Free Halo Simulation Program
© Jukka Ruoskanen 2010
Riihimäki, 25th March 2007
Photo by Jukka Ruoskanen *
Lowitz arcs occur in the vicinity of the 22° halo and consist of a lower-, middle- and upper component. However, these arcs are not always seen together. How is that possible?
Lowitz arcs arise from the same raypath (60° wedge) than all the rest 22° direction halos. The crystal orientation has restrictions, however, so that it spins about a horizontal a-axis, the so called Lowitz axis. This opens the possibility for a family of three separate halo arcs to be formed from the spinning of a simple prism.
The rotation about the Lowitz axis can be restricted to a few tens of degrees, which allows us to suppress the unwanted portions of the three components. If we go even further, we can again modify the prism profile into a rhomboid- or tabular direction, thus adjusting the relative intensities of the components. These consepts in connection with Lowitz arcs were first discussed by Riikonen et. al. (Weather, September 2007 issue). Their ideas are needed also to explain the 25th March 2007 display.
The upper Lowitz component was simulated with a so-called Parry-Lowitz orientation, having tilting angle of 20°. When the tilting angle is decreased, the orientation reduces to a normal Parry orientation. Furthermore, the crystal shape was a bit tabular mostly to make it more believable for such an orientation, but also to reduce the number of rays of lower Lowitz component (although the effect is only minimal).
Five more populations were used to simulate the rest of the display: a random orientation to take care of the 22° and 46° halos and normal plate-, column- and Parry populations. The last population, sixth altogether, was required since there lurk some odd radius halos as well in the photograph. A column crystal with small pyramids in both ends was introduced to explain the 9° and 20° column arcs. A tilting angle of 10° produced acceptable results.
The simulation is still far from good. Is there a hint of the middle Lowitz arc near the parhelia? How would you tackle that issue? The parameter file of the simulation above can be downloaded HERE.
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