Study of Latent and Sensible Heat Entrainment Fluxes in the Convective Boundary Layer with Lidar Synergy

We present work of the Land-Atmosphere Feedback Initiative (LAFI) [1], which aims to quantify and understand land-atmosphere feedback by utilizing synergetic observations and turbulence-permitting simulations. One focus of LAFI is the quantification of entrainment, i.e., the exchange of energy and humidity between the boundary layer and the free atmosphere above.

As a first step, we investigated a proposed similarity relationship of the latent heat entrainment flux at the top of a convective boundary layer. This relation is expressed as the vertical velocity scale times the ratio of the water vapor mixing ratio gradient to the Brunt-Väisälä frequency (summarized in [2]). We studied this equation using several months of turbulence-resolving observations of humidity and vertical wind from Raman and Doppler lidar systems as well as radiosonde temperature measurements from the Atmospheric Radiation Measurement Program's Southern Great Plains site in the US. The analysis, conducted at the lower boundary of the interfacial layer, did not support the proposed formulation. In the search for potential driving variables of the latent heat entrainment flux at this height level, we found a high correlation with the square root of the variances of vertical wind and water vapor mixing ratio.

Further exploration of this connection led to the conclusion that the currently most accurate description of the latent heat entrainment flux is the product of the vertical wind variance and the ratio of the water vapor mixing ratio gradient to the Brunt-Väisälä frequency, confirming recent results over Europe [3].

Within LAFI, these results will be validated through new measurements conducted during a specific field campaign by LAFI in 2025. Here, the profiling of sensible heat fluxes and potential temperature variance is also possible. First results and analog analysis regarding the proposed theoretical descriptions in [2] will be presented at the conference.

 

References:
[1] https://www.lafi-dfg.de/

[2] Wulfmeyer, Volker et al. (2016): Determination of Convective Boundary Layer Entrainment Fluxes, Dissipation Rates, and the Molecular Destruction of Variances: Theoretical Description and a Strategy for Its Confirmation with a Novel Lidar System Synergy. In Journal of the Atmospheric Sciences 73 (2), pp. 667–692. DOI: 10.1175/JAS-D-14-0392.1

[3] Gibert, Fabien; Edouart, Dimitri; Monnier, Paul; Collignan, Julie; Lopez, Julio; Cénac, Claire (2025): Scalar turbulent fluxes and variances in the interfacial layer from lidar observations and assessment of Lagrangian Stochastic Models. In Quart J Royal Meteoro Soc, Article e70016. DOI: 10.1002/qj.70016.