Hierarchical modelling of tropical convective systems using explicit and parametrized approaches

Abstract Cloud systems observed during 1–7 September of GATE are examined in a hierarchical approach, namely: a two‐dimensional cloud‐resolving simulation using a 2 km grid length; two‐ and three‐dimensional simulations using the Kain‐Fritsch convective parametrization and 10, 15 and 25 km grid lengths; and coarse‐grid simulations without any convective parametrization. All simulations are forced by the same objectively analysed time‐varying large‐scale advection of temperature and moisture. The domain‐mean winds are relaxed to the observed wind profiles. Both the cloud‐resolving modelling and the lower‐resolution modelling with parametrized convection realize the three observed cloud system categories (squall line, non‐squall cluster and scattered convection) and transitions among them. In particular, the well‐organized fast‐moving squall‐type cloud system observed on 4 September is realized in a three‐dimensional experiment with parametrized convection. In contrast, the lower‐resolution modelling without any convective parametrization fails to produce the squall‐type convective system during the weakly forced period but successfully represents the non‐squall cluster during strong forcing. This lack of success is mostly attributed to convective triggering and the absence of resolved downdraught‐enhanced surface fluxes. These issues are not as critical during strong large‐scale forcing. The observed evolutions of temperature, water vapour mixing ratio, precipitation and surface moisture flux are realized in all simulations. A common deficiency is the overprediction of upper‐level relative humidity. The simulation with parametrized convection features a comparatively large low‐level water vapour mixing ratio, a surface and upper‐level cold temperature bias and a mid‐tropospheric warm bias. This is mainly attributed to deficiencies in how the Kain‐Fritsch scheme represents convective mass flux, detrainment and entrainment by cumulus congestus.