Modal storm tracks with differential friction

In the second set of experiments, we consider the impact of differential friction. Very little is really known empirically about the dependence of the frictional coefficient on the surface roughness. Therefore, we simply use two different plausible values of frictional coefficients for the land and ocean sectors in this model. Different combinations of them have been tried. The results obtained with the use of a = 0.045 for the land sectors in the model (North America, Euro-Asia) and a = 0.015 for the ocean sectors (Pacific and Atlantic) are representative of this set of experiments. While the drag coefficient over land is three times larger than that over the ocean in this experiment, the domain average value of the drag coefficient in the two experiments is about the same as in the control run. The time mean equilibrated departure stream function field also has dipole structure downstream of each jet similar to that for the case of a = 0.03 everywhere. One difference is that the time mean departure flow in the Atlantic sector is twice as strong as that in the Pacific sector (0.21 vs 0.09; plot not shown for brevity). There are also two equilibrated storm tracks in this case, arising from continual self-sustained transient growth of disturbances. Figure 9 shows the distribution of average kinetic energy of the fluctuating flow component in this experiment. The model storm track downstream of the model Atlantic jet is found to be about 38% more intense than that downstream of the model Pacific jet. Their maximum values are 0.08 vs 0.05 respectively. The key point is that differential friction over land versus over oceans can significantly accentuate the relative intensity of the two storm tracks in favor of the Atlantic storm track for the reason elaborated in the Introduction. This result is a further demonstration that seeding disturbances of the model Atlantic jet are statistically stronger than those of the model Pacific jet, leading to the relative intensity of the two storm tracks.

0 0

Post a comment