Future extremes in surface wind and temperature in southwestern Norway

The article Estimates of changes in surface wind and temperature extremes in southwestern Norway using dynamical downscaling method under future climate recently published in Weather and Climate Extremes discusses future projections for extreme wind and temperature events. In this study, former Nansen Center scientist Dr. Yiwen Xu, provides estimation of extreme surface winds and temperatures use risk management related to the construction of Hardanger Suspension Bridge and the maintenance of Sotra Bridge in southwestern Norway. 

Figure. The statistical distributions of the simulated extreme wind speeds and the Gumbel lines in the 1990s and the 2050s (a) at Sotra Bridge (ST) (b) at the Hardanger bridge (HD). The 95% confidence interval and the Gumbel lines for 64 and 96 sample sizes (c) at Sotra Bridge in the 1990s and (d) at Hardanger bridge in the 2050s. τ is return period in month. The upper bound (ub) and the lower bound (Ib) of the 95% confidence interval are denoted.Figure. The statistical distributions of the simulated extreme wind speeds and the Gumbel lines in the 1990s and the 2050s (a) at Sotra Bridge (ST) (b) at the Hardanger bridge (HD). The 95% confidence interval and the Gumbel lines for 64 and 96 sample sizes (c) at Sotra Bridge in the 1990s and (d) at Hardanger bridge in the 2050s. τ is return period in month. The upper bound (ub) and the lower bound (Ib) of the 95% confidence interval are denoted.Both bridge locations are in areas in Hordaland with highly complex local topography and the limited long term in situ observations at the locations. Accordingly, Dr. Xu has used the Weather Research and Forecasting Model (WRF) to downscale the Norwegian Earth System Model (NORESM) climate projection data from about 2500 km x 1800 km to 1 km x 1 km horizontal grids, resolving process of local wind circulation and variations in surface temperature.  Simulations were performed for the control period, the 1990s, and the projection period, the 2050s, under the high range radiative forcing climate scenario (RCP8.5).  Monthly maximum winds were compared with historical observations at nearby coastal, urban valley, and mountain weather observation stations.  The simulated extreme wind distributions are in good agreement with the observed distributions at the coastal area (Sotra bridge) but are systematically overestimated on the mountain region (Hardanger bridge).  Comparison of the simulated extreme winds between the 1990s and the projections for the 2050s shows that future extreme winds are unlikely to change with statistical significance during the cold winter season but tend to decrease with statistical significance during the warm summer season.  This is mostly consistent with other research results for the same region.  They are possibly the reflections of the shift in the regional storm activities associated with the changes of the North Atlantic Oscillations and the effects of the local mountain topography. Based on the generalized extreme values distributions (Gumbel distribution), extrapolations were made at the two bridge sites and observation stations to project extreme wind speeds in the early and the late 21st century.  The observed U20yr, and U30yr fall into the CI95% (confidence interval) of the simulated Gumbel distributions, indicating that the observed wind do not differ from the simulated return values with statistical significance. The comparison of the extreme temperatures in the 1990s and 2050s shows that Tmax and Tmin will potentially increase, about ~2 to 3 °C, but with a reduced variability for Tmin.  The results indicate that the two bridges may experience increased heat stress and decreased cold stress in the future, while there is expected to be only marginal changes in the wind at the two bridge locations.

 

Citation: Xu, Y., 2019: Estimates of changes in surface wind and temperature extremes in southwestern Norway using dynamical downscaling method under future climate. Weather and Climate Extremes, Vol. 26, 100234, December. doi.org/10.1016/j.wace.2019.100234