Tropical deep convection and its impact on composition in global and mesoscale models – Part 2: Tracer transport

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), chemistry transport, and climate chemistry 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 hours) within the tropical troposphere. Peak convective outflow altitudes range 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 differ 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 the modelling of the halogen burden of the lowermost stratosphere through species such as bromoform, or for the transport of short lived hydrocarbons into the lowermost stratosphere. The modelled tracer profiles are found to be strongly influenced by the convective transport parameterisations, and boundary layer mixing parameterisations of the models. The location of rapid transport into the upper troposphere is similar among the models, and is mostly concentrated over the western Pacific, the Maritime Continent and the Indian Ocean. In contrast, none of the models indicates significant enhancement in upward transport over western Africa. The mean mixing ratios of an idealised CO like tracer in the upper tropical troposphere are found to be sensitive to the surface CO mixing ratios in the regions with the most active convection, revealing the importance of correctly modelling both the location of convective transport and the geographical pollutant emission patterns

Representation of tropical deep convection in atmospheric models – Part 1: Meteorology and comparison with satellite observations

“…The widespread positive biases for the WRF model can be further explained by the underestimation of shallow convection in this region, which therefore pushes mean cloud top height values upwards. This is further supported by the short-lived (lifetime ∼6 h) tracer profiles averaged over the Maritime Continent region (Hoyle et al, 2010) which show that all other models have secondary peaks around 600-700 hPa associated with transport by shallow convection, while there is no such peak for the WRF model. In summary, for the models under investigation, the maxima in precipitation and cloud top height for the Maritime Continent region are not always co-located: coarse resolution models succeed in reproducing the maxima in cloud top height over the Malaysian peninsula, Sumatra and Borneo region but fail to reproduce the maxima in precipitation over the same region; over New Guinea, coarse resolution models fail to reproduce both maxima.…”

“…Therefore the vertical extent of convective transport is not always directly related to the vertical distribution of clouds. Nevertheless, the analysis presented in this section provides a first-order comparison with observations and can additionally be used to interpret the differences in modelled convective transport (Hoyle et al, 2010). The vertical distribution of mid-and high-level clouds in this set of models shows a wide range of values: TOMCAT/pTOMCAT underestimate the percentage of gridboxes with clouds tops above 12 km (or 13 km for West Africa), OSLOCTM2/FRSGCUCI, UMUKCA UCAM nud and, to a smaller extent, WRF tend to overestimate the percentage of gridboxes having clouds with tops above 13-14 km, while pTOMCAT tropical, UM UCAM highres and CATT-BRAMS show cloud heights which are either slightly lower, or within the observed range, depending on the region.…”

“…Additionally, there was no strong ENSO signal for 2005. The comparison of observed convective properties with model results provides a useful benchmark to test model performances; this information can in turn help to interpret the results in Hoyle et al (2010).…”

“…In this paper we analyse results from the latter, and we restrict our analysis to the meteorological parameters associated to convection with the aim of assessing the models' ability to simulate the seasonal cycle, preferential locations, vertical extent and frequency of deep tropical convection. The analysis of tracer distributions in the Tropics (for both short-and long-lived tracers) can be found in the second paper of this series (Hoyle et al, 2010), which focuses on the relative role of convection in the vertical transport of tracers. The section on convective transport of very short-lived tracers in Hoyle et al (2010) is therefore consistent with this paper, although additional model data is used for the analysis of long-lived tracers.…”

“…The analysis of tracer distributions in the Tropics (for both short-and long-lived tracers) can be found in the second paper of this series (Hoyle et al, 2010), which focuses on the relative role of convection in the vertical transport of tracers. The section on convective transport of very short-lived tracers in Hoyle et al (2010) is therefore consistent with this paper, although additional model data is used for the analysis of long-lived tracers.…”