Sensitivity studies using a cloud-resolving model simulation of the tropical west Pacific

It is important to understand the variability of results which can be produced in a cloud-resolving model (CRM) before using its output to develop parametrizations. In the work presented here a broad range of sensitivity experiments are carried out so that the significance of each change to the model can be put into context. Changes to the parametrization of microphysics and radiation in the CRM are considered along with the influence of dimensionality, horizontal resolution and domain size. Further detailed sensitivity studies are then carried out to investigate the impact of the spatial and temporal distribution of the radiative heating. The results presented show that, out of all the sensitivity studies considered, no particular feature or physical parametrization in a CRM dominates. The domain size, horizontal grid length, use of a third dimension and parametrization of microphysics all significantly impact the simulation in some way. Radiative heating has a large impact on the simulation because it dominates over the large-scale forcing above 8 km. However, the magnitude of this impact is strongly dependent on other aspects of the model, such as the choice of microphysical scheme. A diurnal cycle, which is inherent in the large-scale forcing, is not significantly enhanced by radiation in the CRM. However, accounting for the temporal and spatial variability in the radiative heating is important because neglecting this is shown to overestimate the strength of convection.