The calculation of stratospheric air parcel trajectories using satellite data

Abstract

AbstractAir parcel trajectories are calculated for the mid‐stratosphere using data from the stratospheric sounding unit on board NOAA‐6. In the analysis method all the orbital data for each 24 h period are combined into a single global analysis. Three trajectory methods are used‐isobaric, isentropic and quasi‐isentropic‐and the results are compared and contrasted. For the quasi‐isentropic method a radiation model is used, via the thermodynamic equation, to imply cross‐isentrope flow at regular intervals along the trajectory. the positions of the parcels, computed using the three methods, are found in general to be in good agreement when projected on to a horizontal plane. However, the altitude varies according to the trajectory method used and changes by about 1 km along a quiescent trajectory and as much as 2 km along a disturbed trajectory, during a 10‐day period. This has important implications in the study of chemistry along parcel trajectories during disturbed periods since the temperature‐ and pressure‐dependent reactions will proceed at different rates under the different assumptions. Also, the high lapse rate in the vertical mixing ratio profile of ozone implies that the detailed photochemistry will depend critically on the height of the air parcel.Further, potential vorticity and potential temperature are used as quasi‐conservative Lagrangian tracers, to try to determine which trajectory method is most realistic. For the quiescent period studied (June 1979 in the southern hemisphere) the isobaric and isentropic methods produced similar results throughout the ten days of the trajectory but the quasi‐isentropic method was clearly superior. A disturbed period was also studied (the February 1979 stratospheric warming), but the results from the trajectories studied were consistent with the Lagrangian conservation laws for only 6 days, at most. These results illustrate the need for caution, particularly with regard to sensitivity to initial horizontal position, in using trajectories calculated for disturbed periods.