Lagrangian transport schemes, where air parcels following the atmospheric flow carry the information on atmospheric properties, are advantageous for modelling trace gas transport in the stratosphere by reducing the excessive numerical diffusion inherent in standard transport schemes. Within the ESM project, work is devoted to improve stratospheric model transport by coupling the two different Lagrangian transport schemes
• Atmospheric Tracer Transport in a LAgrangian model ATTILA
• Chemical Lagrangian Model of the Stratosphere CLaMS
with the chemistry-climate model EMAC within the MESSy framework. MESSy is a software providing a framework for a standardised, bottom-up implementation of Earth System Models (or parts of those) with flexible complexity. The coupling of the Lagrangian schemes to the MESSy infrastructure was made possible due to a joint collaboration between DLR Oberpfaffenhofen and FZ Jülich. The two Lagrangian transport schemes are based on different formulations of the irregular Lagrangian air parcel grid, with parcel density in ATTILA being controlled by equal air parcel mass, in CLaMS being controlled by an entropy constraint.
Recent work within the ESM project has shown that both Lagrangian transport schemes substantially improve stratospheric tracer transport in EMAC (Brinkop and Jöckel, 2019; Charlesworth et al., 2020), in particular in regions close to the tropopause and around the polar vortex. Calculation methods for the stratospheric age spectrum have been implemented in EMAC and show significantly older air in the lower stratosphere for the Lagrangian transport schemes, related to reduced diffusive transport across the tropopause and across the polar vortex edge. These transport improvements cause reduced cross-tropopause moisture transport, resulting in a dryer lowermost stratosphere in better agreement with stratospheric water vapour observations. Moreover, a Lagrangian convection scheme has been developed for ATTILA to improve model transport in the upper troposphere.
Future work is aimed at developing chemistry-climate coupling for Lagrangian transport, to carry the improvements in the representation of lower stratospheric tracer distributions over to the model dynamics. For that reason, model infrastructure is developed to couple Lagrangian tracers to radiation and related effects on temperature and dynamics are investigated. Clear improvements are expected for model dynamics and for the representation of feedbacks related to stratospheric water vapour and ozone, which will contribute to narrowing down the uncertainty in simulated future warming projections.
References:
Brinkop, S. and Jöckel, P.: ATTILA 4.0: Lagrangian advective and convective transport of passive tracers within the ECHAM5/MESSy (2.53.0) chemistry–climate model, Geosci. Model Dev., 12, 1991–2008, doi.org/10.5194/gmd-12-1991-2019, 2019.
Charlesworth, E. J., Dugstad, A.-K., Fritsch, F., Jöckel, P., and Plöger, F.: Impact of Lagrangian transport on lower-stratospheric transport timescales in a climate model, Atmos. Chem. Phys., 20, 15227–15245, doi.org/10.5194/acp-20-15227-2020, 2020.