For more information about this poster please contact Gerard Devine, School of Earth and Environment, Environment, University of Leeds, Leeds, LS2 9JT Tel:(044) (0113) The influence of subgrid variability on chemical transport in a deep convective environment Gerard Devine 1, Doug Parker 1, Ken Carslaw 1, Jon Petch 2 1 School of Earth and Environment, University of Leeds 2 UK Meteorological Office Deep convective clouds play an important role in the transport of chemical species from the planetary boundary layer (PBL) to the upper troposphere and lower stratosphere. In modelling such transport, global chemical transport models resolve scales larger than that of a typical convective cloud event. However, the subgrid-scale dynamical features of convective cloud systems may be important in controlling the abundance of chemical species in the PBL and ultimately how much is transported vertically. In this study we use a 2-D cloud-resolving model (CRM) to examine the influence of such subgrid-scale features on the concentration and vertical transport of dimethyl sulphide (DMS) in a deep convective environment. Sensitivity Experiments: EXPERIMENTWINDFLUXDMS CRMRESResolved AVWINDAveraged Resolved from averaged winds Resolved AVFLUXResolvedAveragedResolved AVDMSResolved Averaged Convective Simulation: · Model forced using observational data from TOGA-COARE*. Chemistry Setup: Smooth surface Rough surface Breaking Waves · We simulate dimethyl sulphide (DMS), an important pre-cursor gas to sulphur dioxide (SO 2 ) and sulphate aerosol. CRMRES – 10 m wind, DMS flux, and DMS concentration all fully resolved Model Setup: · UK Met Office Large Eddy Model (LEM). · Domain width of 256 km and domain height of 20 km. · Horizontal resolution - 1 km. · Vertical resolution at 20 m near the surface rising to 500 m at domain top. (1) Deriving fluxes of DMS from the ocean surface using a spatially averaged surface wind representative of a global model reduces the domain-mean DMS concentration by approximately 50%. Emission of DMS from the sea surface is greater in the CRM because it resolves the localized high wind speeds embedded in the dynamical structures associated with the convective cloud systems. · 6-day simulation characterized by periods of active convection · The figure opposite shows x-t plots of (a) surface precipitation rate above 1 mm/hr, and (b) 10 m horizontal wind speed (m/s). * Tropical Ocean/Global Atmosphere – Coupled Ocean Atmosphere Response Experiment · Sea-air flux of DMS based on parametrization by Liss and Merlivat [1986]. · Parametrization consists of three wind ‘regimes’ resulting in a piecewise linear relationship (see figure opposite). · Sink term representing oxidation by the hydroxyl radical is also included. AVWIND – 10 m wind vector averaged across the domain at each timestep. DMS flux calculated from resultant average wind. AVFLUX – 10 m wind resolved but an averaged flux field applied across the domain at each timestep AVDMS – 10 m wind field and DMS flux resolved but the resultant DMS concentration in the boundary layer horizontally averaged at each timestep Results Summary: Boundary layer Mid troposphere · Time evolution of average DMS (ppt) in each of three atmospheric regions CRMRES AVWIND AVFLUX AVDMS · Mid and upper troposphere – concentration significantly underpredicted in a model with averaged wind and averaged DMS fields. We have highlighted two issues: (2) The spatial pattern of DMS concentration in the boundary layer is positively correlated with the pattern of convective updraughts. Using a mean DMS concentration field reduces vertical transport to the upper troposphere by more than 50%. The explanation is that secondary convection occurs preferentially on the edges of spreading cold pools, where DMS concentrations are higher than the domain mean. Explanation: On several occasions the mean wind is in a less efficient ‘regime’ than the resolved winds associated with convective activity Upper troposphere · AVWIND Mean Wind Maximum and minimum resolved winds Breaking wave regime Rough surface regime Smooth surface regime · AVDMS Dotted line represents total domain DMS surface flux for CRMRES Dashed line represents total domain DMS surface flux for AVWIND Comparing AVDMS and CRMRES shows strong updraughts are correlated with areas with higher than the horizontally averaged concentration of DMS. Discussion: Through the convective cold pool, convective clouds create an environment in which vertical transport of DMS is enhanced. Low DMS in cold pool Initiation of secondary convective cells at leading edge of spreading cold pool High DMS outside of cold pool Ascending air Timescale for recovery of DMS by enhanced fluxes within the ‘gusty’ conditions of the cold pool is longer than that for secondary initiation Underestimates of the transport due to neglecting these effects may have significant consequences for predictions of the chemical state of the upper troposphere and lower stratosphere. Theta line depicting convective cold pool