Long-term Changes in Wind Speed: Simulating Wind Climate Change using RCM and its Effects on the Wind Resources over Korea

Abstract

Wind information is an important meteorological variable for various scientific communities and industries such as construction, agriculture, and wind energy. In particular wind energy has utmost important for mitigating the risk of climate change and dependence on increasing energy cost. This thesis is a scientific contribution to the advancement of wind climate changes and regional wind-resource potential. It is based on long-term regional climate simulations and spatiotemporal analyses in the wind speed. Regional climate model, Weather Research and Forecasting (WRF) model were utilized to produce wind information, which is distributed homogeneously especially in terms of spatial variation. The spatial and temporal characteristics were investigated using long-term (1982–2011) wind measurements over South Korea in terms of statistical analysis for validation of the model. Ground-based measurements are adjusted using a modified wind-profile power law to standard height, 10 m above ground level. Then, wind climatology, variability, and long-term trends are examined using daily mean wind-speed series. The spatially averaged annual mean wind speeds were found to be characterized by a light breeze and increase trend (0.005 m s-1 a-1) with statistically significance. Wind speed over Korea was generally influenced by the monsoonal flow; wind speeds were higher in DJF and MAM than in JJA and SON. The wind speeds were increased in MAM, JJA, and SON except DJF over the study period. In case of local wind speed trends are seen regardless of seasons. Here we found that the annual trends in the two regions were statistically significant and homogeneous for all time-varying percentiles over 12 months of the year. Wind speed for these regions categorized according to characteristics of geo-spatial information such as elevation, land-use, geographical setting and urbanization. It may indicate that wind-speed changes have large impacts on not only regional circulation but also on geo-spatial features. Multi-year simulations for wind speed were validated focusing on observed wind-speed characteristics from 1982 to 2011 using NCEP reanalysis data (R-2) and wind measurements. Simulated wind speeds are analyzed at turbine hub height level. The typical wind climatology over Korea was represented well by the physics-based RCM simulations. East Asian large-scale circulation and its related low-level flow and surface wind speed over Korea were reproduced well. In particular, the long-term anomalous variability in annual wind speed during the study period also shows a good agreement between measurements and model simulations. These results demonstrate that possibility of climatological assessment based on WRF regional climate simulations over Korea. Wind speed at the turbine hub height of 80m AGL, which is not available in measurements and wind resources based on effective wind speed over 4 m s-1 were also investigated. The wind speed at 80 m AGL was higher than the surface wind speed at 10 m AGL throughout the analysis domain, especially it showed the greater values over mountainous areas due to the overlying flow. In particular the mountainous and coastal areas, with the effective wind speed at 80 m AGL, can use about 70% of total wind speed for the generation of wind power energy. The eastern and southern area and island was identified with high potential areas of wind energy resources of mean wind speeds, 50% wind frequency, and 50% constancy of the direction. The prevailing trend in wind speed over South Korea was characterized by the significant increases (0.007 m s–1 a–1) during the study period. In addition, wind resource, the annual frequency of the effective wind speeds and wind density also shows increase during the study period. These results also indicate that the wind power potential over South Korea, especially eastern and southern area has been in progressive increase in recent time. An attempt was also made to investigate whether the significant changes of wind speed in recent climate would continue in near future over Korea. Projected wind speeds in future were produced using WRF model simulation under emission scenario of Intergovernmental Panel on Climate Change (IPCC) during the late 20th century (1990−1999) and the mid-21st century (2045−2054). The ensemble experiment was designed to provide information with a reasonable level of confidence using different planetary boundary layer physics. The optimal ensemble experiment for wind speed was YSU-MYNN2. Projected wind-speed changes at 80 m above ground level varied from –10.60 to 29.80% depending on the location and season even though changes of annual wind speed were very small. The strong wind speed in the cold season has decreased (–5.45%), whereas the weak wind speed in the warm season has increased (4.54%). A possible cause of the weakened and strengthened winds in cold and warm seasons, respectively, is shown to be the effects of the lower-tropospheric pressure-gradient force accompanied by the pronounced warming. Therefore, wind-speed themselves are climate dependent. Moreover changes of effective wind-speed, its frequency, and density were having similar pattern with the changes of mean wind speed although significant changes were noted over slightly different regions. Finally wind power grade were investigated to assess wind energy potential for the operational use. High wind power grade were noted over west coast and eastern mountainous area in present and near future conditions. In addition wind speed and its temporal variation were examined using new parameterizations and higher spatial resolution model configuration to know effects of high-resolution modeling on predictability of wind speed. The increased vertical mesh model with subgrid drag parameterization represents the largely improved wind speed at 10 m AGL. Wind speeds at 1000 hPa taken from high-resolution modeling experiment shows very slightly improved accuracy in comparison with wind speed taken from control experiment. For temporal variation wind speed exhibits similar tendency between the wind speeds taken from control and high-resolution modeling experiment. Therefore the results indicate that previous WRF wind climate simulations with the configuration of 10 km horizontal resolution may be sufficient to represent the long-term trends of annual wind speed at the turbine height 80 m AGL throughout Korea.