An MPAS Case Study on How Horizontal Resolution Affects Convection in a Warm-Conveyor Belt and Its Upscale Impact on Jet Stream Predictability

Embedded convection within Warm-Conveyor Belts (WCBs) alters the mid-latitude large-scale flow by rapidly redistributing potential vorticity (PV) along the tropopause. Using higher horizontal resolution meshes presents an avenue to more realistically simulate atmospheric convection. However, the extent to which higher-resolution also equates to improved coupling between convection with the large-scale flow remains an open question.

We examine a WCB case over the North West Atlantic in late April 2020, associated with rapid predictability degradation of a Rossby wave in the Global Forecast System (GFS) (~30 km resolution). Utilizing a subset of GFS ensemble members for initialization, simulations are performed using the Model for Prediction Across Scales (MPAS) at resolutions ranging from 60 to 3.75 km. We output diabatic heating components (microphysics, convection, radiation) to provide mechanistic insights into their interaction with the jet stream across resolutions.

The case study shows that MPAS simulations outperform the GFS in forecasting jet stream evolution, linked in-part to MPAS simulations capturing a stronger magnitude and longer duration of microphysics heating within the WCB. To assess the influence of the heating due to convection on the jet stream, we focus on regions of negative PV along the tropopause, which act as fingerprints of diabatically modified air arising from convective clouds.

During the first two days of model integration, as the WCB rapidly intensifies, higher-resolution simulations produce more negative PV due to more pronounced microphysics heating, resulting in a more amplified jet stream than in coarser simulations. As the WCB matures later in the model integration, horizontal resolution becomes less critical for simulating negative PV, with the persistence of heating within the WCB determining jet stream evolution.

Our findings highlight the jet stream's kinematic sensitivity to the model representation of latent heating in WCBs with embedded convection, especially during periods of rapid WCB intensification.