Investigating the impact of land surface heterogeneity on near- surface circulations and fluxes

• Applicant: Professor Dr. Christoph Thomas
• Subject Area: Atmospheric Science
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 514721519

Project P5 is motivated by the need to compute biophysically meaningful surface heat and momentum budgets for heterogenous land surfaces as an essential component in the L-A feedback chain. Over the past decades, progress in understanding and quantifying the impact of surface heterogeneity has been made for the canonical cases of perpendicular and parallel airflows relative to a heterogeneity jump. In contrast, the turbulent structures for the most common case of oblique airflows, here defined as attack angles between 10° and 80° relative to the patch edge, are not mechanistically understood and their impact on surface fluxes escapes quantification. P5’s objective is therefore to detect, understand, and conceptualize these oblique airflows moving across LAFO’s agricultural patches. Our central hypothesis is that zones of enhanced surface fluxes through localized but non-stationary turbulent motions create a transitional boundary layer in which area-averaged fluxes are inflated and enhance L-A feedbacks. The research is guided by four sub-objectives and accompanying sub-hypotheses. We analyze spatially explicit measurements from high-resolution fiber-optic distributed sensing (FODS) of air temperatures in the surface and crop layers, wind speed and turbulence kinetic energy on scales of seconds and decimeters over hundreds of meters. FODS also observes soil temperature and moisture, and the distribution of solar radiation at scales of decimeters and tens of seconds combined with classic sensor networks above the patches planted in winter wheat and maize. Led by P5, the FODS cable-arrays are installed by the LAFI team and operated within P1. The scientific work program of P5 consists of four work packages (WP): First, we post-field calibrate the raw FODS quantities into the mentioned quantities using a task-specific software package developed by us. WP2 stratifies the observations according to the airflow’s angle of attack and performs the analysis of the observed turbulent spatiotemporal structures and fluxes. Its results are combined with those from the improved surface energy balance (P6), remotely sensed surface (P2), and evapotranspiration (P4) to evaluate our central hypothesis. The lidar scans above the canopy from P1 provide unique synergies. WP3 synthesizes the mechanistic understanding of surface heat and momentum fluxes into a conceptual framework for oblique flows. Within WP4 we engage in the team and skill building efforts. The outlook for phase 2 includes incorporating different land surface heterogeneity, carbon budgets, and FODS sensor improvements. This project addresses LAFI’s objectives O1 to O3, OS, and OE in case of an extreme event. We will participate in CCWG-DL and CCWG-SenSyn. In addition to the above mentioned cross-project collaborations, we collaborate with P11 for the within plant-canopy profiles of light and temperature.

• DFG Programme: Research Units
• Subproject of: FOR 5639: Land-Atmosphere Feedback Initiative (LAFI)