On the Cell Broadening of Mesoscale Cellular Convection in a Well‐Mixed Boundary Layer

Abstract Understanding what controls the horizontal scale of mesoscale cellular convection is central to explaining cloud self‐organization and its radiative impacts. Using a hierarchy of large‐eddy simulations, we investigate cell broadening in a well‐mixed boundary layer by isolating the roles of radiation, cloud microphysics, and water vapor. In cloudy simulations, cellular convection develops large aspect ratios (∼20–30) and forms donut‐like open cells with thick cloud walls, where open cells are defined dynamically based on the water‐vapor perturbation field. Even when radiative and microphysical effects are entirely removed, water vapor alone still self‐organizes into cellular structures with aspect ratios of ∼10. In these vapor‐only simulations, cell size is regulated primarily by surface sensible heat flux under the necessary condition of surface moisture supply, while remaining largely insensitive to the absolute magnitude and vertical distribution of water vapor. We identify mixing‐induced cooling across the entrainment interface layer (EIL) as the key mechanism enabling cell broadening without cloud‐related feedbacks. Within this idealized framework, we introduce Δ s l /Δ z i , the mean gradient of liquid water static energy across the EIL, as a post‐hoc and process‐oriented diagnostic to quantify the relative strength of mixing‐induced cooling across the EIL. Composite stream‐function analysis reveal qualitatively similar mesoscale overturning circulations in cloudy and vapor‐only simulations, highlighting the role of cooling effects across the EIL in cellular organization. These results reveal a pathway for cell broadening in water‐vapor‐only conditions and underscore the need for future work on the nonlinear interplay among mixing, radiation, and microphysics in regulating mesoscale cellular organization.