Trajectory mapping: A tool for validation of trace gas observations

We investigate the effectiveness of trajectory mapping (TM) as a data valid ation tool. TM combines a dynamical model of the atmosphere with trace gas observations to provide more statistically robust estimates of instrument p erformance over much broader geographic areas than traditional techniques a re able to provide. We present four detailed case studies selected so that the traditional techniques are expected to work well. In each case the TM r esults are equivalent to or improve upon the measurement comparisons perfor med with traditional approaches. The TM results are statistically more robu st than those achieved using traditional approaches since the TM comparison s occur over a much larger range of geophysical variability. In the first c ase study we compare ozone data from the Halogen Occultation Experiment (HA LOE) with Microwave Limb Sounder (MLS). TM comparisons appear to introduce little to no error as compared to the traditional approach. In the second c ase study we compare ozone data from HALOE with that from the Stratospheric Aerosol and Gas Experiment II (SAGE II). TM results in differences of less than 5% as compared to the traditional approach at altitudes between 18 an d 25 km and less than 10% at altitudes between 25 and 40 km. In the third c ase study we show that ozone profiles generated from HALOE data using TM co mpare well with profiles from five European ozonesondes. In the fourth case study we evaluate the precision of MLS H2O using TM and find typical preci sion uncertainties of 3-7% at most latitudes and altitudes. The TM results agree well with previous estimates but are the result of a global analysis of the data rather than an analysis in the limited latitude bands in which traditional approaches work. Finally, sensitivity studies using the MLS H2O data show the following: (1) a combination of forward and backward traject ory calculations minimize uncertainties in isentropic TM; (2) although the uncertainty of the technique increases with trajectory duration, TM calcula tions of up to 14 days can provide reliable information for use in data val idation studies; (3) a correlation coincidence criterion of 400 km produces the best TM results under most circumstances; (4) TM performs well compare d to (and sometimes better than) traditional approaches at all latitudes an d in most seasons; and (5) TM introduces no statistically significant biase s at altitudes between 22 and 40 km.