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VOCA Modeling Experiment


Matt Wyant, Rob Wood, Chris Bretherton

Department of Atmospheric Sciences, University of Washington

with help from Scott Spak, Jerome Fast, Louisa Emmons & Bill Gustafson

email: mwyant at

Last Updated 7/07/2010

Results due 9/30/2010

PDF file of this specification


I.                    Introduction

II.                 Experiment Design

III.               Geoengineering Experiment

IV.              Submission of Results

V.                 Aerosol/Chemistry Specification

VI.              Required Outputs

VII.            References

VIII.         Useful Links (with URLs)

IX.              List of Changes to this Document



I. Introduction

The VOCALS modeling experiment aims to compare and evaluate model simulations of the South East Pacific, using the extensive set of measurements made during the REx field campaign of Oct-Nov 2008.

The PreVOCA experiment highlighted significant differences in model simulations of   October of 2006, particularly in cloud fraction, liquid water path and boundary layer depth (A paper describing the results of PreVOCA is published in ACP).  Sixteen modeling groups participated including operational forecast models, regional models, and climate models run in forecast mode. However, very few models had interactive treatment of aerosols or predicted cloud-droplet effective radius.   

This experiment will cover the entire REx observation period Oct 15 – Nov 15 2008. We will focus more on aerosol cloud interactions and chemical/aerosol transport than PreVOCA did. Our goal is to address the following questions:

·        Do models simulate the variation of droplet concentration Nd along 20S?

·        Is anthropogenic sulfate the main contributor to Nd variation?

·        What controls Nd in remote ocean regions?

·        What is the simulated indirect effect due to anthropogenic aerosol perturbation of clouds and radiative fluxes in the VOCALS region?

II. Experiment Design

To enable convenient comparison of models and observations, all models are requested to interpolate outputs to the same horizontal grid and resolution. We will have two main study regions for this experiment. The coarser outer region is designed to compare meteorology for a large number of models, as was done in PreVOCA. The finer inner region is meant for higher resolution models and/or models with large numbers of chemical species where simulating the entire outer domain is impractical. Modelers are encouraged to submit results in both domains if applicable (in separate files labeled ‘outer’ and ‘inner’). Modelers are free to extend their model domains outside of the study regions to whatever dimensions they deem appropriate so long as output is provided within the study regions on the designated grid.  The study region grids are specified in the following table:

Study Region

Latitude Range

Longitude Range

Averaging Resolution


40S – 0S

65W – 110W

1x1 degree


35S – 12S

68.5W – 88W

0.25 x 0.25  degrees


For example, outer-region horizontally-averaged output should be provided at latitudes 40S, 39S, 38S etc. and at longitudes 65W, 66W, 67W etc.  

All output should be at 3 hour time resolution, either averaged over 00-03 UTC, 03-06 UTC, etc., or instantaneous at 0130, 0430, etc. This allows good comparison with GEOS and MODIS/A-Train data. Please provide output covering the period from 0Z 15 Oct 2008 through 0Z 16 November 2008. Please start your runs before15 Oct 2008 to allow for your model dynamics and tracer fields to spin-up before the study period. Since models with chemical transport may require long period for spin-up, we have provided MOZART files starting at 0Z GMT 5 October so that a 10-day spin-up is possible.  

While atmosphere-ocean coupled models are welcome to participate, we anticipate that most models will use their own time-varying specified SST based on observations. Depending on the number and type of regional models that plan on participating, we may try to coordinate and create a standardized SST field for regional models.

Use your native model vertical grid.

While we expect participating regional models to be run continuously, global models can be run in forecast mode if desired. Forecast outputs should be concatenated into a single continuous time series.

There are two primary modes of runs we are requesting for this experiment: One is basically meteorology-only (MET), where aerosol interactions are nonexistent or minor (e.g. specified aerosol concentration that only affects radiation). Chemical/aerosol transport may optionally be included but the advected species affect the simulations minimally.  The other (AERO) has interactive aerosols that can affect cloud droplet concentration and may be consumed by precipitation processes. These runs may also include primary production of aerosols. While many models may only be capable of MET runs, we encourage models capable of running both types of runs to do so. If you are performing both types of experiments, append the appropriate output files with ‘AERO’ or ‘MET’.

You are welcome to submit the results of additional sensitivity tests that you choose to perform and we would be happy to help analyze them.


III. Geoengineering Experiment

We also want to simulate a uniformly polluted case to see sensitivity of the physical system to drastically changed droplet concentration. For this experiment, run for the entire study period with a cloud droplet concentration of Nd = 375 cm-3 over the entire study region (below 600hPa). We are interested in the radiative and microphysical impact of this forcing on the atmosphere for various models. No chemistry is needed, and explicit aerosol effects should be limited to their direct radiative effect.  Please append ‘GEO’ to output files for this experiment.


IV. Submission of Results

If you are interested in participating please email Matt as soon as possible. Let us know which runs you are likely to submit, so we can gauge the level of interest in the experiment and notify you of any specification updates, corrections, or emissions changes.   

Results and model documentation for VOCALS should be submitted no later than September 30, 2010. Submitting short and/or preliminary results earlier than that date is encouraged so that potential problems can be discovered in output formatting or run setup. Please send model documentation and instructions for how we can download your run output files (e.g. via ftp) to mwyant at  Preliminary analysis of the runs will be available by November 30, 2010 at this website. Runs will be archived and a password-protected collection of diagnostic plots will be available for participants to view. 

V. Aerosol/Chemistry Specification

The following information is applicable to all models with chemical and aerosol transport. If you are planning on submitting runs with chemical transport, tools are available for regridding of data and setup of runs from Scott Spak or Jerome Fast.  We are most interested in analyzing SO4, sea salt, black carbon, organic carbon, DMS, SO2, CO, and O3 but you are welcome to include additional aerosol/chemical species. 

A. Boundary Conditions

For initial and lateral boundary conditions of aerosol and gaseous species, Louisa Emmons has prepared a run of MOZART-4 over the study period. This 6-hourly data is available for download here. Each file represents one day starting at 0Z Oct 5. Further description of these runs, including the definitions of the aerosol/chemical species can be found in Emmons et al. (2009). Both Jerome Fast and Scott Spak have tools for applying these files to WRF-Chem and other models so please contact us if you are interested in using these tools.

Please use emissions data from natural and anthropogenic sources compiled by Scott Spak and available at These are available either as comma separated values (gridded except for the point sources) as gridded netcdf files. For all gridded data a 12 km x 12 km grid is used, and there is a link for tools to regrid the data. For either the comma-separated-value or netcdf files, combine the sources in all the files provided. Assume these emissions are constant in time. Suggested proportions for vertical distribution of emissions are provided at the website. Please use these if possible.

Scott Spak has been experimenting with daily estimates of biomass burning from satellite. These are not required for VOCA, but if you are interested in using them please let us know.   

B. Sea-Salt Fluxes

If possible, for fluxes of sea-salt aerosol from the sea-surface, please use the following parameterization from Gong et al. (1997) and Monahan et al. (1986) neglecting spume formation:





where r is the equilibrium radius at 80% relative humidity in microns, and dF/dr is the flux in kg m-2 s-1 μm-1, and U10N  is the stability-corrected 10-meter wind speed in m s-1. If needed assume that r is equal to the dry diameter. If you are including sea-salt aerosol flux below 0.1 μm in your runs, use the following simple specification designed to crudely match the polynomial fit of Clarke et al. (2006) for ultrafine aerosols:




Note that there is a discontinuity at r = 0.1 μm (which looks fairly small in logarithmic space!).

C. DMS Flux

If you are including fluxes of DMS from the ocean surface to the atmosphere, please use a geographically uniform ocean surface DMS concentration (C) of 2.8nM L-1 or 2.8x10-6 M m-3. For the flux of DMS please use the following parameterization based on Nightingale et al.(2000) (Note: A fixed DMS Schmidt number of 920 at 20 degrees Celsius (Saltzman et al. 1993) is incorporated into our specified formula): Flux  = k * C  where k has units of m s-1 and is given by k = 10-7 x [5.0 U10N2 + 7.5 U10N] where U10N  is the stability corrected 10-m wind speed in m s-1.

D. Dust

For simplicity we do not plan to perform much analysis of dust in the simulations. You are welcome to include dust in your simulations if you choose, and if you do please include it in your model output (emissions rates and mass concentrations) along with other aerosol species and document how your dust emissions are parameterized.

VI. Required outputs

Each model should submit a brief MS Word, pdf, or plain text documentation file and netcdf output file(s). Items specified in bold have been added or revised from the Pre-VOCA model output specification.

A. Documentation File

The documentation file should include:
D1) the name and email address of the contributor
D2) the official version number and model name
D3) literature reference for your model (if available) plus any major changes made since that reference.
D4) Forecast/analysis approach used to make model output fields
D5) Horizontal model resolution, model domain description, grid projection, details of grid spacing
D6) Describe the vertical coordinate system in your model. Give the number of model vertical levels, corresponding values of sigma, how your sigma is defined, and the model top pressure. Provide enough information so that the full time-dependent model pressure and geopotential height fields can be computed based on your output fields (or alternately provide the full pressure and geopotential height fields).
D7) Timestep
D8) PBL scheme type (e.g. nonlocal surface-forced K-profile, moist TKE, etc.) as you would summarize it in the table of a paper.
D9) Microphysics scheme type
D10) Cloud fraction scheme type

D11) Radiative scheme and cloud-overlap schemes used in radiation calculations. Provide model-top pressure level (for use in interpreting TOA fluxes provided below).
D12) Aerosol scheme description
D13) What form, if any, of aerosol-cloud interaction is included

D14) Model Chemistry scheme/transport scheme

B. Model Output File

The output file, in netcdf format, should include the following field listed below(ignore chemical/aerosol species not present in your model). Please include enough information so that the pressure levels and geopotential height in your model can be reconstructed for all times and all columns.

1) 1D fields:

time [days] since 00Z 15 Oct. 2008
lat [deg N]
lon [deg E]
sigma [0-1] of each model vertical level, averaged over all horizontal gridpoints and times given in the output file (we assume sigma = p/ps. If you use a different system for sigma please specify what it is).
emission_profile [0-1] for non sea-salt species, the fraction of total surface emissions released at each vertical model level

2) 2D fields (time x lat x lon):

TS [K] surface radiative skin temperature (= SST over ocean for most models)
ZS [m] surface height (may be less than zero over ocean due to spectral ringing)
land fraction [0-1]

PS[Pa]  - Model surface pressure (allows computation of pressure levels over land)
SLP [Pa] - validate against Quikscat/reanalyses
T (2 m) [K] – compare vs. stratus buoy at 20S 85W
q (2 m) [kg/kg] specific humidity – compare vs. stratus buoy at 20S 85W
u (10 m) [m/s] – compare vs. Quikscat and analyses (u and v components should be    rotated to the same lat-lon grid used for averaging)
v (10 m )
u (850 hPa) - compare vs. analyses
v (850 hPa)
w (850 hPa) – or omega [Pa/s]
SHF [W/m2]
LHF [W/m2]
Surface momentum fluxes in x and y directions [Pa]

DMSFlux [M m-2 s-1]

SSFlux [kg m-2 s-1] Sea-salt flux

BCEmiss [kg m-2 s-1] Black Carbon emission rate

OCEmiss [kg m-2 s-1] Organic Carbon emission rate

SO2Emiss [kg m-2 s-1] SO2 emission rate

COEmiss [kg m-2 s-1] CO emission rate
LWUPTOA, SWUPTOA [W/m2] TOA upwards radiative fluxes (not net fluxes) (compare with ISCCP)
LWDNSFC, SWDNSFC – downwards fluxes at surface (not net fluxes) compare vs. ISCCP and at buoy

LWCF, SWCF [W/m2] – cloud forcing, if available
WVP[kg/m2] - compare vs. SSMI/TMI
LWP[kg/m2] - compare vs. TMI/AMSR
CLDLOW - low cloud fraction lcf below 700 hPa
PRECT [kg H­20/(m2 s)] surface precipitation.

PFLUX [kg H20/(m2 s)] - precipitation flux at 500m for comparison with radar
AOD [0-1] Aerosol optical depth (only include columns where the 3-hr mean cloud fraction is less than 0.6 at all vertical levels, otherwise set to missing value)


For the following 2D fields, we are attempting to retrieve stratocumulus-top properties for comparison to satellite measurements. At each timestep for each model column, choose the cloud-top level to report from as the highest grid-point below 700hPa where the cloud water exceeds 0.1 g/kg. (if the cloud liquid water content is less than 0.1 g/kg in the entire column, report a missing value) When the 3-hour average is computed, if the low-cloud fraction over the same 3-hour period is less than 0.8, report a missing value.

TCLDTOP [K] low cloud IR brightness temperature proxy
PCLDTOP [Pa] low cloud height proxy
REFFTOP [μm] effective radius proxy
NDTOP [cm-3] cloud droplet concentration

3) 3D Fields (time x height x lat x lon):

T [K]

qv [kg/kg]   specific humidity

ql [kg/kg]    cloud liquid water content

qi [kg/kg]     cloud ice water content
cf (cloud fraction in each grid cell)
reff [μm](only report in grid cells where 3-hr time mean cloud water > 0.1 g/kg, otherwise use missing value)

N [cm-3] cloud droplet number concentration (same cloud water restriction as reff)

CCN  [cm-3] number concentration, 0.1% supersaturation.

u [m/s]

v [m/s]

w [m/s] (or omega[Pa/s])

Z[m]* – geopotential height – if not able to be computed using other provided data

P [Pa]* - pressure


* These two output fields only need to be provided if they can’t be computed from other fields/information provided.


For the following aerosol species provide total mass-mixing ratio [kg kg-1], total mass-mixing ratio less than 1 micron dry diameter (PM1) if they are included in your simulations.

  • SO4  (H2SO4/SO4)
  • SS    (Sea salt)
  • BC   (Black Carbon)
  • OC   (Organic Carbon)


e.g. SS, SS_PM1, SO4, SO4_PM1, etc.


Also provide the following data for the sum of all aerosols (include all species of aerosols in your simulation, e.g. dust, not just the ones listed above):


SUMAERO  [kg kg-1]  total mass

SUMAERO_PM1 [kg kg-1]

SUMAERO_PM25  [kg kg-1]  PM2.5

SUMAERO_N   Number concentration [cm-3]


For the following gas species provide mass mixing ratio [kg/kg] if they are included in your simulations.


  • DMS
  • SO2
  • CO
  • O3


If you provide atmospheric DMS concentration, but are using your own DMS ocean concentration, please specify what you are using or include the concentration in the output file.


Feel free to provide additional aerosol/chemical species output.

VII. References

Clarke, A. D., S. R. Owens, and J. Zhou, 2006: An ultrafine sea-salt flux from breaking waves: Implications for cloud condensation nuclei in the remote marine atmosphere. J. Geophys. Res., 111, D06202, doi:10.1029/2005JD006565.

Emmons, L. K., and co-authors, 2009: Description and evaluation of the Model for Ozone and Related chemical Tracers, version 4 (MOZART-4). Geosci. Model Dev. Discuss., 2, 1157-1213.

Gong, S. L., L. A. Barrie, and J.-P. Blanchet, 1997: Modeling sea-salt aerosols in the atmosphere. 1. Model development. J. Geophys. Res., 102, D3, 3805-3818.

Monahan, E. C., D. E. Spiel, and K. L. Davidson, 1986: A model of marine aerosol generation via whitecaps and wave disruption, in Oceanic Whitecaps and Their Role in Air-Sea Exchange Processes, edited by E. C. Monahan and G. Mac Niocaill, pp. 167-193, Springer, New York.

Nightingale, P. D., G. Malin, C. S. Law, A. J. Watson, P. S. Liss, M. I. Liddicoat, J. Boutin, and R. C. Upstill-Goddard, 2000: In situ evaluation of air-sea gas exchange parameterizations using novel conservative and volatile tracers. Glob. Biogeochem. Cyc., 14, 373-387.

Saltzman, E. S., D. B. King, K. Holmen, and C. Leck, 1993. Experimental determination of the diffusion coefficient of dimethylsulfide in water. JGR, 98 (C9), 16841-16846.

Wyant, M. C., R. Wood, C. S. Bretherton, C. R. Mechoso, J. Bacmeister, M. A. Balmaseda, B. Barrett, F. Codron, P. Earnshaw, J. Fast, C. Hannay, J. W. Kaiser, H. Kitagawa, S. A. Klein, M. Köhler, J. Manganello, H.-L. Pan, F. Sun, S. Wang, and Y. Wang, 2010: The PreVOCA Experiment: Modeling the lower troposphere in the Southeast Pacific. Atmos. Chem. Phys., 10, 4757-4774.

VIII. Useful Links

IX. List of Changes to this Document

7/07/2010 Section V:  Biomass burning emissions data will not be specified for VOCA but may be available on request.

7/07/2010 Sections I and VII. The reference to the now-published PreVOCA ACP paper has been updated.


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