NCAR/ACD FLEXPART backtrajectories for SAS NOMADSS C130 flights 2013-11-07 Christoph Knote, ACD, NCAR 3450 Mitchell Lane Boulder, CO, 80301 USA +1 303 497 1874 knote@ucar.edu 1.0) Overview This dataset contains output of FLEXPART, a Lagrangian Particle Dispersion Model (LPDM). It provides information on the history of air sampled along the C130 flight track by releasing "particles" (infinitesimally small parcels of air) from the plane location and following its path backwards in time. Advection, turbulence and convection are the processes considered. A particle itself is "inert", processes like deposition, emission or chemical conversion are not considered. Thereby the result could be called "airmass history", as the particles behave like air. Its location back in time and space since released is called "backtrajectory". The chaotic nature of the atmosphere and uncertainties in model estimates of wind vector, turbulence and convection requires a statistical approach to estimate all probable backtrajectories. Hence not only one, but a large number of particles (1e5 - 1e6) are released over a short time period (1 hr) and followed back in time. While the exact trajectory for each particle is used internally, the main model result is the number of particles and the time they spent within a 3D lat/lon/altitude grid, called a "sensitivity" or "residence time" field. This is calculated at discrete intervals (every hour) in time since release. These fields are the raw output of the model calculations. For ease of use, deterministic backtrajectories as mass weighted mean of the plume at each hour, and 5 plume cluster centroids (Stohl et al., Atm. Env., 2002) are calculated as well. 2.0) Model description The FLEXPART model (http://flexpart.eu) in version 9.02 has been used. FLEXPART has been originally developed by A. Stohl (NILU, Norway) and is continually improved by an international team of developers. The model has been modified to allow the output of NetCDF data instead of the native binary format. This code has been kindly provided by Stephan Henne / Dominik Brunner of Empa, Duebendorf, Switzerland. 3.0) Model setup The main FLEXPART configuration file can be found in Appendix A.1. Meteorological forcing data from analyses every 6 hours (interlaced with the 3 hour forecast fields) of the Global Forecasting System (GFS) of the National Center for Environmental Prediction (NCEP) (http://www.nco.ncep.noaa.gov/pmb/products/gfs/) have been used. These are provided on a 0.5 x 0.5 degrees regular latitude/longitude grid. Release points have been defined every five minutes along the GPS flight track of the C130 for each research flight. Between 15.000 and 30.000 particles per point are released instantaneously and followed back in time for approximately 72 hours. The resulting footprint is hence representative for air sampled at the start date (given as filename and in header information / plots). The sampling grid used in model output is at 0.2 x 0.2 degrees resolution with 18 vertical levels (see A.2), roughly covering the Northern Hemisphere from the date line to Greenwich. 4.0) Data format File name conventions: bt--.(png|ict|nc) with YYYYmmdd_HHMM the start time of the particles. bt-RF16-20130708_1503.png is hence the plot output of particles released during research flight 16 at 15:03 UTC on July 8th 2013. Three distinct outputs are generated: * NetCDF output: CF-compliant NetCDF version 4 (using standard HDF compression) files that contain a 2D (lat, lon) field for model orography ("oro"), and a 4D (time, level, lat, lon) field of the "residence time" (or sensitivity) at each point in time in space ("conc", in units "s m3 kg-1"). Stohl et al., ACP, 2005 discuss this fields in detail. Variables: time - seconds since start time of simulation (coincides with end of particle release interval) lon/lat - longitude / latitude of output grid cell centers level - height above ground of output grid levels (level top) oro - surface altitude in meters conc - residence time / sensitivity field for particles released * ICARTT output: a text file containing backtrajectories in ICARTT/NASA AMES format. Given are for each hour the mass-weighted center trajectory locations (lon, lat, altitude) and the centroids (lat, lon, altitude) for 5 clusters including uncertainty (root mean square) and fraction of total particle number pertaining to each cluster. The methods to derive these trajectories are described in Stohl et al., Atm. Env., 2002. * Plot output: an overview plot showing a horizontal integration (s m2 kg-1) and vertical average (s m3 kg-1) of the "conc" field in the NetCDF file, and additionally the trajectories (center / clusters) given in the ICARTT output. 5.0) Data remarks What to use when? The atmosphere is chaotic - a deterministic backtrajectory can be very misleading (see e.g. Stohl et al., Atm. Env., 2002) as it does not capture stochastic processes like turbulence or convection. For all quantitative assessments the user is strongly urged to use the residence time / sensitivity fields in the NetCDF output instead of the trajectories in the ICARTT file. Higher time/space resolution? More detailed calculations with higher spatial and temporal resolutions are possible by using output of a regional model (e.g. WRF). This can easily be done on a per request basis - just let me know. Why NetCDF v4? NetCDF version 4 is used as sparse matrix-type compression is available there. Backtrajectories are usually very spares (lots of "empty cells"), hence such a compression algorithm is very efficient here: a NetCDF v4 backtrajectory file uses ~25 MB, where as the same file as NetCDF v3 needs ~2.1 GB (sic). All standard tools (nco, ncl, cdo, IDL, R, MatLab, IGOR) can work with NetCDF v4, but some older installations might need to be re-installed / re-compiled. 6.0) References Stohl, A., Eckhardt, S., Forster, C., James, P., Spichtinger, N., & Seibert, P. (2002). A replacement for simple back trajectory calculations in the interpretation of atmospheric trace substance measurements. Atmospheric Environment, 36(29), 4635-4648. Stohl, A., Forster, C., Frank, A., Seibert, P., & Wotawa, G. (2005). Technical note: The Lagrangian particle dispersion model FLEXPART version 6.2. Atmospheric Chemistry and Physics, 5(9), 2461-2474. A.1) COMMAND file +++++++++++++ HEADER +++++++++++++++++ +++++++++++++ HEADER +++++++++++++++++ +++++++++++++ HEADER +++++++++++++++++ +++++++++++++ HEADER +++++++++++++++++ +++++++++++++ HEADER +++++++++++++++++ +++++++++++++ HEADER +++++++++++++++++ +++++++++++++ HEADER +++++++++++++++++ -1 3600 3600 300 9999999 300 SYNC 5.0 CTL 4 IFINE 13 IOUT 0 IPOUT 1 LSUBGRID 1 LCONVECTION 0 LAGESPECTRA 0 IPIN 1 IOFR 0 IFLUX 0 MDOMAINFILL 1 IND_SOURCE 2 IND_RECEPTOR 0 MQUASILAG 0 NESTED_OUTPUT 2 LINIT_COND INITIAL COND. FOR BW RUNS: 0=NO,1=MASS UNIT,2=MASS MIXING RATIO UNIT A.2) OUTGRID file ******************************************************************************** * * * Input file for the Lagrangian particle dispersion model FLEXPART * * Please specify your output grid * * * ******************************************************************************** 1. ------.---- 4X,F11.4 -178.1000 GEOGRAFICAL LONGITUDE OF LOWER LEFT CORNER OF OUTPUT GRID OUTLONLEFT (left boundary of the first grid cell - not its centre) 2. ------.---- 4X,F11.4 -20.1000 GEOGRAFICAL LATITUDE OF LOWER LEFT CORNER OF OUTPUT GRID OUTLATLOWER (lower boundary of the first grid cell - not its centre) 3. ----- 4X,I5 900 NUMBER OF GRID POINTS IN X DIRECTION (= No. of cells + 1) NUMXGRID 4. ----- 4X,I5 500 NUMBER OF GRID POINTS IN Y DIRECTION (= No. of cells + 1) NUMYGRID 5. ------.--- 4X,F10.3 0.200 GRID DISTANCE IN X DIRECTION DXOUTLON 6. ------.--- 4X,F10.3 0.200 GRID DISTANCE IN Y DIRECTION DYOUTLAT 10. -----.- 4X, F7.1 100.0 LEVEL 1 HEIGHT OF LEVEL (UPPER BOUNDARY) 10. -----.- 4X, F7.1 250.0 LEVEL 2 HEIGHT OF LEVEL (UPPER BOUNDARY) 10. -----.- 4X, F7.1 500.0 LEVEL 2 HEIGHT OF LEVEL (UPPER BOUNDARY) 10. -----.- 4X, F7.1 1000.0 LEVEL 2 HEIGHT OF LEVEL (UPPER BOUNDARY) 10. -----.- 4X, F7.1 1500.0 LEVEL 3 HEIGHT OF LEVEL (UPPER BOUNDARY) 10. -----.- 4X, F7.1 2000.0 LEVEL 3 HEIGHT OF LEVEL (UPPER BOUNDARY) 10. -----.- 4X, F7.1 3000.0 LEVEL 4 HEIGHT OF LEVEL (UPPER BOUNDARY) 10. -----.- 4X, F7.1 4000.0 LEVEL 5 HEIGHT OF LEVEL (UPPER BOUNDARY) 10. -----.- 4X, F7.1 5000.0 LEVEL 6 HEIGHT OF LEVEL (UPPER BOUNDARY) 10. -----.- 4X, F7.1 6000.0 LEVEL 7 HEIGHT OF LEVEL (UPPER BOUNDARY) 10. -----.- 4X, F7.1 7000.0 LEVEL 7 HEIGHT OF LEVEL (UPPER BOUNDARY) 10. -----.- 4X, F7.1 8000.0 LEVEL 7 HEIGHT OF LEVEL (UPPER BOUNDARY) 10. -----.- 4X, F7.1 9000.0 LEVEL 8 HEIGHT OF LEVEL (UPPER BOUNDARY) 10. -----.- 4X, F7.1 10000.0 LEVEL 8 HEIGHT OF LEVEL (UPPER BOUNDARY) 10. -----.- 4X, F7.1 12500.0 LEVEL 8 HEIGHT OF LEVEL (UPPER BOUNDARY) 10. -----.- 4X, F7.1 15000.0 LEVEL 8 HEIGHT OF LEVEL (UPPER BOUNDARY) 10. -----.- 4X, F7.1 20000.0 LEVEL 8 HEIGHT OF LEVEL (UPPER BOUNDARY) 10. -----.- 4X, F7.1 50000.0 LEVEL 9 HEIGHT OF LEVEL (UPPER BOUNDARY)