Local Land-Atmosphere Coupling (LoCo) Project
The LoCo Perspective in Bulletin of the American Meteorological Society (BAMS)
The original structure of the GEWEX Global Land/Atmosphere System Study (GLASS) was designed to support four modes of land-surface modeling: (1) local-scale off-line; (2) large-scale off-line; (3) local-scale coupled; and (4) large-scale coupled (van den Hurk et al., 2011). To date, each of these has been addressed through organized, community-wide intercomparison studies, such as the Project for the Intercomparison of Land Surface Parameterization Schemes (PILPS), the Global Soil Wetness Project (GSWP), and the Global Land-Atmosphere Coupling Experiment (GLACE), with the exception of local land-atmosphere coupling (LoCo). The LoCo Project has instead evolved and, in recent years, gained momentum through process-level modeling and observational studies that focus on the development and application of coupling diagnostics. This has lead to the development of coupling metrics “cheat sheets” and the Coupling Metrics Toolkit (CoMeT), which is a set of fortran modules containing the most widely used coupling diagnostics.
The motivation for LoCo has been clear for some time, in that the results of PILPS and GSWP are limited by the lack of atmospheric feedback when running in uncoupled mode. Although the results of GLACE provide an assessment of current global circulation model (GCM) coupling coherence, they cannot isolate and evaluate the processes implied in the coupling that lead to model development. In terms of accurately representing the relationship between soil moisture (SM) and precipitation (P) and coupling strength in models (particularly in a changing climate), and to have the proper understanding and related improvement, it is necessary to carefully examine and quantify the full series of interactions and feedbacks (i.e., links in the chain) that include the planetary boundary layer (PBL) feedback. In simplified form, the coupling process chain may be presented as:
The equation above emphasizes that the impact of SM anomalies (ΔSM) on cloud development and subsequent precipitation (ΔP) depends on the sensitivities of: (a) the surface fluxes (EFsm) to SM; (b) PBL evolution to surface fluxes; (c) entrainment fluxes at the PBL-top (ENT) to PBL evolution; and (d) the collective feedback of the atmosphere (through the PBL) on ambient weather (2-meter temperature and humidity). As a result, there are numerous pathways composed of positive and negative feedback loops inherent in this chain. Equation 1 has omitted the role that additional inherent and external factors (e.g., canopy interception and large-scale convergence) can play in modulating the strength of each link, and therefore must be addressed in LoCo studies as well.
An accurate definition of the “local” versus “non-local” (or global/large-scale) coupling is also needed. The realm of LoCo has been defined by GLASS as “the temporal and spatial scale of all land-surface related processes that have a direct influence on the state of the PBL.” The fundamental processes that fall into this realm include the direct moistening/drying and heating/cooling of the PBL and the feedback exerted by this PBL change on surface fluxes (through PBL growth and entrainment); the subsequent formation and disappearance of PBL clouds; the triggering and fuelling of convection; and the accumulation of hydrological anomalies in the soil reservoir and their subsequent impacts on the energy balance. Inherent in this definition is the importance of the diurnal interaction (e.g., convective PBL evolution) in contrast to the seasonal and long-term perspective of GLACE.
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