Effective Incidence Angles

Two things regarding my previous posts:

1. I used a generic location for the radar location just for debugging.
   
2. I used wave conditions that were fairly small.

Now, in order to get more representative results, I am running a more chllenging wave condition, that of August 11, 2007, at 7 am (Hmo=1.38 m, Tp=7.5 s). I also put Funwave to high strain, since I decreased my grid size to 1 m, rather than 3. The only thing lingering in the air is the actual antenna height, which I don't have because the GPS readings were crazy (it reported 0 m at the antenna base), so I am using a conservative value of 5 m.

Since the antenna is further inland, the grazing angle is even smaller, and that combined with the steeper waves gave a lot of shadowing, which was something expected for nearshore applications.

But the other interesting thing was to compute the actual surface slope, which combined with the local grazing angle (et every grid point), can be used to get the effective incidence or grazing angle. This is relevant, because LGA effects are usually considered to be relevant in the range GA<20 Traditional Bragg theory seems to be best suited to the range 20 < GA < 70. One of the results of our preliminary analysis of field data showed that large waves (steep) yield increased returns, especially under the appropiate water and wind conditions. Wave breaking on the other hand, was rather insensitive to the ambient conditions (although calibrated data is needed to corrboarate this). So, we speculated that the high returns were due to Bragg-like effects. Now the question is wheter the effective angles are compatible with the range were Bragg is known to be the main mechanism.

And it seems that they are.

Here is a snap fo the free surface for this scenario. The upper panel is the free surface, where wave peaks esceeded 1 m. The second panel is the surface slope, with zero value being a horizontal surface (thus crests have zero angle). The third panel is the effective grazing angle, which is the result of the sum of the local grazing angle and the slope. It can be seen that in the front of the waves, local grazing angles can reach 20 deg for these waves, thus no longer being in the LGA regime. Moreover, the choice of z_radar=5 m is a conservative one, since a higher elevation value would shift all the values even more outside the LGA regime.



Hence, it looks reasonable that Bragg mechanisms are responsible for large returns in the front of the waves.  The confirmation of this requires modeling the radar return and compare it with the measurements.