On the Vertical Mixing Of Heat And Long Term Surface Temperature Trends

There is a new paper [thanks to Jos de Laat for alerting us to it!]

Somnath Baidya Roy, and Justin J. Traiteur, 2010: Impacts of wind farms on surface air temperatures. PNAS. http://www.pnas.org/cgi/doi/10.1073/pnas.1000493107

with the abstract

“Utility-scale large wind farms are rapidly growing in size and numbers all over the world. Data from a meteorological field campaign show that such wind farms can significantly affect near-surface air temperatures. These effects result from enhanced vertical mixing due to turbulence generated by wind turbine rotors. The impacts of wind farms on local weather can be minimized by changing rotor design or by siting wind farms in regions with high natural turbulence. Using a 25-y-long climate dataset, we identified such regions in the world. Many of these regions, such as the Midwest and Great Plains in the United States, are also rich in wind resources, making them ideal candidates for low-impact wind farms.”

While their conclusions are important with respect to how wind turbine farms can alter surface air temperatures, their results have an even broader importance.

An excerpt from their paper is

Data from the field campaign show that near-surface air temperatures downwind of the wind farm are higher than upwind regions during night and early morning hours, whereas the reverse holds true for the rest of the day (Fig. 2A). Thus, this wind farm has a warming effect during the night and a cooling effect during the day. The observed temperature signal is statistically significant for most of the day according to the results of a Mann–Whitney Rank Sum Test

As we indicated in our paper

Pielke Sr., R.A., and T. Matsui, 2005: Should light wind and windy nights have the same temperature trends at individual levels even if the boundary layer averaged heat content change is the same? Geophys. Res. Letts., 32, No. 21, L21813, 10.1029/2005GL024407.

 and reported by my colleague Dick McNider

In the Dark of the Night – the Problem with the Diurnal Temperature Range and Climate Change by Richard T. McNider

long term alterations in the vertical distribution of heat changes can result in long term temperature trends which are a function of height, even IF the average lower tropospheric temperature change were zero. The strength of the winds in the layer of the atmosphere near the surface has been proposed as one mechansim to influence the vertical distribution of heat in this layer.

The figure below, from the Roy and Traiteur 2010 paper illustrates this effect very clearly

 

Figure 2a: Observed temperature in and near the San Gorgonio wind farm for near-surface air-temperature patterns at the San Gorgonio wind farm during the field campaign. 

Without the wind turbines, the temperature plots would presumably be identical.

This study indicates an error in the findings of Parker in his paper

Parker, D.E., 2004: Large-scale warming is not urban. Nature, 432, 290, doi:10.1038/432290a.

Parker wrote in his abstract

Controversy has persisted over the influence of urban warming on reported large-scale surface-air temperature trends. Urban heat islands occur mainly at night and are reduced in windy conditions. Here we show that, globally, temperatures over land have risen as much on windy nights as on calm nights, indicating that the observed overall warming is not a consequence of urban development.

Since the figure presented above does show a dependence of temperatuare anomalies as a function of wind speed (due to the wind turbines in this case), this raises serious issues with the conclusions in Parker (2004) regarding his claim that “temperatures over land have risen as much on windy nights as on calm nights”. 

We look forward to additional important research studies by Somnath Baidya Roy, and Justin J. Traiteur!

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