Global review and synthesis of trends in observed terrestrial near-surface wind speeds: Implications for evaporation

See Zeng et al. 2019 for updated surface wind speed science.

Key Findings

• Shows that terrestrial stilling is widespread across the globe
• Confirms declining rates of evaporative demand
• Highlights the contribution wind speed has made to these declining evaporative rates

• States that in a globally warming climate, observed rates of atmospheric evaporative demand have declined over recent decades
• States that several recent studies have shown that declining rates of evaporative demand are primarily governed by trends in the aerodynamic component (primarily being the combination of the effects of wind speed (u) and atmospheric humidity) and secondarily by changes in the radiative component
• States a number of these studies also show that declining rates of observed near-surface u (termed ‘stilling’) is the primary factor contributing to declining rates of evaporative demand
• Establishes first objective: to review and synthesize the literature to assess whether stilling is a globally widespread phenomenon
• Analyses 148 studies reporting terrestrial u trends from across the globe (with uneven and incomplete spatial distribution and differing periods of measurement) and found that the average trend was −0.014 m s−1 a−1 for studies with more than 30 sites observing data for more than 30 years, which confirmed that stilling was widespread
• States that a second objective was to confirm the declining rates of evaporative demand by reviewing papers reporting trends in measured pan evaporation (Epan) and estimated crop reference evapotranspiration (ETo); average trends were −3.19 mm a−2(n = 55) and −1.31 mm a−2 (n = 26), respectively
• Says a third objective was to assess the contribution to evaporative demand trends that the four primary meteorological variables (being u; atmospheric humidity; radiation; and air temperature) made
• Finds the results from 36 studies highlighted the importance of u trends
• Quantifies the sensitivity of rates of evaporative demand to changes in u and how the relative contributions of the aerodynamic and radiative components change seasonally over the globe