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

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 COSMOiso 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 COSMOiso, 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.

Stable water isotope signals and their relation to stratiform and convective precipitation in the tropical Andes

“…Fractionation processes during soil evaporation are account for by TERRAiso, an isotope-enabled prognostic multilayer soil model that is coupled to COSMO iso (Christner et al, 2018). COSMO iso has previously been used in numerous different studies (Aemisegger et al, 2015;Breil et al, 2020;Christner et al, 2018;Lee et al, 2019;Pfahl et al, 2012;Thurnherr et al, 2021) including tropical and subtropical regions (de Vries et al, 2022;Villiger et al, 2023).…”

Water isotopic characterisation of the cloud-circulation coupling in the North Atlantic trades. Part 2: The imprint of the atmospheric circulation at different scales

“…The data from three convection-resolving COSMO iso simulations, described and evaluated in Villiger et al (2023), are used.…”

“…Cloud base is identified using the same procedure as in Villiger et al (2023), i.e., with the following steps:…”

“…The COSMO iso,1km and COSMO iso,5km grid points at cloud base in the domain 54.5-61 • W and 11-16 • N are assigned to three categories representing features from the overturning circulation associated with clouds, clear-sky regions with drywarm anomalies, and clear-sky regions of no particular interest (see details in Villiger et al, 2023, note that here we do not differentiate between precipitating and non-precipitating clouds and only take the vertical distribution of cloud water into account). The following definitions are applied to assign the data points:…”

“…Evidence for the latter mechanism is the change in the subsidence rate. Since cloud base altitude is approximately constant (Villiger et al, 2023), more negative ∆z indicates that the air parcels originate from higher altitudes (cf. analysis of Risi et al, 2019).…”