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For more information about this poster please contact Gerard Devine School of Earth and Environment Environment University of Leeds Leeds LS2 9JT Email gerardenvleedsacuk Tel(044) (0113) 3431598
The influence of subgrid variability on chemical transport in a deep convective environment
Gerard Devine1 Doug Parker1 Ken Carslaw1 Jon Petch21School of Earth and Environment University of Leeds
2UK 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
EXPERIMENT WIND FLUX DMS
CRMRES Resolved Resolved Resolved
AVWIND AveragedResolved from
averaged windsResolved
AVFLUX Resolved Averaged Resolved
AVDMS Resolved Resolved Averaged
Convective Simulation Model forced using observational data from TOGA-COARE
Chemistry Setup
Smooth
surface
Rough surfac
e
Breaking Waves
We simulate dimethyl sulphide (DMS) an important pre-cursor gas to sulphur dioxide (SO2) and sulphate aerosol
CRMRES ndash 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 mmhr and (b) 10 m horizontal wind speed (ms)
Tropical OceanGlobal Atmosphere ndash Coupled Ocean Atmosphere Response Experiment
Sea-air flux of DMS based on parametrization by Liss and Merlivat [1986]
Parametrization consists of three wind lsquoregimesrsquo resulting in a piecewise linear relationship (see figure opposite)
Sink term representing oxidation by the hydroxyl radical is also included
AVWIND ndash 10 m wind vector averaged across the domain at each timestep DMS flux calculated from resultant average wind
AVFLUX ndash 10 m wind resolved but an averaged flux field applied across the domain at each timestep
AVDMS ndash 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 troposphe
re
Time evolution of average DMS (ppt) in each of three atmospheric regions
CRMRESAVWINDAVFLUXAVDMS
Mid and upper troposphere ndash
concentration significantly underpredicted in a model
with averaged wind and averaged DMS fields
Mid and upper troposphere ndash
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 lsquoregimersquo than the resolved winds associated with
convective activity
Upper troposphe
re
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
lsquogustyrsquo 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