Assimilated ozone from EOS‐Aura: Evaluation of the tropopause region and tropospheric columns

Abstract

Retrievals from the Microwave Limb Sounder (MLS) and the Ozone Monitoring Instrument (OMI) on EOS‐Aura were included in the Goddard Earth Observing System version 4 (GEOS‐4) ozone data assimilation system. The distribution and daily to seasonal evolution of ozone in the stratosphere and troposphere during 2005 are investigated. In the lower stratosphere, where dynamical processes dominate, comparisons with independent ozonesonde and Measurement of Ozone and Water Vapour by Airbus In‐Service Aircraft (MOZAIC) data indicate mean agreement within 10%. In the troposphere, OMI and MLS provide constraints on the ozone column, but the ozone profile shape results from the parameterized ozone chemistry and the resolved and parameterized transport. Assimilation of OMI and MLS data improves tropospheric column estimates in the Atlantic region but leads to an overestimation in the tropical Pacific and an underestimation in the northern high and middle latitudes in winter and spring. Transport and data biases are considered in order to understand these discrepancies. Comparisons of assimilated tropospheric ozone columns with ozonesonde data reveal root‐mean‐square (RMS) differences of 2.9–7.2 Dobson units (DU), which are smaller than the model‐sonde RMS differences of 3.2–8.7 DU. Four different definitions of the tropopause using temperature lapse rate, potential vorticity (PV), and isentropic surfaces or ozone isosurfaces are compared with respect to their global impact on the estimated tropospheric ozone column. The largest sensitivity in the tropospheric ozone column is found near the subtropical jet, where the ozone‐ or PV‐determined tropopause typically lies below the lapse rate tropopause.