Year: 2025
Type: article
Abstract: . Observations from airborne field campaigns are used to study the interplay between boundary-layer thermals and clouds in the trades. The size distributions of thermal and cloud-base chords inferred from turbulence and horizontal lidar-radar measurements are robustly described by the sum of two exponentials. Analytical calculations and statistical simulations demonstrate that the two exponentials result from objects merging, respectively representing the populations of merged- and unmerged-object chords. They also show how circulations induced by convective objects facilitate the merging process. The observed day-to-day variability of these populations at cloud base can thus be tied to the variability of thermal merging across the depth of the subcloud layer. Clouds rooted in unmerged thermals are small and shallow while those rooted in merged thermals are wider and deeper. An intricate interplay between thermal- and cloud-merging arises: when thermal merging is weak, thermal number density is high and cloud bases merge easily, leading to strong mesoscale mass fluxes and "Gravel" shallow mesoscale organizations. In contrast, when thermal merging is strong, clouds are fed by sparser but wider thermals, leading to longer cloud lifetimes but weaker cloud merging, weaker mesoscale mass fluxes, and "Flower" mesoscale organizations. This interplay between thermal- and cloud-merging imposes an upper bound on cloud coverage and suggests a negative feedback on the growth of mesoscale circulations. Thermal merging also controls observed size distributions of thermals in deep convective regimes. The merging process thus appears to be a fundamental player in the mesoscale organization of convection.
Institutions: Centre National de la Recherche Scientifique, École Polytechnique, École Normale Supérieure - PSL, Sorbonne Université, Laboratoire de Météorologie Dynamique, Laboratoire d'Aérologie, Institut de Recherche pour le Développement, Université de Toulouse, Max Planck Institute for Meteorology, Commissariat à l'Énergie Atomique et aux Énergies Alternatives, CEA Paris-Saclay - Etablissement de Saclay, Laboratoire des Sciences du Climat et de l'Environnement, CEA Paris-Saclay, Université de Versailles Saint-Quentin-en-Yvelines, Université Paris-Saclay, Institut Pierre-Simon Laplace, Laboratoire atmosphères, milieux, observations spatiales
Language: English
Subfield: Atmospheric Science
Field: Earth and Planetary Sciences
Domain: Physical Sciences
Open Access status: gold
Estimated APC paid: $1,725
OpenAlex ID: W4416896488