Advancing Understanding of Planetary Boundary Layer Height: Insights from the ACTIVATE Field Campaign and Comparative Assessment of Estimation Algorithms

The planetary boundary layer (PBL) is the lowest layer of the atmosphere and controls exchanges of fluxes of energy and diffusion of pollutants. The vertical thickness of the PBL often plays an essential role in atmosphere-surface interactions. Over the ocean, understanding of the marine boundary layer is limited due to the scarcity of observational data. Typically, the daytime PBL consists of a mixed layer (ML), which is usually close to PBL. However, over the ocean, when there are thick clouds, MLH is lower than PBL, with MLH and PBLH usually taken as the cloud base and top, respectively. Dropsondes have been widely used to retrieve PBLH over the ocean due to their high precision. NASA’s Aerosol Cloud meTeorology Interactions oVer the western ATlantic Experiment (ACTIVATE) over the Northwest Atlantic Ocean from 2020 to 2022 used various instruments to observe the structure of PBL, including dropsondes. We use thermodynamic profiles to compute PBLH from 506 dropsondes in this study. There are five common ways to retrieve PBLH from dropsonde thermodynamic profiles: the parcel method, the gradient of potential temperature, relative humidity (RH), and specific humidity(q), and the bulk Richardson number method. We evaluate the performance of these five methods under four different scenarios: decoupled vs. well-mixed, cloudy vs. clear sky. The reasons each method in various scenarios has different performance are identified. Besides dropsondes, data from additional observation instruments will be used to provide insights: for instance, cloud fraction from the airborne High-Spectral-Resolution Lidar – Generation 2 (HSRL-2) and the cloud in-situ data collected onboard the Falcon aircraft are used for cloud condition. Furthermore, we use the dropsonde-derived PBLH to evaluate the performance of lidar-retrieved MLH from HSRL-2.