Lagrangian transport simulations using the extreme convection parametrization: an assessment for the ECMWF reanalyses

Abstract. Atmospheric convection plays a key role in tracer transport from the planetary boundary layer to the free troposphere. Lagrangian transport simulations driven by global meteorological input data such as the European Centre for Medium-Range Weather Forecasts (ECMWF's) ERA5 and ERA-Interim reanalysis typically lack proper explicit representations of convective up- and downdrafts because of the limited spatiotemporal resolution of the input data. Lagrangian transport simulations for the troposphere can be improved by applying parametrizations to better represent the effects of unresolved convective transport in the global meteorological reanalysis data. Here, we implemented and assessed the effects of the extreme convection parametrization (ECP) in the Massive Parallel Trajectory Calculations (MPTRAC) model. The ECP is conceptually simple. It requires the convective available potential energy (CAPE) and the height of the equilibrium level (EL) of the meteorological data for input. Assuming that unresolved convective events yield well-mixed vertical columns of air, the ECP randomly redistributes the air parcels vertically between the surface and the EL, if CAPE is present. We analyzed statistics of explicitly resolved and parametrized convective updrafts and found that the frequencies of strong updrafts due to the ECP, i.e., 20 K potential temperature increase over 6 h or more, increase by 2 to 3 orders of magnitude for ERA5 and 3 to 5 orders of magnitude for ERA-Interim compared to the explicitly resolved updrafts. To assess the effects of the ECP on tropospheric tracer transport, we conducted transport simulations for the artificial tracer e90, which is released globally near the surface and has a constant e-folding lifetime of 90 days throughout the atmosphere. The e90 simulations were conducted for the year 2017 with both, ERA5 and ERA-Interim data. Next to sensitivity tests on the choice of the CAPE threshold, an important tuning parameter of the ECP, we suggest a possible improvement of the ECP method, i.e., to take into account the convective inhibition (CIN) indicating the presence of warm, stable layers that prevent convective updrafts in the real atmosphere. While ERA5 has higher spatiotemporal resolution and explicitly resolves more convective updrafts than ERA-Interim, we found there is still a need for both reanalyses to apply a convection parametrization such as the ECP to better represent tracer transport from the planetary boundary layer into the free troposphere on the global scale.