Comments On An ECMWF New Report On Surface Wind Trends

There is a recent study by ECMWF [the European Centre for Medium Range Forecasts] on long-term trends in surface winds – h/t to  Paolo Mezzasalma.

The news report on the study is titled

Wind stilling over the continents of the Northern hemisphere in the last 30 years

The article starts with the text [bold face added]

“A study published on 17 October 2010 in the journal Nature Geoscience shows that over the past three decades, surface wind speeds seem to have noticeably decreased in several regions of the world, such as the United States, China, Australia, and in several European countries. Given the often inadequate quality and heterogeneity of wind data measured by anemometers, no long-term study of the evolution of wind speeds on a global scale had been carried out so far.

 

However, after a detailed and thorough statistical analysis of the inconsistencies of wind measurements taken at 5412 stations resulting in the rejection of 85 % of them, an analysis of the remaining data revealed a major trend: over most land surfaces  of the Northern hemisphere mid latitudes, winds have decreased (see figure below). The study was carried out jointly by the Laboratoire des Sciences du Climat et de l\u2019Environnement (LSCE) and the European Centre for Medium-Range Weather Forecasts (ECMWF). It also shows that over Asia, moderate to strong winds  have decreased most rapidly.”

This study also attempts an explanation of the reasons for this decrease. Using a variety of data (datasets from reanalyses carried out by ECMWF or other smaller scale simulations, satellite and radiosonde observations), the authors show that this decrease in surface wind speed can be largely explained by an increase in vegetation and, to a smaller extent, by changes in the general atmospheric circulation over the past few decades.

Since, wind speed affects the vertical mixing of heat, this study implies there will be an effect on the surface temperature trends at these locations also. Most, or all, of these sites presumably are where  surface temperatures, used as part of the construction of a global annual average surface temperature trend, are obtained.

The study also documents that the surface landscape changes at these sites sufficiently to alter the winds. They write “the authors show that this decrease in surface wind speed can be largely explained by an increase in vegetation and, to a smaller extent, by changes in the general atmospheric circulation over the past few decades.” 

Such an increase in vegetation would also alter the temperatures. We have assessed this issue for locations in Colorado in our paper

Hanamean, J.R. Jr., R.A. Pielke Sr., C.L. Castro, D.S. Ojima, B.C. Reed, and Z. Gao, 2003: Vegetation impacts on maximum and minimum temperatures in northeast Colorado. Meteorological Applications, 10, 203-215.

The abstract of our paper reads [boldfaced added]

“A daily 850–700 mb layer mean temperature, computed from the National Center for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR) reanalysis, and satellite-derived greenness values, as defined by NDVI (Normalised Difference Vegetation Index), were correlated with surface maximum and minimum temperatures at six sites in northeast Colorado for the years 1989–98. The NDVI values, representing landscape greenness, act as a proxy for latent heat partitioning via transpiration. These sites encompass a wide array of environments, from irrigated-urban to short-grassprairie. The explained variance (r2 value) of surface maximum and minimum temperature by only the 850–700 mb layer mean temperature was subtracted from the corresponding explained variance by the 850–700 mb layer mean temperature and NDVI values. The subtraction shows that by including NDVI values in the analysis, the r2 values, and thus the degree of explanation of the surface temperatures, increase by a mean of 6% for the maxima and 8% for the minima over the period March–October. At most sites, there is a seasonal dependence in the explained variance of the maximum temperatures because of the seasonal cycle of plant growth and senescence. Between individual sites, the highest increase in explained variance occurred at the site with the least amount of anthropogenic influence. This work suggests the vegetation state needs to be included as a factor in surface temperature forecasting, numerical modeling, and climate change assessments.”

This another reason why there is a divergence between surface and lower tropospheric temperatures as we identified in our papers

Klotzbach, P.J., R.A. Pielke Sr., R.A. Pielke Jr., J.R. Christy, and R.T. McNider, 2009: An alternative explanation for differential temperature trends at the surface and in the lower troposphere. J. Geophys. Res., 114, D21102, doi:10.1029/2009JD011841.

Klotzbach, P.J., R.A. Pielke Sr., R.A. Pielke Jr., J.R. Christy, and R.T. McNider, 2010: Correction to: “An alternative explanation for differential temperature trends at the surface and in the lower troposphere. J. Geophys. Res., 114, D21102, doi:10.1029/2009JD011841″, J. Geophys. Res., 115, D1, doi:10.1029/2009JD013655.

The finding by ECMWF that there is long-term trend in wind speeds that is “largely explained by an increase in vegetation“, supports the conclusion of significant effects on the assessment of the global  annual land average surface temperature trend that are the result of effects other than warming (or cooling) of the remainder of the troposphere.

Comments Off on Comments On An ECMWF New Report On Surface Wind Trends

Filed under Climate Change Metrics

Comments are closed.