1. Stability classes from operational ceilometers
Gabriele Rau, Martin Piringer

2. Outline
Motivation
Stability classes from human cloud cover observations
Stability classes from ceilometer using cloud base height information
Results: Comparison of stability classes
Summary of findings

3. Motivation
Dispersion models usually need atmospheric stability information as a meteorological input parameter
Klug-Manier scheme used here (German AKTerm-format)
Stability classes from cloud coverage data is one of the most often used schemes for the parameterization
The number of fully automatic meteorological stations (without human observations) is increasing
Methods other than human observations to quantify cloud coverage – e.g. based on instruments such as ceilometers –will come into use, despite the different approaches of how to define cloud cover
CL51 at ZAMG

4. Aim of the investigation
A human observer will take into account the whole visible sky
A ceilometer can only detect clouds directly above the instrument, but with a high temporal resolution
Can the mean value of ceilometer soundings over a defined period of time (i.e. an hour) be used as a substitute for observed cloud coverage data in order to determine stability classes?
At 4 sites in Austria Klug-Manier-classes derived from observed cloud data (for the year 2014) were compared with Klug-Manier-classes from ceilometer data
At one site (ZAMG at Vienna), a single ceilometer CL51 is in use, at the airports (AustroControl) several (up to five instruments) ceilometers (CL31) can be combined to improve the cloud cover information

5. Cloudiness from Ceilometers
Ceilometer does not deliver total cloud cover, but cloud cover from up to 3 single layers where clouds are detected (sky-condition-algorithm of manufacturer)
At airports, data of more than one ceilometer can be combined to decrease the effect of a point measurement
Two algorithms for total cloud cover from ceilometers were tested (for CL51 only):
Total cloud cover = Max. of observed cloud cover (CeiloMax). This method was used for all further investigations
Total cloud cover = Sum of observed cloud cover from all layers (CeiloSum)
Habe ergänzt, dass nur für CL51 zwei Methoden getestet wurden. Was die ACG für ihre CL31 hernimmt, wissen wir nicht so genau. Hab ergänzt, dass für die weiteren Sachen CeiloMax genommen wurde (weil es bei 8/8 weniger Abweichungen hatte)

6. Cloudiness - human observation vs. ceilometer
Independent of site:
Ceilometer over-estimates 0/8 and 8/8 cloudiness
Ceilometer under-estimates 2/8 to 7/8, partly over- and underestimates 1/8
This is a result of observer instructions: 0/8 only, if there is no visible cloud;
8/8 only, if there is no visible cloud-free space
Amount of agreement: 36 to 40 %; +/- 1 okta: ~ 70 %
Wien durch Vienna ersetzt, Hohe Warte durch ZAMG

7. Definition of Klug-Manier stability classes
Klug-Manier classes are numbered from I to V and are classified according to atmospheric stability as follows:
Stability classes V and IV comprise very unstable and unstable conditions, meaning good vertical mixing in the boundary layer. They do not occur during night-time. Class V occurs only between May and September.
Stability classes III/2 and III/1 are classified as neutral. III/2 occurs predominantly at daytime, III/1 predominantly at night-time and during sunrise and sunset. These classes are typical for cloudy and/or windy conditions.
Stability classes II and I comprise stable and very stable conditions, mostly, but not exclusively at night. They occur with reduced vertical mixing; horizontal transport over long distances is possible.

8. Basic scheme to determine Klug-Manier stability classes
For *), **) and ***) several additional rules apply. Depending on the season a variety of further adaptations have to be considered.
The stability class is determined for each hour via wind speed, cloud cover,
base height of cloud as well as month and daytime
Low wind speed, low cloud cover: stable during night, unstable during daytime
High wind speed, high cloud cover: neutral day and night
Englische Beschriftung aus der VDI (heisst dort wind velocity). Die Wolkenart spielt eigentlich keine Rolle, wichtig ist nur die Höhe (hohe oder niedrige Wolken)
Das ist so natürlich unvollständig, gewisse Klassen (V) treten erst nach Durchführung aller Korrekturen und Ausnahmen auf.

9. Results: comparison of stability classes for Vienna
Nur beide ceilos und synop, Schwechat-Synop ist echt mit der Beobachtung von Schwechat gemacht. in der VDI nennen sie es dispersion category, das habe ich übernommen
Agreement observation to ceilometer: 84% (ZAMG) to 86.6 % (Airport)

10. Results: comparison of stability classes for Linz
Nur ceilo und synop, in der englischen Version der VDIheißt es dispersion category.
Agreement observation to ceilometer: 86.2%

11. Results: comparison of stability classes for Salzburg
Agreement observation to ceilometer: 88.1%

12. Details of the comparison I
In Schwechat sind es 5 Geräte (soweit wir wissen)
Upper table: Vienna/ZAMG, one ceilometer CL51 Lower table: Vienna/Airport, several ceilometers CL31 Causes of good agreement: cloudiness and wind speed are split into only a few classes, wind speed data are the same

13. Details of the comparison II
Very good agreement for the airports at Linz (upper table) and Salzburg (lower table)

14. Interpretation
A good agreement between stability classes from visual cloudiness observations and ceilometer-derived cloudiness is obtained
In 84 to 88 % of all cases, the same stability class is determined
The agreement of stability classes is better than that of the cloud information
This result is to be expected as the cloudiness and wind speed data are summarized to only a few classes
Ceilometer cloud information can be used equally well as visual observation to determine stability classes

15. Thank you for your attention! Questions?
Thank you for your attention!
Questions?