Spatial structure of assimilated ozone in the upper troposphere and lower stratosphere

Abstract

Ozone distributions display a rich spatial structure in the upper troposphere and lower stratosphere (UTLS). This study uses in situ aircraft observations to examine how accurately such spatial distributions of ozone are represented in an assimilation of NASA's EOS‐Aura ozone data using meteorological fields from the Goddard Earth Observing System, Version 4. Total ozone columns from the Ozone Monitoring Instrument (OMI) and stratospheric profiles from the Microwave Limb Sounder (MLS) are assimilated. The results demonstrate that the dominant large‐scale ozone distribution is well‐constrained (to realistic concentrations) near 250 hPa in the UTLS. Spatial patterns and the spectral power of variations are similar in the model simulations and the assimilation. The spectral power of the analysis increments is concentrated at large scales. Thus transport, rather than direct assimilation of small‐scale features, is responsible for the spatial ozone structure in the UTLS. Consistent with the documented behavior of the Lin‐Rood transport scheme, we demonstrate that the assimilated ozone represents realistic features on spatial scales of about four‐to‐six model grid boxes (500–800 km) and that smaller scales present in the aircraft observations are represented too weakly in the modeled and assimilated ozone fields. For the assimilated ozone, this result is robust over a range of realistic model‐error‐covariance length scales. The results indicate that observations of ozone on spatial scales that constrain the large‐scale gradients are suited to producing global analyses of UTLS ozone that represent features at higher spatial resolution.