Discussion of overlap effects and overlap measurements


There are significant differences between both CL51 instruments below 500m. The profile of CL51CG decreases towards ground but profile of CL51RAO increases. See examples:

20150614PS time15 16zr00 03km Smooth01 Ceilos 5 6 20150614PS time19 20zr00 03km Smooth01 Ceilos 5 6

This behaviour improved significantly when CL51RAO was updated to TOPROF firmware, see example of July 16:

20150716ps time15 16zr00 03km smooth01 ceilos 5 620150716ps time21 23zr00 03km smooth01 ceilos 5 6


During the To-PROf meeting in Granada, it was mentioned that a constant artefact could be seen on most CHM15k instruments. It was pointed out that this is very likely due to an improperly corrected overlap.

It is visible during the Ceilinex campaign (for example the 06/07/2015 between 23:00 and 24:00)


Artefact on CHM profiles

Yann Poltera and Maxime Hervo from MeteoSwiss developed an algorithm to correct this overlap. No horizontal measurements or external references are needed.

This new overlap function calculated the 20150603 remove the artefacts observed on CHM15k ceilometers.

Artefacts CHM140101


Artefacts CHM100110

This algorithm still needs to be validated against other methods.

Christoph Münkel says:
Jul 17, 2015 02:17 PM
CL51RAO had been running on an old firmware version 1.023. The update to version 1.029 delivered from Vaisala to DWD in December 2012 had unfortunately not been installed. So during the first weeks of CeiLinEx, CL51RAO was still using an old overlap function that had been updated (and not changed since) for version 1.024.
Matthias Wiegner says:
Aug 04, 2015 05:44 PM
Intercomparisons of the 4 Lufft ceilometers show good agreement when arbitrarily normalized at 1 km. The overlap is as expected: lower ovl of the CHXs compared to the CHMs. The CHX RAO has the best performance in this respect, however, in particular during daytime, even the top of the boundary layer might not be detectable due to the increased noise.
Manuel Lara says:
Aug 31, 2015 09:46 PM
Two questions:
Does the overlap corretion consider the tilt of the instrument?
Would we get any benefit for the overlap tilting the instrument a sizable amount, lets say 15 degrees? (Or this have nothing to do?)
Thanks, Manuel Lara
Frank Wagner says:
Sep 03, 2015 04:52 PM
The overlap correction is done using the "range". Tilting does not affect range. It is only important when range is transformed into height.
Tilting the instrument results in a better height resolution at the expence of not measuring data in the far range.
Tilting the instrument decreases the HEIGHT (not range) of the full overlap which is beneficial for retrievals.
15 degrees doesn't change much because cos(15) = 0.9659.