Which Processes Govern the Lower Tropical Troposphere CO2-Exchange in ECMWF-IFS?

The exchange of CO₂ in the lower troposphere is governed by surface and atmospheric processes operating across a wide range of spatial and temporal scales. Over tropical regions such as the Amazon rainforest, daily occurring shallow-to-deep convective clouds are anticipated to significantly modulate the diurnal cycle and spatial variability of CO₂. Yet, the physical processes controlling the exchange at the surface and across the interface of the boundary layer and free troposphere remain poorly understood and insufficiently quantified, limiting accurate estimates of CO₂-exchange over the Amazon rainforest under a changing climate.

To address this research gap, we examine the lower tropical troposphere CO₂-exchange across shallow-to-deep convective regimes over the Amazon rainforest. More specifically, we develop an analytical framework that adopts well-mixed conditions, assuming vertical uniformity of state variables and greenhouse gases within the atmospheric boundary layer. Utilising this framework, we reconstruct and attribute the diurnal (from sunrise to sunset), day-to-day and spatial lower tropospheric CO₂-exchange into seven physical components: (I) free tropospheric background mole fraction, (II) entrainment and detrainment during the night-to-day transition, (III) surface fluxes (including plant assimilation and soil respiration), (IV) entrainment and detrainment from convective boundary layer development as determined by the lapse rate of CO₂, (V) boundary layer dilution associated with clouds, (VI) cloud mass flux and (VII) advection. Moreover, by inverting the expression we obtain a first-order, physics-based estimate of net ecosystem exchange that helps interpret the sensitivity of existing inverse modelling estimates. The framework is applied to the global storm-resolving Integrated Forecasting System (IFS) of the ECMWF across three horizontal resolutions (25 km, 9 km, and 4.4 km) and evaluated using comprehensive observations and turbulence and cloud resolving large-eddy simulations from the CloudRoots-Amazon22 campaign (dry season 2022).

Our initial findings indicate that we satisfactorily reproduce and attribute the relative importance of each physical component to the diurnal CO₂-exchange in the lower tropical troposphere under clear, shallow convective, and deep convective conditions in ECMWF-IFS. However, larger uncertainties persist in the early morning and late afternoon. Ongoing work aims to further disentangle the role of the physical processes in controlling the CO₂-exchange over the Amazon rainforest across time, space and model resolutions.