Dynamical Impacts of Warm-Starting Operational Weather Models over Africa*

Weather models which allow explicit convection can add value to weather forecasting by improving the intensity and timing of precipitating systems and their dynamics. This is particularly valuable in the tropics, where moist diurnal convection dominates. In West Africa, convection can become organised to form mesoscale convective systems (MCSs), thunderstorms hundreds of kilometres in size which are crucial for supplying water but may have devastating impacts, and while convection-permitting models improve forecasts, issues remain in implementation. A major problem is initialising weather models where there are sparse measurements to use for data assimilation, such as in Africa. Currently, operational convection-permitting models are initialised using lower-resolution global models, but these have no fine dynamical structure and as a result there is a spin-up time of around 12-18 hours before the model begins to accurately reflect precipitation.

A “warm-starting” method has been trialled in the UK Met Office’s Tropical Africa Model. Warm-starting the model initialises it with large-scale features from the global model but retains fine-scale fields below a certain length scale from previous runs of the high-resolution model, and improves precipitation representation. We investigate why this is effective by examining the atmospheric fluid dynamics at initialisation and beyond, comparing the warm-start method to initialising with only the global model.