Evaluating Tropical Tropospheric Ozone and Water Vapor in MERRA-2, ERA-Interim, and CAM-Chem using Observations from the Western Pacific

Tropical tropospheric ozone (O₃) is a significant greenhouse gas that absorbs both longwave and shortwave radiation (Anderson et al., 2016) . Previous studies have shown an anti-correlation between O₃ and RH (i.e Anderson et al., 2016; Hayashi et al., 2008). During the CONTRAST campaign, Pan et al. (2015) observed a bimodal O₃ distribution with the primary and secondary modes located at 18 ppbv and 60 ppbv, respectively. The secondary enhanced O₃ mode was no longer observed when RH was above 45%, showing an anti-correlated relationship between O₃ and water vapor. In order to represent O₃ and water vapor in atmospheric models, this relationship and the controlling mechanisms must be understood and depicted. Data from the CONTRAST campaign will be compared to CAM-Chem, a climate-chemistry model, MERRA-2, an atmospheric reanalysis simulation, and ERA-Interim, also an atmospheric reanalysis simulation.

Bimodal O₃ distributions are seen in frequency histograms of the model and reanalysis data when following the methods of Pan et al. (2015). When RH is greater than 45%, only the primary mode, located at a lower O₃ concentration, is seen for the model and reanalysis data. This shows that the model and reanalysis simulations do represent the anti-correlation between O₃ and water vapor. Layer normalized frequency plots of CONTRAST and collocated MERRA-2 and ERA-Interim data show that MERRA-2 produces similar O₃ trends to the observations, while ERA-Interim produces different O₃ trends located at higher concentrations. Global plots of dry air frequency and average O₃ interpolated on the 320 K isentropic level, shows that high O₃ concentrations (> 60 ppbv) are associated with high frequencies of dry air (RH < 45%) for the CAM-Chem model. High O₃ concentrations and high dry air frequencies do not spatial match for the MERRA-2 and ERA-Interim reanalysis simulations.