CEOS Intercal Nordic campaign at Sodankylä
Objectives of CEOS Intercal Brewer and Dobson intercomparison campaigns
Ground-based instruments (Dobson and Brewer) as well as satellite ozone retrievals present four major issues that are not fully understood: (1) seasonal differences between Dobson and Brewer, (2) stray light in single monochromator Brewers, (3) algorithm differences and (4) ozone cross section effects. Dobson and Brewer comparisons show a seasonal pattern with largest differences in late autumn and winter (high solar zenith angle). This difference is partially explained by the different sensitivity of the Brewer and Dobson algorithms to the temperature of the ozone layer.
Monthly averages out of the daily means of the relative difference are drawn (blue curve), orange and red curves represent the differences, corrected by the influence of the temperature dependence of the Dobson absorption coefficients and differences in slant path calculation.
During the SAUNA 1 and SAUNA 2 campaigns 2006-2007 at Sodankylä the single Brewers exhibited striking column dependences at high SZA. This behavior is due in part to instrumental stray-light and calibration issues of the single monochromator instruments (Singles: blue and green crosses, Doubles: blue, red and turquoise dots on the plot).
As opposed to satellites, which rely on ozone profile and temperature climatologies in their inversions, ground-based instrumentations use a fixed effective ozone altitude and temperature in their algorithms. This difference in implementation leads to differences between ground-based and satellites and also between ground-based time-series themselves (since they use different values - also contributing to the seasonality of Brewer/Dobson seasonality). A useful fix for the satellite community would be to assess the implementation of latitude dependent climatology in the ground-based algorithms.
Another issue on the satellite validation is the use of different ozone cross sections in the retrieval algorithms. This problem is outside the scope of the project, but the implementation of changes on the ground-based algorithms mentioned above requires a reevaluation of the calibration of the instrument. This means that the calibration history has to be carefully documented and raw data files must be stored for further reanalysis.
The issues described above can readily be improved by performing calibration from a well characterized double monochromator instrument after operating the instruments side-by-side for periods of 2 weeks over a large range of solar zenith angles and varying ozone column amounts. Nordic campaign will provide useful data to assess all these points. FMI-ARC has exceptionally diversified observation programme to provide auxiliary measurements, which help in this regard. Especially atmospheric vertical profiles (radiosoundings and ozone soundings) will provide valuable precise data for the further analysis of observed differences.
The mission will take place from March 4 - 25, 2011. Arrival and configuration of the ground-based instruments is scheduled for March 7-8. The formal intercomparison begins on March 9 and continues through March 23. Instruments will be prepared for shipping and participants will depart on March 24-25. During the effective intercomparison period the sky measurements are taken always when good conditions for sun or moon observations exist. On the less-good-sky days we need to reserve sufficient time for the calibration-lamp and HeCd-laser measurements. Team Meetings will be held at main building Polaria of FMI-ARC. Weather briefings and measurement schedules will be the main focus at first. Once the intercomparison period is underway, scientific results from the most recent measurements will be the focus of the meetings. Data will be exchanged for further collaborative analysis in the field. For the presence of the teams and participation see tables below.
AEMET RBCC-E: Alberto Redondas, Juan José Rodriguez (Brewer #185)
DMI: Paul Eriksen (Brewer #053)
DWD RDCC-E: Ulf Köhler, Herbert Munier (Dobson #064)
FMI Jokioinen: Tapani Koskela, Kimmo Rikkonen (Brewer #107)
FMI Sodankylä: Esko Kyrö, Rigel Kivi (Brewer #037)
SMHI/SLU Vindeln: Weine Josefsson, Mikaell Ottosson Löfvenius (Brewer #006, Dobson #30)
*************MAP of FMIARC *****************
Local scientific and technical staff has experience from the earlier SAUNA projects 2006-2007 (http://fmiarc.fmi.fi/SAUNA/ ) to host Brewer–Dobson campaigns. The main campaign site is in and around the Sounding station (yellow square). For the photos of the SAUNA operations see http://fmiarc.fmi.fi/SAUNA/ops.html
Contact persons of various issues that may rise (e-mail Forename.email@example.com)
Guest rooms and logistics: Riitta Aikio, Secretary
Scientific support: Esko Kyrö, the PI of the campaign, Rigel Kivi, the Co-PI
Technical and IT support: Osmo Aulamo, Head of operations