Windfarms, climate and weather: the influence of surface roughness anomalies on the general circulation.Dan Kirk-Davidoff Absract Large scale wind farms can substantially increase the effective surface roughness of the earth. Keith et al. (2004) showed results of model experiments in which surface roughness anomalies of varying strenght were imposed. The reported equilibrium temperature anomalies patterns had amplitudes of up to 1K. In the first part of the talk, we explain these results by first running experiments using the NCAR Community Atmospheric Model (CAM) in a simplified geometric setting ("aquaplanet") and then using insight gained from these runs to explain changes in the surface energy budget in the full model runs as consequences of straightforward changes in the model wind fields. Temperature anomalies are found to result from a complicated balance of advective and radiative changes. We further show that the magnitude of the temperature anomalies scales with the horizontal length scale of the roughened region. In the second part of this talk, we use CAM to simulate the effect of a sudden change in surface roughness. The model is run for ten years with a large simulated wind farm installation in place. At monthly intervals, branch runs are launched in which the wind farm roughness is reduced by about 1/2. We then observed the divergence of the original run and the branch over the following 30 days. We find a characteristic pattern of Rossby wave radiation follows the instantaneous change in surface roughtness. This leads to, for instance, noticeable changes in storm size and strength downstream of the roughness anomaly within 5 days of the beginning of the branch run. |