Prediction of the timing of convective initiation along a dryline in a high-resolution model

The timing of convective initiation is one of the less accurately predicted aspects of numerical weather prediction. Even in very good cases where a high-resolution numerical model is able to produce excellent forecasts in the location of initiation, the timing of initiation can be off by a couple of hours, especially with convection that develops along a dryline where boundary layer processes play an important role.

In our previous work studying convective initiation near a dryline, we found the immediate trigger for convection to be the interaction of horizontal convective rolls with the mesoscale convergence associated with the dryline and with the inversion layer atop the moist boundary layer.

In this work, we examine factors which influence the exact timing of convective initiation near drylines. The effects of soil moisture, surface properties, and the modeling of turbulence in the boundary layer are examined. Resolution of convective rolls and details of the erosion of the capping inversion are facilitated by using an unusually high vertical resolution, in addition to a high horizontal resolution. A mesoscale grid is run at a 3 km horizontal resolution with nested grids of 1 km and 250 m resolutions. In addition, a 60 m vertical resolution is used to resolve the boundary layer. The detailed structure of the dryline is thus modeled with unprecedented resolution. The model is also run either with a PBL parameterization for turbulence or with a subgrid-scale turbulence closure model. For the cases examined, the modeling of turbulence was found to be more important for the prediction of convective initiation than accurate surface properties.