Sea breeze dynamics and convection initiation: the influence of convective parameterisation in weather and climate model biases

There are some long established biases in atmospheric models that originate from the representation of tropical convection. Previously it has been difficult to separate cause and effect because errors are often the result of a number of interacting biases. Recently we have gained the ability to run multi-year global climate models simulations with grid-spacings small enough to switch the convective parameterisation off, which permit the convection to develop explicitly. There are clear improvements to the initiation of convective storms and the diurnal cycle of rainfall in the convection-permitting simulations, which enables a new process-study approach to model bias idenfication. In this study multi-year global atmosphere-only climate simulations with and without convective parameterisation are undertaken with the Met Office Unified Model and are analysed over the Maritime Continent region, where convergence from sea breeze circulations is key for convection initiation. The analysis shows that although the simulation with parameterised convection is able to reproduce the key rain-forming sea breeze circulation, the parameterisation is not able to respond realistically to the circulation. A feedback of errors also occurs; the convective parameterisation causes rain to fall in the early morning, which cools and wets the boundary layer, reducing the land-sea temperature contrast and weakening the sea breeze. This is, however, an effect of the convective bias, rather than a cause of it. Improvements to how and when convection schemes trigger convection will improve both the timing and location of tropical rainfall and representation of sea breeze circulations.