Winter and spring observations of stratospheric chlorine monoxide from Ny-Alesund, Spitsbergen, in 1997/98 and 1998/99

Observations of stratospheric chlorine monoxide (ClO) were performed at the Arctic station of the "Network for the detection of Stratospheric Change" (NDSC). A ground based millimeter wave radiometer was routinely operated in Ny-Alesund, Spitsbergen (78.9 degreesN, 11.9 degreesE). In 1998 a period w ith enhanced ClO in a layer around 22 km altitude was detected from Februar y 16 to March 9. Calculations using the three dimensional chemical transpor t model SLIMCAT are consistent with the observations. In 1999 no significan t chlorine activation was observed while SLIMCAT calculations predicted a r ather strong chlorine activation in February 1999, when the polar vortex wa s situated above Ny-Alesund. Deviations of SLIMCAT calculations for 1999 fr om the observations could be attributed to differences in temperatures obta ined from the United Kingdom Meteorological office (UKMO) used in the model , and temperatures obtained from other sources. UKMO temperatures were comp ared to temperatures obtained from the National Center for Environmental Pr edictions (NCEP), the European Centre for Medium Range Weather Forecast (EC MWF) and radio soundings in Ny-Alesund. A low bias of 1 to 2 K in UKMO temp eratures was detected during the preconditioning phase of the vortex air at the altitude were the ClO activation was modeled. No systematic deviation from the radio sonde temperatures were found in the other data sets. In UKM O data temperature differences reach up to 8 K during the investigated time period.