Seven groups have participated in an intercomparison study of calculations of radiative forcing (RF) due to stratospheric water vapour (SWV) and contrails. A combination of detailed radiative transfer schemes and codes for global-scale calculations have been used, as well as a combination of idealized simulations and more realistic global-scale changes in stratospheric water vapour and contrails. Detailed line-by-line codes agree within about 15 % for longwave (LW) and shortwave (SW) RF, except in one case where the difference is 30 %. Since the LW and SW RF due to contrails and SWV changes are of opposite sign, the differences between the models seen in the individual LW and SW components can be either compensated or strengthened in the net RF, and thus in relative terms uncertainties are much larger for the net RF. Some of the models used for global-scale simulations of changes in SWV and contrails differ substantially in RF from the more detailed radiative transfer schemes. For the global-scale calculations we use a method of weighting the results to calculate a best estimate based on their performance compared to the more detailed radiative transfer schemes in the idealized simulations. Zusammenfassung Sieben Forschungsgruppen haben an einem Ergebnisvergleich hinsichtlich des Strahlungsantriebes von Kondensstreifen und stratosphärischem Wasserdampf teilgenommen. Dies umfasst einen Vergleich sowohl von aufwändigen Strahlungsübertragungsmodellen mit Parametrisierungsschemata aus globalen Klimamodellen als auch einen Vergleich von idealisierten und realistischen Mustern von Kondensstreifen und stratosphärischem Wasserdampf. Aufwändige Linienmodelle weichen für den langwelligen (LW) und kurzwelligen (SW) Strahlungsantrieb um etwa 15 % voneinander ab, in einem Fall beträgt die Abweichung 30 %. Da die LW und SW Komponenten sowohl für Kondensstreifen als auch für stratosphärischen Wasserdampf ein unterschiedliches Vorzeichen haben, können sich die Differenzen in den Komponenten bzgl. des Netto-Strahlungsantriebes kompensieren und verstärken. Daher sind relativ gesehen die Modellunsicherheiten beim Netto-Strahlungsantrieb am größten. Einige der Modelle, die in globalen Klimasimulationen zur Anwendung kommen, weichen für die Strahlungsantriebe von Kondensstreifen undÄnderungen des stratosphärischen Wasserdampfes deutlich von den Ergebnissen aufwändigerer Strahlungsmodelle ab. Um eine beste Abschätzung für die Berechnungen des globalen Strahlungsantriebes geben zu können, wichten wir die Ergebnisse der einzelnen Modelle unter Berücksichtigung der Abweichungen, die in den idealisierten Vergleichsrechnungen im Vergleich zu den Ergebnisse der detailliertesten Modelle festgestellt wurden.
The CT25K ceilometer is a general-purpose cloud height sensor employing lidar technology for detection of clouds. In this paper it is shown that it can also be used to retrieve aerosol optical properties in the boundary layer. The authors present a comparison of the CT25K instrument with the aerosol lidar system and discuss its good overall agreement for both the range-corrected signals and the retrieved extinction coefficient profiles. The CT25K aerosol profiling is mostly limited to the boundary layer, but it is capable of detecting events in the lower atmosphere such as mineral dust events between 1 and 3 km. Assumptions needed for the estimation of the aerosol extinction profiles are discussed. It is shown that, when a significant part of the aerosol layer is in the boundary layer, knowledge of the aerosol optical depth from a sun photometer allows inversion of the lidar signal. In other cases, surface observations of the aerosol optical properties are used. It is demonstrated that additional information from a nephelometer and aethalometer allows definition of the lidar ratio. Extinction retrievals based on spherical and randomly oriented spheroid assumptions are performed. It is shown, by comparison with the field measurements during the United Arab Emirates Unified Aerosol Experiment, that an assumption about specific particle shape is important for the extinction profile inversions. The authors indicate that this limitation of detection is a result of the relatively small sensitivity of this instrument in comparison to more sophisticated aerosol lidars. However, in many cases this does not play a significant role because globally only about 20% of the aerosol optical depth is above the boundary layer.
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