Parametrized diabatic processes in numerical simulations of an extratropical cyclone

The parametrization of diabatic processes in numerical models is critical for the accuracy of weather forecasts and for climate projections. A novel approach to the evaluation of these processes in models is introduced in this contribution. The approach combines a suite of on‐line tracer diagnostics with off‐line trajectory calculations. Each tracer tracks accumulative changes in potential temperature associated with a particular parametrized diabatic process in the model. A comparison of tracers therefore allows the identification of the most active diabatic processes and their downstream impacts. The tracers are combined with trajectories computed using model‐resolved winds, allowing the various diabatic contributions to be tracked back to their time and location of occurrence. We have used this approach to investigate diabatic processes within a simulated extratropical cyclone. We focus on the warm conveyor belt, in which the dominant diabatic contributions come from large‐scale latent heating and parametrized convection. By contrasting two simulations, one with standard convection parametrization settings and another with reduced parametrized convection, the effects of parametrized convection on the structure of the cyclone have been determined. Under reduced parametrized convection conditions, the large‐scale latent heating is forced to release convective instability which would otherwise have been released by the convection parametrization. Although the spatial distribution of precipitation depends on the details of the split between parametrized convection and large‐scale latent heating, the total precipitation amount associated with the cyclone remains largely unchanged. For reduced parametrized convection, a more rapid and stronger latent heating episode takes place as air ascends within the warm conveyor belt.