Representing equilibrium and non-equilibrium convection in large-scale models

TitleRepresenting equilibrium and non-equilibrium convection in large-scale models
Publication TypeMiscellaneous
Year of Publication2013
AuthorsBechtold, P, Semane, N, Lopez, P, Chaboureau, JP, Beljaars, ACM, Bormann, N
Secondary TitleTechnical Memorandum
Number705
Abstract

A new diagnostic convective closure, which is dependent on the convective available potential energy (CAPE), is derived under the quasi-equilibrium assumption for the free troposphere subject to boundary-layer forcing. The closure involves a convective adjustment time-scale for the free troposphere, and a coupling coefficient between the free troposphere and the boundary-layer based on different time-scales over land and ocean. Earlier studies with the ECMWF Integrated Forecasting System (IFS) have already demonstrated the model's ability to realistically represent tropical convectively-coupled waves and synoptic variability with use of the 'standard' CAPE closure, given realistic entrainment rates. A comparison of low-resolution seasonal integrations and high-resolution short-range forecasts against complementary satellite and radar data shows that with the extended CAPE closure it is also possible, independently of model resolution and time step, to realistically represent non-equilibrium convection such as the diurnal cycle of convection and the convection tied to advective boundary-layers, though representing the late night convection over land remains a challenge. A more in depth regional analysis of the diurnal cycle and the closure is provided for the continental United States and particularly Africa, including comparison with data from satellites and a cloud resolving model (CRM). Consequences for global numerical weather prediction (NWP) are not only a better phase representation of convection, but also better forecasts of its spatial distribution and local intensity.