Add ICON visibility variable

[patrick-zippenfenig]

DWD adds visibility to their model output on April 23rd

Near-surface visibility diagnostic
Starting Tuesday, April 23rd 2024, at 09 UTC the output variable VIS (near-surface
visibility) will be available from ICON-D2 and ICON-EU.
An interest in near-surface visibility forecasts was put forward by the aviation forecasting
offices of DWD. This parameter is routinely used, together with ceiling (lowest level, where
cloud cover exceeds 50%) to forecast conditions near airports and for VFR (visual flight rule)
aviation in general. Further there was an interest in having these two parameters (VIS and
ceiling) as routine verification metrics, since they yield relevant information about the
boundary layer, especially in stable conditions. While ceiling was already available as output
parameter, a visibility diagnostic had to be implemented in the ICON model. After
implementation and further calibration, visibility has been monitored for a three month period
in winter 2023/2024. Its behavior was found to be consistent with expectations, in particular
the frequency of events compares well with observations.
The visibility diagnostic itself consists of two parts:

1. Visibilities below about 1-2 kilometers are dominated by hydrometeors. Here warm fog is
   dominant, while precipitation also contributes, at least over the ICON-D2 and ICON-EU
   domains. The visibility dependence on hydrometeor partial densities is modeled according to
   [1]. The partial densities are available at runtime.

2024_04_23_icon_VIS.pdf

[patrick-zippenfenig]

9z run from icon-d2 and -eu does not include visibility yet...