[1] The upper troposphere/lower stratosphere (UTLS) region plays an important role in the climate system. Changes in the structure and chemical composition of this region result in particularly large changes in radiative forcings of the atmosphere. Quantifying the processes that control UTLS composition (e.g., stratosphere-troposphere exchange) therefore represents a crucial task. We assess the influence of uncertainties in the atmospheric mixing strength on global UTLS distributions of greenhouse gases (water vapor, ozone, methane, and nitrous oxide) and associated radiative effects. The study is based on multiannual simulations with the Chemical Lagrangian Model of the Stratosphere (CLaMS) driven by ERA-Interim meteorological data and on a state-of-the-art radiance code. Mixing, the irreversible part of transport, is controlled by the local horizontal strain and vertical shear of the atmospheric flow. We find that simulated radiative effects of water vapor and ozone, both characterized by steep gradients in the UTLS, are particularly sensitive to uncertainties of the atmospheric mixing strength. Globally averaged radiative effects are about 0.72 and 0.17 W/m 2 for water vapor and ozone, respectively. For ozone, the largest impact of mixing uncertainties is observed in the extra-tropical lower stratosphere.Citation: Riese, M., F. Ploeger, A. Rap, B. Vogel, P. Konopka, M. Dameris, and P. Forster (2012), Impact of uncertainties in atmospheric mixing on simulated UTLS composition and related radiative effects,

[1] Recent satellite observations and dynamical studies have demonstrated the existence of filamentary structures in chemical tracer fields in the stratosphere. It is also evident that such features are often below the spatial resolution of the highest-resolution Eulerian models that have been used up to the present time. These observations have motivated the development of a novel Chemical Lagranigan Model of the Stratosphere (CLaMS) that is based on a Lagrangian transport of tracers. The description of CLaMS is divided into two parts: Part 1 (this paper) concentrates on the Lagrangian dynamics, i.e., on the calculation of trajectories and on a completely new mixing algorithm based on a dynamically adaptive grid, while part 2 describes the chemical integration and initialization procedure. The mixing of different air masses in CLaMS is driven by the large-scale horizontal flow deformation and takes into account the mass exchange between the nearest neighbors determined by Delaunay triangulation. Here we formulate an isentropic, i.e., two-dimensional version of the model and verify the mixing algorithm using tracer distributions measured during the space shuttle CRISTA-1 experiment where highly resolved stratospheric structures were observed in early November 1994. A comparison of the measured Southern Hemispheric N 2 O distribution with CLaMS results allows the intensity of simulated mixing to be optimized. The long-term robustness of the transport scheme is investigated in a case study of the 1996-1997 Northern Hemisphere polar vortex. This study further provides a dynamical framework for investigations of chemical arctic ozone destruction discussed in part 2.

[1] The three-dimensional (3-D) formulation of the Chemical Lagrangian Model of the Stratosphere (CLaMS-3d) is presented that extends the isentropic version of CLaMS to cross-isentropic transport. The cross-isentropic velocities of the Lagrangian air parcels are calculated with a radiation module and by taking into account profiles of ozone and water vapor derived from a HALOE climatology. The 3-D extension of mixing maintains the most important feature of the 2-D version as mixing is mainly controlled by the horizontal deformations of the wind fields. In the 3-D version, mixing is additionally driven by the vertical shear in the flow. The impact of the intensity of mixing in the 3-D model formulation on simulated tracer distributions is elucidated by comparing observations of CH 4 , Halon-1211, and ozone from satellite, balloon, and ER-2 aircraft during the SOLVE/ THESEO-2000 campaign. CLaMS-3d simulations span the time period from early December 1999 to the middle of March 2000, with air parcels extending over the Northern Hemisphere in the vertical range between 350 and 1400 K. The adjustment of the CLaMS3d mixing parameters to optimize agreement with observations was obtained for strongly inhomogeneous, deformation-induced mixing that affects only about 10% of the air parcels per day. The optimal choice of the aspect ratio a defining the ratio of the mean horizontal and vertical separation between the air parcels was determined to be 250 for model configuration with a horizontal resolution r 0 = 100 km. By transporting ozone in CLaMS-3d as a passive tracer, the chemical ozone loss was inferred as the difference between the observed and simulated ozone profiles. The results show, in agreement with previous studies, a substantial ozone loss between 380 and 520 K with a maximum loss at 460 K of about 1.9 ppmv, i.e., of over 60% locally, from December to the middle of March 2000. During this period, the impact of isentropic mixing across the vortex edge outweighs the effect of the spatially inhomogeneous (differential) descent on the tracer/ ozone correlations in the vortex. Mixing into the vortex shifts the early winter reference tracer/ozone correlation to higher values, which may lead to an underestimate of chemical ozone loss, on average by 0.4 and 0.1 ppmv in the entire vortex and the vortex core, respectively.

We compare global water vapor observations from Microwave Limb Sounder (MLS) and simulations with the Lagrangian chemical transport model CLaMS (Chemical Lagrangian Model of the Stratosphere) to investigate the pathways of water vapor into the lower stratosphere during Northern Hemisphere (NH) summer. We find good agreement between the simulation and observations, with an effect of the satellite averaging kernel especially at high latitudes. The Asian and American monsoons emerge as regions of particularly high water vapor mixing ratios in the lower stratosphere during boreal summer. In NH midlatitudes and high latitudes, a clear anticorrelation between water vapor and ozone daily tendencies reveals a large region influenced by frequent horizontal transport from low latitudes, extending up to about 450K during summer and fall. Analysis of the zonal mean tracer continuity equation shows that close to the subtropics, this horizontal transport is mainly caused by the residual circulation. In contrast, at higher latitudes, poleward of about 50°N, eddy mixing dominates the horizontal water vapor transport. Model simulations with transport barriers confirm that almost the entire annual cycle of water vapor in NH midlatitudes above about 360K, with maximum mixing ratios during summer and fall, is caused by horizontal transport from low latitudes. In the model, highest water vapor mixing ratios in this region are clearly linked to horizontal transport from the subtropics.

Abstract. The European Centre for Medium-Range Weather Forecasts' (ECMWF's) next-generation reanalysis ERA5 provides many improvements, but it also confronts the community with a “big data” challenge. Data storage requirements for ERA5 increase by a factor of ∼80 compared with the ERA-Interim reanalysis, introduced a decade ago. Considering the significant increase in resources required for working with the new ERA5 data set, it is important to assess its impact on Lagrangian transport simulations. To quantify the differences between transport simulations using ERA5 and ERA-Interim data, we analyzed comprehensive global sets of 10-day forward trajectories for the free troposphere and the stratosphere for the year 2017. The new ERA5 data have a considerable impact on the simulations. Spatial transport deviations between ERA5 and ERA-Interim trajectories are up to an order of magnitude larger than those caused by parameterized diffusion and subgrid-scale wind fluctuations after 1 day and still up to a factor of 2–3 larger after 10 days. Depending on the height range, the spatial differences between the trajectories map into deviations as large as 3 K in temperature, 30 % in specific humidity, 1.8 % in potential temperature, and 50 % in potential vorticity after 1 day. Part of the differences between ERA5 and ERA-Interim is attributed to the better spatial and temporal resolution of the ERA5 reanalysis, which allows for a better representation of convective updrafts, gravity waves, tropical cyclones, and other meso- to synoptic-scale features of the atmosphere. Another important finding is that ERA5 trajectories exhibit significantly improved conservation of potential temperature in the stratosphere, pointing to an improved consistency of ECMWF's forecast model and observations that leads to smaller data assimilation increments. We conducted a number of downsampling experiments with the ERA5 data, in which we reduced the numbers of meteorological time steps, vertical levels, and horizontal grid points. Significant differences remain present in the transport simulations, if we downsample the ERA5 data to a resolution similar to ERA-Interim. This points to substantial changes of the forecast model, observations, and assimilation system of ERA5 in addition to improved resolution. A comparison of two Lagrangian trajectory models allowed us to assess the readiness of the codes and workflows to handle the comprehensive ERA5 data and to demonstrate the consistency of the simulation results. Our results will help to guide future Lagrangian transport studies attempting to navigate the increased computational complexity and leverage the considerable benefits and improvements of ECMWF's new ERA5 data set.