The surface-boundary layer connection across spatial scales of irrigation-driven thermal heterogeneity: An integrated data and modeling study of the LIAISE field campaign

Abstract

Irrigation in semi-arid regions induces thermal heterogeneity across a range of spatial scales that impacts the partitioning of energy at the surface, the development of the atmospheric boundary layer, and the bidirectional interactions between the atmosphere and the surface. In this analysis, we use data from the Land Surface Interactions with the Atmosphere in the Iberian Semi-Arid Environment (LIAISE) experiment combined with a coupled land–atmosphere model to understand the role of the scales of irrigation-induced, thermal heterogeneity on the surface fluxes and consequently, the development of the diurnal convective boundary layer. The surface heterogeneity is characterized by Bowen ratios that range from ∼0.01 in the irrigated areas to ∼30 in the non-irrigated areas; however, the observed boundary-layers dynamics in both locations are similar. In this analysis, we address the questions of how the surface fluxes impact the development of the boundary-layer dynamics and how the boundary layer influences the diurnal cycle of surface fluxes. To interpret the observations, we introduce a heterogeneity scaling scheme where length scales range from local scale (∼100 m) to regional scale (∼10 km) to investigate the role of scale on surface representation in numerical models and to address the discrepancy between surface observations and their representation in weather and climate models. We find that at the surface, both the available energy and its partitioning depend on spatial scale. The observed boundary-layer properties can be explained through the composite of surface fluxes at the regional scale. Surface fluxes at the local scales are unable to replicate the observed boundary layer — even when including large-scale contributions. We find that non-local boundary layer processes like advection are important for partitioning energy at the local scale. We explore the connection between surface fluxes and the development of the boundary layer and the potential non-local effects on boundary-layer development.