observations with low temporal resolution (radiosondes) or on modelling outputs (reanalysis). However, growing interest in the integration of PBLH measurements at surface energy flux tower sites has highlighted the temporal, spatial, and financial trade-offs that come with different PBL height measurement techniques. Of the available techniques, ceilometers offer a promising solution, as they have the potential for wide-range integration at a comparatively low cost. However, there is no publicly available, cross-platform algorithm for the detection of the PBL using ceilometer data. In this study, we developed a Python-based algorithm that can be applied to Lidar backscatter profiles from different ceilometer types. We use this algorithm to create a 5-year time series of PBLH estimates from a semi-arid high desert ecosystem in Arizona. The ceilometer PBLH estimates were compared to PBLH estimates using three independent approaches: reanalysis data, nearby National Weather Service radiosonde observations, and on-site field radiosonde campaigns. We use these estimates to validate the PBL heights derived using the newly developed algorithm. The time series of PBLH measurements is then used in combination with surface energy flux observations to investigate the relationship between PBL growth dynamics and the surface energy balance measured at a nearby flux tower. The maximum daily mean PBLH occurred in June, reaching 3100 m when the median Bowen ratio for the month was 2.6. The PBLH then decreased by 200 m in July, and another 300 m in August when the Bowen ratios dropped to 0.7 and 0.2, respectively. The results show that strong land-atmosphere coupling is characteristic for the site, with both the terrestrial (i.e. soil moisture to surface flux) and the atmospheric (i.e., surface flux to boundary layer atmosphere) legs playing an important role for the seasonal dynamics of PBLH. This study demonstrates how ceilometer-derived PBLH estimates at surface energy flux tower sites can be used to improve the current understanding of land-atmosphere coupling.