Sensitivity of flower trade-wind cloud organisation to mesoscale atmospheric heterogeneities

Année de publication

2026

The sensitivity of cloud organisation to mesoscale atmospheric heterogeneities is investigated considering a case of flower shallow convection, typical of the trade-wind regions. To do so, the February 2, 2020, convective scene, observed during the Elucidating the Role of Clouds-Circulation Coupling in Climate (EUREC4A) field campaign, is simulated with the Meso-NH model and a 100 m horizontal grid spacing. The sensitivity of the cloud organisation to the mesoscale atmospheric heterogeneities is tested with one reference and eight additional large-eddy simulation experiments, where some of the initial and forcing fields are horizontally averaged: wind components, water vapour mixing ratio, and potential temperature. Though insensitive to dynamical and thermal heterogeneities, the cloud organisation is very sensitive to the humidity mesoscale heterogeneities, especially in the cloud layer, where they form moist patches, and not in the sub-cloud layer. Absence of cloud-layer mesoscale humidity heterogeneity leads to weak-to-absent mesoscale convective circulation and weak convection. The following chain of processes explains this sensitivity: clouds and rain develop more in moist patches, then cold pools develop progressively, initiating mesoscale circulations. Monotonic relations, and even proportionality, are found between the maximal sizes of the moist patches, the rain patches, the cold pools, and the mesoscale circulation. The new clouds that form along the cold-pool edges reinforce the pre-existing moist patches. This leads to positive feedback loops on humidity mesoscale heterogeneity that grows and drives the cloud mesoscale organisation. This study clearly hierarchs the importance of thermodynamical heterogeneities for cloud organisation at the mesoscale, specifically highlighting the major role of humidity heterogeneities in the cloud layer.</div>

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