Testing Storm-Track Sensitivity to Resolution and Climate Change Using High-Resolution Global Simulations

Extratropical cyclones gain energy from the thermal gradient created between the warm subtropics and the cold poles and from the latent heat released in the formation of precipitation. Projections of global warming indicate that surface temperatures will warm faster near the poles leading to a weaker temperature gradient, thus weakening extratropical cyclones. At the same time, climate models project only small changes in relative humidity, such that the moisture content of the atmosphere will increase with warming, leading to a strengthening of extratropical cyclones. These competing effects make it difficult to project how extratropical cyclone activity will change in the future. Because extratropical storms play a vital role in mid-latitude weather and climate, it is of great interest to determine how these storms and their storm-track activity will change as Earth warms.

We address this question through analyses of a set of global model simulations carried out as part of the UPSCALE project in the United Kingdom, and a separate set of simulations using the Model for Prediction Across Scales (MPAS). The UPSCALE experiments include current and future (RCP8.5) climate conditions at resolutions of 130 km, 60 km, and 25 km, while the MPAS simulations are carried out for current and future climates at a resolution of 15 km in the Northern Hemisphere. The current generation of global climate models have a limited ability to capture the dynamics of extratropical cyclones, as they do not resolve the mesoscale features within cyclones which account for most precipitation and release of latent heat. At the same time, high-resolution, limited-area models constrain the global circulation, prohibiting the operation of feedbacks present in the climate system, and thus could exaggerate the effects of model resolution and global warming on the storm tracks. The highest-resolution UPSCALE simulations and the MPAS simulations have sufficient resolution to capture the mesoscale release of latent heat in cyclones while avoiding the constraints imposed by regional models. In addition, the coarser UPSCALE simulations allow a direct comparison of the effects of resolution and the effects of climate change within a single model.

Extratropical cyclone activity is assessed using the local deepening rate (LDR) defined by Kuwano-Yoshida (2014) as the average of 1-day surface pressure falls (rises in surface pressure are excluded), normalized by the sine of latitude. This storm track diagnostic provides a measure of cyclone activity without the need for temporal filtering or feature-tracking; the activity of strong storms can be assessed by using thresholds greater than zero for pressure falls prior to averaging.

Preliminary results show only modest changes in extratropical cyclone activity with climate and resolution in the UPSCALE runs, and these changes differ among basins. In addition to the LDR, we will report how latent heating and stationary wave patterns change with warming and resolution, and these results will be compared between the UPSCALE and MPAS simulations.