sebvi
commented
4 years ago Hello @tomkralidis , it seems to me that this proposal is the follow up of the proposal from Yves after IPET=CM II meeting. At that time, we had a long discussion (emails around June 21 2018 to June 25 2018). It seems that this proposal is the orginal one, not the revised version sent by Yves on June 22 2018. As explained in these emails (I can forward them to you if you want), I am against the units chosen for sky transparency. I can attached to this issue the revised document sent by Yves.
tomkralidis
SibylleK
chenxiaoxia2019
amilan17
Branch
https://github.com/wmo-im/GRIB2/tree/issue-25
Summary and purpose
New GRIB2 Code Table 4.2 entries for the physical atmospheric properties of seeing and sky transparency
Action proposed
The tables annexed herewith contain proposed additions to Table 4.2 of the GRIB2 section of the Manual on Codes, for new physical atmospheric forecast parameters, namely sky transparency (humidity component) and seeing. Interpretation notes are included.
Discussions
Initially discussed at Offenbach 2018 report
Seeing and transparency diagnostic forecast parameters have been produced for many years from NWP output at the Canadian Meteorological Centre. They are popular with amateur and professional astronomers, and were found useful in other applications where it is important to characterize the properties of sky light at a given time in the future. While up to now these fields were made available as non-georeferenced raster graphics, there is now a requirement to make these fields available as georeferenced data and as GRIB files. Seeing is the term used in astronomy to qualify the steadiness or turbulence of the atmosphere. Turbulence causes rapid and random fluctuations of the optical path through the atmosphere. The twinkling of stars, for example, occurs in poor seeing conditions. The data product is a categorical index that provides a qualitative indication of transparency conditions (i.e. from "excellent" to "very poor"). Sky transparency qualifies the effect of atmospheric composition on the viewing experience. Light travelling through the atmosphere is subject to scattering, the amount of which depends on the presence of water vapour, aerosols or other constituents. Ideal transparency conditions produce a black night sky conducive to viewing faint astronomical objects, almost like being in outer space. In poor transparency conditions, which may occur even in cloud-free conditions, the deep sky background is grayish (not black), faint details are washed out and contrast is reduced. The data product is a categorical index that provides a qualitative indication of seeing conditions (i.e. from "excellent" to "very poor"). More information can be found in these pages:
http://www.skyandtelescope.com/astronomy-blogs/imaging-foundations-richard-wright/seeing-vs-transparency-difference/
https://weather.gc.ca/astro/seeing_e.html
https://weather.gc.ca/astro/transparence_e.html
Detailed proposal
Proposed new entries for Code Table 4.2 Product Discipline 0 – Meteorological products, parameter category 19: physical atmospheric properties
Code Table 4.214 Environmental Factor Qualifier
Notes
(4) Parameter 38: In astronomy, Sky transparency means the effect on the viewing experience caused by the scattering of light through atmospheric water vapour, aerosols or other constituents. Ideal transparency conditions produce a black night sky conducive to viewing faint astronomical objects, almost like being in outer space. In poor transparency conditions, which may occur even in cloud-free conditions, the deep sky background is grayish (not black), faint details are washed out and contrast is reduced.
(5) Parameter 39: Seeing means the steadiness or turbulence of the atmosphere in the context of astronomical observation. Turbulence causes rapid random fluctuations of the optical path through the atmosphere. The twinkling of stars, for example, occurs in poor seeing conditions.