Homogenization and Assessment of Observed Near-Surface Wind Speed Trends across Sweden, 1956–2013

Abstract

Multidecadal variability of observed near-surface wind speed from 24 stations across Sweden has been analyzed for 1956–2013, with a focus on 1979–2008 (incorporating an additional 9 stations) for comparison with previous studies. Wind speed data have been subjected to a robust data processing protocol, consisting of quality control, reconstruction, and homogenization, by using geostrophic wind speed series as reference. The homogenized dataset displays a significant (at p < 0.05) downward trend for 1956–2013 (−0.06 m s−1 decade−1) and an even larger decreasing trend for 1979–2008 (−0.14 m s−1 decade−1). However, differences have been observed seasonally, with significant decreasing values in spring, summer, and autumn and a small downward trend in winter for 1956–2013. Most interestingly, a nonsignificant wind speed increase has been detected in winter for 1979–2008, which contrasts with the marked “stilling” reported for this season in much of midlatitude regions. The decreasing rate in wind speed is larger for coastal stations and in the southern part of Sweden. Decreasing trends were found at 91.7% of the stations during summer, whereas 58.3% of the stations displayed decreasing trends in winter. On the contrary, increasing trends occurred in 41.7% of the stations for winter and in only 8.3% for summer. The possible impact of the North Atlantic Oscillation (NAO) index has also been investigated, showing evidence that the small increasing trend in winter for 1979–2008 is hypothetically associated with the positive tendency of the NAO index during the last decades. These results reveal the influence of large-scale atmospheric circulation on wind speed variability across Sweden.

Homogenization of Tianjin monthly near-surface wind speed using RHtestsV4 for 1951–2014

“…Coastal locations exhibited higher wind speeds than interior stations in all months of the year, with maximum monthly wind speeds occurring in the winter months. Minola et al (2016) found similar results. Figure 7 shows the spatial distribution of trends in annual and seasonal time series based on the raw and adjusted monthly mean wind speed.…”

“…Accordingly, to obtain the best results, here the monthly candidate and its reference series in the calendar order were transformed into series in a calendar year with the same month; this technique has not been used in previous studies on the homogenization of near-surface wind speed (Wan et al 2010;Li et al 2011;Cusack 2013;Minola et al 2016;Azorin-Molina et al 2014, 2016. The change signal in the climate time series may be a systematic error, but it may also be a periodic variation in local climate (Li 2016).…”

“…Therefore, the method uses observations from nearby stations in the Tianjin network ( Fig. 1), instead of using the geostrophic wind speed series reflecting atmospheric circulation (Wan et al 2010;Minola et al 2016) or model simulation data not affected by human activities on the ground (Azorin-Molina et al 2014).…”

“…There are a few, rare cases showing wind speed homogenization in China and worldwide, but they have focused on the detection and correction of mean annual wind speed time series, which are less affected by seasonal variability (Liu 2000;Cao et al 2010;Xin et al 2012). Recently, the observed daily or monthly near-surface wind speed series over Canada, Beijing, Netherlands, Sweden, Spain, and Portugal have been homogenized by Wan et al (2010), Li et al (2011), Cusack (2013), Minola et al (2016), and AzorinMolina et al (2014AzorinMolina et al ( , 2016, respectively. The corresponding climatology, wind speed trends, and derivative climatic factors (e.g., windstorms or extremes) were also assessed.…”

“…Moreover, in Cusack (2013), a reference wind is defined as the average near-surface wind from four other stations instead of the geostrophic wind. Wind speed data in Sweden was also homogenized by Minola et al (2016) using geostrophic winds based on air pressure observations from three weather station sites as reference, similar to the work by Wan et al (2010). Daily wind speed observations from 12 stations in the Greater Beijing Area for 1960-2008 were homogenized using the Multiple Analysis of Series for Homogenization method (MASH) by Li et al (2011); they found that regional and local climate characteristics of wind speed in Beijing area were objectively reproduced using the adjusted data.…”