A Mass Flux Convection Scheme with Representation of Cloud Ensemble Characteristics and Stability-Dependent Closure

TLDR

The UK Meteorological Office’s large‑scale numerical models use a convection scheme that is described in this study. The study evaluates the scheme’s ability to represent convective processes across GATE, BOMEX, ATEX experiments and an 11‑layer climate model. The scheme represents an ensemble of convective clouds with a bulk model, using a stability‑dependent closure that links initial mass flux to the convecting layer’s stability, and is tested in the aforementioned experiments and climate model. The scheme produces realistic heating rates, establishes quasi‑equilibrium without forcing balance, captures latent heat release patterns, simulates upper‑level divergent circulation (though weak in northern summer), yields reasonable zonal tropospheric temperature, and matches observed outgoing longwave radiation–sea‑surface‑temperature relationships, though it underestimates the Indian Monsoon.

Abstract

The convection scheme used at the UK Meteorological Office in large-scale numerical models is described. The scheme uses a “bulk” cloud model to represent an ensemble of convective clouds and aims to represent shallow, deep and midlevel convection. A simple closure is employed, the initial convective mass flux being related to the stability of the initial convecting layer. The ability of the scheme to represent convective processes in a variety of situations is evaluated using GATE, BOMEX, and ATEX data. In each case realistic heating rates are simulated and although the closure of the scheme does not demand a balance between convective and large-scale forcings as in many other types of convection scheme (for example the Arakawa–Schubert scheme), a quasi-equilibrium is established while retaining realistic atmospheric structure. The performance of the scheme in an 11-layer atmospheric general circulation model used in climate research at the UK Meteorological Office is also evaluated by comparing aspects of the simulated tropical flow from a recent 4-year integration with observed data. The scheme simulates the main areas of latent heat release and their variation throughout the year, although the Indian Monsoon is poorly simulated. The upper level divergent circulation is also well simulated, although too weak in northern summer. The zonally averaged tropospheric temperature structure is reasonable indicating that the interaction of convective and radiative processes is reasonably modeled. The variation of outgoing longwave radiation (a proxy for convective rainfall in the tropics) with sea surface temperature agrees with recent observational studies.