Uncertainty in recent near-surface wind speed trends: a global reanalysis intercomparison

Abstract

Reanalysis products have become a tool for wind energy users requiring information about the wind speed long-term variability. These users are sensitive to many aspects of the observational references they employ to estimate the wind resource, such as the mean wind, its seasonality and long-term trends. However, the assessment of the ability of atmospheric reanalyses to reproduce wind speed trends has not been undertaken yet. The wind speed trends have been estimated using the ERA-Interim reanalysis (ERA-I), the second version of the Modern Era Retrospective-Analysis for Research and Applications (MERRA-2) and the Japanese 55-year Reanalysis (JRA-55) for the period 1980-2015. These trends show a strong spatial and seasonal variability with an overall increase of the wind speed over the ocean and a tendency to a decline over land, although important disagreements between the different reanalyses have been found. In particular, the JRA-55 reanalysis produces more intense trends over land than ERA-I and MERRA-2. This can be linked to the negative bias affecting the JRA-55 near-surface wind speeds over land. In all the reanalyses high wind speeds tend to change faster than both low and average wind speeds. The agreement of the wind speed trends at 850 hPa with those found close to the surface suggests that the main driver of the wind speed trends are the changes in large-scale circulation.

Synergetic Satellite Trend Analysis of Aerosol and Warm Cloud Properties ver Ocean and Its Implication for Aerosol‐Cloud Interactions

“…Note that in the remote ocean region, aerosols are dominated by sea salt. Given that sea salt emissions are highly dependent on the surface wind speed (Chin et al, 2002), these opposite trends in AOD and AE may be partly due to a strong increase in near‐surface wind speed over tropical oceans (e.g., Torralba et al, 2017; Young et al, 2011). Using ECMWF data, we find an obvious increase in surface wind speed over tropical oceans in the Southern Hemisphere (supporting information Figure S1).…”

What global reanalysis best represents near‐surface winds?

“…This methodology followed in the computation of decadal trends is identical to the approach used in Torralba et al . () to facilitate the comparison of results.…”

“…Results for ERA‐Interim (Figure a), JRA55 (Figure c), and MERRA2 (Figure d) are similar to those obtained by Torralba et al . () for the 1980–2015 period and the reader is referred there for complete information. Here, the addition of the newly released ERA5 reanalysis (Figure b) shows good agreement with its predecessor ERA‐Interim (Figure a) as well as MERRA2 (Figure d).…”

“…An asterisk indicates that the trends are significant at the 95% confidence level: no asterisk indicates that the trends are not significant. One asterisk (∗) means that only one of the reanalyses has significant trends, two asterisks (∗∗) inform us that two reanalyses have significant trends, and so on (adapted from Torralba et al )…”

“…According to Torralba et al . (), the extrapolation of surface winds in the JRA55 reanalysis from the lowest model level is carried out assuming neutral stability. In areas where this level is located too high above the surface (such as forested zones), this assumption may lead to a significant reduction of wind speeds after extrapolation to the surface level.…”