Representation of tropical deep convection in atmospheric models – Part 2: Tracer transport

Abstract

Abstract. The tropical transport processes of 14 different models or model versions were compared, within the framework of the SCOUT-O3 (Stratospheric-Climate Links with Emphasis on the Upper Troposphere and Lower Stratosphere) project. The tested models range from the regional to the global scale, and include numerical weather prediction (NWP), chemical transport, and chemistry-climate models. Idealised tracers were used in order to prevent the model's chemistry schemes from influencing the results substantially, so that the effects of modelled transport could be isolated. We find large differences in the vertical transport of very short-lived tracers (with a lifetime of 6 h) within the tropical troposphere. Peak convective outflow altitudes rangeCorrespondence to: C. R. Hoyle (christopher.hoyle@env.ethz.ch) from around 300 hPa to almost 100 hPa among the different models, and the upper tropospheric tracer mixing ratios differ by up to an order of magnitude. The timing of convective events is found to be different between the models, even among those which source their forcing data from the same NWP model (ECMWF). The differences are less pronounced for longer lived tracers, however they could have implications for modelling the halogen burden of the lowermost stratosphere through transport of species such as bromoform, or short-lived hydrocarbons into the lowermost stratosphere. The modelled tracer profiles are strongly influenced by the convective transport parameterisations, and different boundary layer mixing parameterisations also have a large impact on the modelled tracer profiles. Preferential locations for rapid transport from the surface into the upper troposphere are similar in all models, and are mostly concentrated over the western Pacific, the Maritime Continent and the Indian Published by Copernicus Publications on behalf of the European Geosciences Union. Ocean. In contrast, models do not indicate that upward transport is highest over western Africa.

OMI tropospheric NO<sub>2</sub> profiles from cloud slicing: constraints on surface emissions, convective transport and lightning NO<sub><i>x</i></sub>

“…These observations are in line with reports in Williams et al (2010) showing that the underestimation of upper tropospheric O 3 in TM4 relative to observations over Africa may be linked to a too weak convective uplift using the Tiedtke scheme. The studies of Tost et al (2007), Barret et al (2010) and Hoyle et al (2011) corroborate this finding, indicating that the vertical extent of tropical convection and associated transport of CO and O 3 in the middle and upper troposphere is underestimated in Tiedtke-based models. Accurately constraining the convective transport in CTMs should contribute to the determination of the vertical distribution of lightning NO x , since knowledge of the extent of mixing of air into the cloud as a function of altitude is required to separate the NO x produced by lightning from that produced by upward transport (Dickerson, 1984).…”

“…Most convective parameterizations are tested against temperature and humidity profiles from radiosondes (Folkins et al, 2006), but chemical tracers provide additional constraints. A number of studies have tried to quantify the effect of different convective schemes on tropospheric CO and O 3 profiles using satellite-based climatologies for comparison with model data (Mahowald et al, 1995;Barret et al, 2010;Hoyle et al, 2011), finding the largest discrepancies in the tropical middle and upper troposphere. Even though NO 2 may appear unsuitable as a tracer of air motion because of its high reactivity with other NO y members (such as N 2 O 5 , HNO 3 , PAN, NO − 3 and HNO 4 ) and the presence of time-varying sources (mainly surface emissions and lightning NO x , but also aircraft and stratospheric inflows), its short lifetime makes it attractive to study very fast transport mechanisms like convection.…”

The impact of overshooting deep convection on local transport and mixing in the tropical upper troposphere/lower stratosphere (UTLS)

“…Particularly, the strength of Hector and the maximum overshooting altitude were affected. The sensitivity to surface and boundary layer schemes was also shown by Hoyle et al (2011), who compared the tropical transport processes of 14 different (lower-resolution) models. In our case, the QNSE surface and boundary layer schemes gave the best results in reproducing Hectors' cloud top altitude, compared to the CPOL radar and aircraft measurements.…”

Transport of short-lived species into the Tropical Tropopause Layer

“…Tost et al (2010) have shown that different parameterisations of convection lead to substantial differences in simulated chemical profiles in an Eulerian model. In a comparison of vertical transport of idealised tracers in a range of Eulerian models, Hoyle et al (2011) reported a substantial impact on calculated tracer profiles when different convective parameterisations were employed (see also earlier work by Lawrence and Rasch, 2005). In a Lagrangian representation, Pisso et al (2010) added a parameterisation of deep convection to their model and found that the fraction of an idealised VSLS emitted at the surface, that then goes on to reach the upper TTL, increased by up to an order of magnitude.…”