Key aspects of stratospheric tracer modeling using assimilated winds

Abstract

Abstract. This study describes key aspects of global chemistry-transport models and their impact on stratospheric tracer transport. We concentrate on global models that use assimilated winds from numerical weather predictions, but the results also apply to tracer transport in general circulation models. We examined grid resolution, numerical diffusion, air parcel dispersion, the wind or mass flux update frequency, and time interpolation. The evaluation is performed with assimilated meteorology from the "operational analyses or operational data" (OD) from the European Centre for MediumRange Weather Forecasts (ECMWF). We also show the effect of the mass flux update frequency using the ECMWF 40-year re-analyses (ERA40).We applied the three-dimensional chemistry-transport Tracer Model version 5 (TM5) and a trajectory model and performed several diagnoses focusing on different transport regimes. Covering different time and spatial scales, we examined (1) polar vortex dynamics during the Arctic winter, (2) the large-scale stratospheric meridional circulation, and (3) air parcel dispersion in the tropical lower stratosphere.Tracer distributions inside the Arctic polar vortex show considerably worse agreement with observations when the model grid resolution in the polar region is reduced to avoid numerical instability. The results are sensitive to the diffusivity of the advection. Nevertheless, the use of a computational cheaper but diffusive advection scheme is feasible for tracer transport when the horizontal grid resolution is equal or smaller than 1 degree. The use of time interpolated winds improves the tracer distributions, particularly in the middle and upper stratosphere. Considerable improvement is found both in the large-scale tracer distribution and in the polar regions when the update frequency of the assimilated winds is increased from 6 to 3 h. It considerably reduces the vertical dispersion of air parcels in the tropical lower stratosphere.Correspondence to: B. Bregman (bregman@knmi.nl) Strong horizontal dispersion is not necessarily an indication of poor wind quality, as observations indicate. Moreover, the generally applied air parcel dispersion calculations should be interpreted with care, given the strong sensitivity of dispersion with altitude.The results in this study provide a guideline for stratospheric tracer modeling using assimilated winds. They further demonstrate significant progress in the use of assimilated meteorology in chemistry-transport models, relevant for both short-and long-term integrations.

A revised linear ozone photochemistry parameterization for use in transport and general circulation models: multi-annual simulations

“…The tendency for excessive equator to pole circulation intensity by the ECMWF analyses has been already diagnosed by analysis of satellite and soundings ozone data (de Laat et al, 2007), and with the calculation of stratospheric "age of air" (Bregman et al, 2006). The age of air is defined as the time that a stratospheric air mass has been out of contact with the troposphere.…”

Model evaluation of CO2 and SF6 in the extratropical UT/LS region

“…[74] In previous studies it was shown that increasing the update frequency of the applied meteorology fields from 6 h to 3 h significantly improved the modeled stratospheric tracer fields [Legras et al, 2005;Berthet et al, 2006;Bregman et al, 2006]. Here we investigate the impact for the UT/LS for the integration period considered.…”

“…The stratospheric tracer distributions were improved significantly by including iterative advection [Bregman et al, 2006]. For a detailed description of the new model version TM5 the reader is referred to Krol et al [2005].…”

Mass-conserving tracer transport modelling on a reduced latitude-longitude grid with NIES-TM

“…Consequently, it is possible to run simulations without reducing the time step. Although Bregman et al (2006) described the pitfalls of polar model grid averaging in global model studies of polar tracer transport in the stratosphere, numerical advection schemes that use a reduced grid generally perform well in model performance tests and are efficient solvers in various applications (Peterson et al, 1998). …”

“…The influence of grid resolution on atmospheric composition simulation is a topic of continuous interest. Marchand et al (2003) and Bregman et al (2006) reported that grid resolution has a strong influence on the distribution of the tracer mixing ratio in the Polar Regions. In contrast, van den Broek et al (2003) reported a negligible improvement in methane transport model results with increasing horizontal grid resolution to 1 • × 1 • in a study on the effect of spatial resolution in the Polar Regions.…”