Water isotopic characterisation of the cloud-circulation coupling in the North Atlantic trades. Part 1: A process-oriented evaluation of COSMO_iso simulations with EUREC⁴A observations

Abstract. Stable water isotope observations have the potential to provide information on cloud processes in the trade-wind region, in particular when combined with high-resolution model simulations. In order to evaluate this potential, nested convection-resolving COSMO_iso simulations with horizontal grid spacings of 10, 5, and 1 km were carried out in this study over the tropical Atlantic for the time period of the EUREC⁴A field experiment. To keep the conditions in the domain as close as possible to the real meteorology, we applied a spectral nudging of horizontal winds towards reanalysis data. The comparison to airborne in situ and remote sensing observations shows that the three simulations are able to distinguish between different mesoscale cloud organisation patterns as well as between periods with comparably high and low rain rates. Precipitation, cloud fraction and liquid water content are sensitive to the grid spacing. Cloud fraction and liquid water content show a better agreement with aircraft observations with higher spatial resolution. Contrastingly, temperature, humidity, and isotopes in vapour remain fairly unaffected by the model resolution. A low-level cold-dry bias, including too depleted vapour in the subcloud and cloud layer and too enriched vapour in the free troposphere, is found in all three simulations. Furthermore, the simulated secondary isotope variable d-excess in vapour is overestimated compared to observations. Special attention is given to the cloud base level, the formation altitude of shallow cumulus clouds, which are rooted in the thermals of the subcloud layer. The temporal variability of the simulated isotope variables at cloud base agrees reasonably well with observations, with correlations of the flight-to-flight data as high as 0.69 for δ²H and 0.74 for d-excess. A close examination of different mesoscale cloud base features, including clouds and clear-sky dry-warm patches, and their isotopic characteristics shows that i) these features are represented faithfully in the model with similar frequency of occurrence, isotope signals and specific humidity anomalies as found in the observations (+2 ‰ to +5 ‰ [+2 g kg⁻¹] for precipitating clouds vs. −3 ‰ to −4 ‰ [−2 g kg⁻¹] in dry-warm patches for δ²H [q]) and ii) the δ²H of cloud base vapour at the hourly time scale is mainly controlled by mesoscale transport and not by local microphysical processes while the d-excess is mainly controlled by large-scale drivers. Overall, this evaluation of COSMO_iso, including the isotopic characterisation of cloud base features, suggests that the simulations can be used for investigating the role of atmospheric circulations on different scales for controlling the formation of shallow cumulus clouds in the trade-wind region, as will be done in part 2 of this study. Additionally, we provide explicit recommendations for adaptations of the modelling setup to be tested in future research.