Future Extreme Winter Windstorms in the Northeastern US: A Storyline-Based Pseudo-Global Warming Approach

Winter windstorms are a major hazard in the Northeastern US in the contemporary climate. This region has a high population density and major infrastructure assets in many locations and is therefore vulnerable to winter windstorms. Further, changes of these windstorms in terms of intensity, frequency, spatial extent, and associated co-occurrence of other hazards; extreme precipitation and/or freezing rain as a result of global climate non-stationarity may result in significant socioeconomic consequences. Different approaches may be applied to study future windstorms and their associated hazard including 1) storylines of historically important socioeconomically consequential events with pseudo-global warming (PGW) perturbations applied to examine the degree to which such events may evolve in the future and the linkages between the thermodynamic perturbations and both the hydroclimate and dynamic responses, 2) downscale of long-term transient simulations with regional climate models applied at moderate resolution to sample across a range of target events (e.g. CORDEX simulations, see Letson et al. 2024 Historical and Future Windstorms Affecting the Northeast US: Their Impacts and Origins this conference and/or 3) large ensemble of simulations from global climate models across different initial conditions and/or climate forcings (e.g., analyses of Max Planck Institute Earth System Model (MPI-ESM-LR) large ensemble output, see Coburn et al. 2024, Northeastern Windstorms and Midlatitude Cyclones in the MPI Large Ensemble, this conference).

In this presentation, we focus on results from simulations using a storyline-based approach performed with the Weather Research and Forecasting (WRF) model version 4.3.3. We selected three windstorm storylines from the top 10 most powerful and wide-spread windstorms in the Northeastern US during the 40 years from 1979-2018 based on an analysis of ERA5 reanalysis output (Letson et al. 2021: Intense windstorms in the Northeastern United States. Natural Hazards and Earth System Sciences, 21, 2001-2020). These storylines focus on mid-latitude cyclones that typify those in the historical record and are thus; one Alberta Clipper and two Colorado Lows. A small ensemble of control simulations is built for each of these typical winter windstorm storylines from the contemporary climate and then the optimal model configuration is used in a series of PGW experiments.

For each storyline, we have downscaled the ERA5 reanalysis using a double nest and an innermost domain with 3km grid spacing. We have performed a suite of simulations using different combinations of planetary boundary layer (PBL) schemes and microphysical (MP) schemes which are two key dimensions of simulation uncertainty in wind speed and precipitation. The PBL schemes included are Mellor-Yamada-Nakanishi-Niino (MYNN) level 2.5 with Eddy-Diffusion Mass-Flux (EDMF) option on and Yonsei University (YSU), and the MP schemes used are Milbrandt-Yau (MILB) and Morrison (MORR). Each ensemble member is evaluated comprehensively across 24 properties and a total of 47 individual skill scores considering the model performance in terms of temporal variations, spatial locations, and spatial coverage. Then the optimal model configuration (YSU-MILB) is used to assess how these storylines may evolve in a warmer climate.

In the PGW experiments, we applied only thermodynamic perturbations to the model’s initial and boundary conditions (e.g. temperature, moisture, and ice coverage of the Great Lakes). Specifically, two sets of perturbations are used 1) a 3.5K mean surface air temperature perturbation under the high-forcing shared socioeconomic pathway scenario SSP5-8.5 by 2100 based on the Intergovernmental Panel on Climate Change Sixth Assessment Report (IPCC-AR6), and 2) a 5.96K temperature perturbation derived based on the perturbation over the Northeast US from the Department of Energy (DoE) Energy Exascale Earth System Model version 1 in the SSP5-8.5 scenario. To maintain the static stability regime, the temperature perturbation is uniformly applied to the temperature-related fields while keeping the relative humidity constant, but the Great Lakes are completely de-iced. Using a cyclone-tracking approach, Diabatic Potential Vorticity (DPV) and 900 hPa wind speeds, as expected, have the highest values on the western flank of the sea level pressure minimum. However, although storm-averaged DPV does increase in the PGW simulations, these areas are not preferentially influenced by the application of the PGW experiments and thus there is no evidence of storm-sharpening. Indeed changes in wind speeds in the lower troposphere and near-surface exhibit very complex perturbation patterns and modest change (PGW minus control) with, for example, peak wind speeds at 10-m lying within +/- 1.5 m/s of those in the control simulations. As expected, precipitation exhibits a stronger response to the thermodynamic changes in PGW simulations with increasing total precipitation within a 500 km radius of the cyclone centroid and a transition from snow to rainfall.