Water isotopic characterisation of the cloud–circulation coupling in the North Atlantic trades – Part 1: A process-oriented evaluation of COSMO_iso simulations with EUREC^4A observations

• Main Authors: Villiger, Leonie, id_orcid:0 000-0002-8595-2339, Dütsch, Marina, Bony, Sandrine, Lothon, Marie, Pfahl, Stephan, id_orcid:0 000-0002-9872-6090, Wernli, Heini, id_orcid:0 000-0001-9674-4837, Brilouet, Pierre-Etienne, Chazette, Patrick, Coutris, Pierre, Delanoë, Julien, Flamant, Cyrille, Schwarzenboeck, Alfons, Werner, Martin, Aemisegger, Franziska
• Format: Article in Journal/Newspaper
• Language: English
• Published: Copernicus 2023
• Subjects: North Atlantic
• Online Access: https://hdl.handle.net/20.500.11850/648569; https://doi.org/10.3929/ethz-b-000648569

Naturally available, stable, and heavy water molecules such as HDO and H218O have a lower saturation vapour pressure than the most abundant light water molecule H216O; therefore, these heavy water molecules preferentially condense and rain out during cloud formation. Stable water isotope observations thus 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 COSMOiso (isotope-enabled Consortium for Small Scale Modelling; ) simulations with explicit convection and horizontal grid spacings of 10, 5, and 1ĝ€¯km were carried out in this study over the tropical Atlantic for the time period of the EUREC4A (Elucidating the role of clouds-circulation coupling in climate; ) field experiment. 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 comparatively high and low rain rates. Cloud fraction and liquid water content show a better agreement with aircraft observations with higher spatial resolution, because they show strong spatial variations on the scale of a few kilometres. 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, which is the formation altitude of shallow cumulus clouds. 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 0.7 for I2H and d-excess. A close examination of isotopic characteristics under precipitating clouds, non-precipitating clouds, clear-sky and dry-warm patches at the altitude of cloud ...