A two-dimensional Ising model is presented in order to reproduce cloud field organization at the mesoscale (i.e., from 10 up to 10 km). This model takes into account different interactions between an air mass and its environment. It is shown that this novel and original approach allows to reproduce quite well satellite observations. Although created initially for pristine (i.e., natural) oceanic conditions, this model can be adapted to any other situations, mutatis mutandis.

In this paper, a non-linear bulk model inspired from species competition dynamics is proposed in order to describe the physics of water vapour-aerosol-cloud-rain interactions. Despite the complexity of such interactions, certain non-trivial aspects of the macro behavior of a cloud are predictable without concerning the full complexity of the dynamical system. The model is for warm clouds (no ice) and it consists in a set of three non-linear differential equations. This species-competition model is confronted with in situ measurements in different situations: for pristine environments (1) at the high-altitude Maïdo Observatory (Indian Ocean, Reunion Island) and (2) in a pure oceanic context around the Reunion Island (aeromarine field campaign) and for a continental urban context (3) in Lille (North of France). Compared with observations (radar, radiometric measurements), it is shown that, whatever the situation considered, the model reproduces efficiently the macro features of clouds, like cloud occurrences, cloud water content magnitudes, and cloud-rain links. The model is adapted to a two-dimensional (horizontal) grid and its predictions are compared with the era5 reanalyses above the Hauts-de-France region and in the pristine Southwest Indian Ocean (around Reunion Island). Furthermore, satellite data enable to corroborate that the model clearly shows promising results for the horizontal cloud field organization. The spatiotemporal resolution of this model is adaptable according to the needs and it can be used in any region and at the desired altitude provided that data be available for implementation. In this paper, it is also suggested that the model may be fruitful to derive from measurements, by means of an optimization scheme, different theoretical parameters not easy to determine. The model runs on a laptop with a relatively short time (of the order of the minute); it is not intended to supplant comprehensive models, but it may be fruitful to understand the essential mechanisms in warm cloud formation.

We present the results of the AEROMARINE field campaign which took place in the boreal spring 2019 off the coast of Reunion island in the South West Indian Ocean basin. The southern Indian Ocean is of major interest for the study of marine aerosols, their distribution and variability [1]. Nine instrumented light plane flights and a ground-based microwave radiometer were used during the AEROMARINE field campaign. These measurements were compared with the long-term measurements of the AERONET sun-photometer (based in Saint Denis, Reunion Island) and various instruments of the high altitude Maido Observatory (2200m above sea level, Reunion island). These results were analyzed using different model outputs: (i) the AROME mesoscale weather forecast model to work on the thermodynamics of the boundary layer, (ii) the FLEXPART-AROME Lagrangian particle dispersion model to assess the geographical and vertical origin of air masses, and (iii) the chemical transport model CAMS (Copernicus Atmosphere Monitoring Service) to work on the aerosol chemical composition of air masses. These measurements allowed us to determine the background concentration of natural marine aerosols and to highlight that (1) the atmospheric layers above 1500m are in the free troposphere and are mainly composed of aerosols from the regional background and (2) that the local environment (ocean or island) has little impact on the measured concentrations. Marine aerosols emitted locally are mostly measured in the lower atmospheric layers (below 500m). The daytime marine aerosol distributions in the free troposphere measured by the aircraft were compared to the aerosol distribution measured at the high altitude Maido observatory at night when the observatory is located in the free troposphere. We also found that the CAMS reanalyses overestimated the aerosol optical depth in this region. Finally, our study confirms, with no ambiguity, that the AERONET station in Saint Denis (Reunion island) can be considered as a representative marine station in the tropics [2]. References [1] I. Koren, G. Dagan, and O. Altaratz. From aerosol-limited to invigoration of warm convective clouds. Science, 344 (6188) : 1143–1146, 2014. [2] P. Hamill, M. Giordano, C. Ward, D. Giles, and B. Holben. An aeronet-based aerosol classification using the mahalanobis distance. Atmospheric Environment, 140 : 213–233,2016.