Objective

Validation of groundbased and satellite total ozone measurements are usually performed at mid latitudes where total ozone is near 300 DU. In general, agreement is within 2% between instruments, giving confidence to the methods and algorithms under these conditions. However total column ozone retrievals at high latitudes show persistent differences of 5–10%, especially under conditions of low sun, high total column ozone and high column variability. Satellite and ground-based measurements must be compared under a greater variety of ozone column amounts and profile shapes if such differences are to be resolved.

The objective of the Sodankylä Total Column Ozone Intercomparison is to assess the comparative performance of the ground-based instruments and algorithms which measure total column ozone at large solar zenith angles and high total column ozone amounts. Observations made by space based instruments aboard several satellites is tightly integrated into the intercomparison strategy of the mission as well. The effect of ozone and temperature profile on total column measurements will also be explored using LIDAR and ozone sonde observations. The scientific findings from the SAUNA mission should improve the absolute accuracy of future total ozone observations made under the extreme conditions of high ozone and large solar zenith angles.

Brewer UV Instruments

Ozonesonde launch

Sounding station at FMI-ARC in Sodankylä

Background top

The SAUNA mission follows the TOMS³F measurement campaign of 2001, which was conducted at the University of Alaska, Fairbanks and looked at similar high latitude, high total ozone amount retrievals. The SAUNA mission will work to build on findings from the smaller Fairbanks campaign by confirming its findings and increasing the breadth and number of ground based and satellite measurements with a highly international team of investigators.

Sodankylä, Finland is located 120 km north of the Arctic Circle. The early springtime at this high latitude provides the ideal large solar zenith angles for the mission, and total ozone is consistently higher than 400 DU over Sodankylä at this time of year. The SAUNA mission is set for March 20-April 14, 2006 to take advantage of these geophysical conditions.

Map of Finland showing location of Sodankyla

Finland and the location of Sodankylä, 120 km north of the Arctic Circle (67.37 °N, 26.63 °E).

Sodankylä is also the site of the Finnish Meteorological Institute's Arctic Research Centre, FMI-ARC. The Arctic Research Centre has an established groundbased ozone measurement program and an excellent infrastructure for an large instrument intercomparison campaign.

Scientific Questions top

Ozone algorithm retrievals are best understood at low and middle latitude stations such as Hohenpeissenberg, Germany, where intercomparisons between groundbased instruments can be within 2%. At high latitudes and large solar zenith angles, however, total ozone measurements made by groundbased instruments total may be as large as 10%.

The primary scientific question that the SAUNA mission will aim to answer is

Why do total column ozone measurements from groundbased and satellite instruments and their respective retrieval algorithms continue to differ at mid and high latitudes, and where large solar zenith angles and high total ozone amounts are typical?

In addressing this question, the following will be explored:

» What combination of instruments and algorithms should be used to measure total ozone at the 1% level under mid to high latitude, large solar zenith angle, high ozone conditions?
» How can the absolute accuracy of groundbased and satellite measurements under these conditions be improved?
» What particular differences exist in the nature of measurements made by satellite and ground instruments over Sodankylä?
» What can explain the observed differences between the satellite DOAS and TOMS-type retrievals?

Groundbased Dobson vs. Brewer total ozone measurements are within 2% at Hohenpeissenberg, Germany, Latitude 47.8 °N. (U. Koehler, DWD)

Differences between the DOAS and TOMS satellite ozone retrieval algorithms can be as large as 10% at mid and high latitudes (ESA, 2002). The SAUNA mission should illuminate the nature of these differences by supplying data from groundbased validation data at these conditions coincident with the satellite measurements.

The SAUNA mission dates of March 20-April 14, 2006 will have an ideal solar zenith angle range to a minimum of near 60° and up to three daily satellite overpasses due to the high latitude of the field site. At this time of year total ozone amounts of over 400 DU are frequently measured over Sodankylä and days are typically could free.

Springtime levels of total column ozone are consistently above 400 DU over Sodankylä. (E. Kyrö, FMI-ARC)

Schedule top

The mission will take place from March 20-April 14, 2006. Arrival and configuration of the ground-based instruments is scheduled for March 20-26. The formal intercomparison begins March 27 and continues through April 12. Instruments will be prepared for shipping and participants will depart on April 13-14.

Science Team Meetings will be held twice a week at facilities on-site at FMI-ARC. All members should attend. Early meetings will focus on the status of the instruments as they are configured, and on measurement schedules for the mission, including coordination of groundbased measurements with satellite instrument overpasses. 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.

Short daily briefings will also be held to present short status updates and disseminate information to the teams.

Science Team Meeting Set-up / Break-down Intercomparison Period