ICARTT Analysis Products: Description of Products
Meteorological Fields Used
Two meteorological input data sets have been used here: 1x1 degree data on 26 pressure levels from the Global Forecast System (GFS) model of the National Center for Environmental Prediction (NCEP), and data from the ECMWF model. The ECMWF data has 60 model levels and was retrieved fully mass-consistently from the T511 spherical harmonics data at ECMWF. The gridded data has 1x1 degree resolution globally, but a 0.36x0.36 degree nest is used in the region 108W-18E and 18N-72N.
All calculations have been done with the particle dispersion model FLEXPART. For emission input, the EDGAR 1995 version 3.2 emission inventory on a 1 x 1 degree grid is used outside North America. Over most of North America, the inventory of Frost and McKeen (2004) is used. This inventory has a resolution of 4 km and also includes a list of point sources. Previous experience has shown that Asian emissions of CO are underestimated (probably by as much as a factor of 2 or more) in the EDGAR inventory, while American CO emissions may be overestimated.
Backward simulations are done from along the flight tracks and the ship cruise, and from the locations of the measurement stations. Whenever an aircraft changes its position by more than 0.2 degrees, a backward simulation is initiated. Also, whenever it changes its altitude by 50 m below 300 m, 150 m below 1000 m, 200 m below 3000 m, or 400 m above, a new backward simulation is initiated. For surface sites, backward simulations are done every 3 hours, for the ship every hour or whenever it changes position by more than 0.1 degrees. Every simulation consists of 40.000 particles released in the volume of air sampled. The backward simulations are done with full turbulence and convection parameterizations. Their theory is described by Seibert and Frank (Source-receptor matrix calculation with a Lagrangian particle dispersion model in backward mode, Atmos. Chem. Phys. 4, 51-63, 2004), and an application to aircraft measurements was presented by Stohl et al. (A backward modeling study of intercontinental pollution transport using aircraft measurements, J. Geophys. Res., 108, 4370, doi:10.1029/2002JD002862, 2003). The volume volume of air sampled is defined by small four-dimensional boxes covering the latitudes, longitudes, altitudes, and times covered, depending on the geometry of the sampling. For surface stations, the "boxes" are actually 1-dimensional (i.,e., points in space from which particles are released over a 3-hour interval). Output is produced every 24 hours (particle positions plus so-called "residence times" accumulated over the 24 hours, see below). The "residence times" are stored on a 3-d grid with five levels (0-150 m, 150-300 m, 300-1000 m, 1000-3000 m, and above).
Products available from backward simulations
Plots are produced using both ECMWF and GFS data and you can switch back and forth to compare the results. Plots are also shown for four plotting regions: Global, Atlantic, North America, and the North American east coast. You can always toggle between projection regions. The resolution of the output fields is 1 degree with the GFS data, but 0.25x0.33 degrees with the ECMWF data for the North American and east coast domains. Once you have selected one of the products (see below), you can enter a particular flight (or cruise, or time period) at a time of your choice and, from there, you can navigate back and forth along the flight. You can also change the product displayed (and the data source used) for the active time by a simple mouse-click, or you can go back to the flight overview to enter at a different time, or you can go back to the main page.
Retroplume summary This is perhaps the most complex product and uses a technique described by Stohl et al. (A replacement for simple back trajectory calculations in the interpretation of atmospheric trace substance measurements, Atmos. Environ., 36, 4635-4648, 2002) to display 5-dimensional data. Every 24 hours, particle positions are assigned to one of 5 groups using a clustering algorithm. At the position of every cluster a circle is drawn with the circle's radius scaled with the number of particles the cluster represents (i.e., the fraction of sampled air for which it is representative). The color of the circle indicates the altitude, and the number on top gives the time backward in days. Times are only approximate, as model output is only available every 24 hours, whereas 70 releases are always made in each individual simulation. The retroplume's centroid is also displayed by a trajectory, but as plumes get complex back in time, the centroid may not be very representative of the true plume position. It takes some time to get acquainted, but once you know how it can be used, this product tells you where the air sampled was at what time and at what altitude, all in one plot. Also shown are timeseries of the mean altitude of the retroplume (and the five clusters, red circles in the time series, size again indicating the relative fraction of sampled air it represents), the fraction of particles in the boundary layer, and the fraction of particles in the stratosphere (2 pvu polewards from 30 degree, thermal tropopause in the tropics).
Column residence time This product shows the vertically integrated residence time of the particles. It is recommended to inspect this product first, because it always shows the entire retroplume and gives the quickest impression where the air did come from (but without altitude information). Strictly, this is not a residence time, but the response an emission release of unit source strength would have at the receptor (i.e., at the measurement point) assuming no chemical transformations, deposition, etc. This response function is proportional to the residence time of all particles over a unit area (hence the name I have chosen), but involves scaling with the specific volume of air. The unit shown is nanoseconds times meters divided by kilograms. The numbers superimposed on the shading are the days back in time for the retroplume centroid (see above). They give an approximate indication of where the plume was at what time (but note that the centroids become poorly representative for the plume if the plume shape is too complex. Numbers typically become unrepresentative when they leave the main stream of particles (i.e., a well confined streamer in the column residence time) or if there are multiple such streamers.
You may notice, especially with the high-resolution ECMWF data, that individual particle trajectories become visible as "lines" of low values of residence times. This is due to the logarithmic scale used and typically occurs far backward in time when particle trajectories have already diverged strongly and the 40.000 particles used are not many enough to fully characterize the retroplume's complexity. Also note that low values of residence times often can be found appearantly "downwind" of the measurement location. This normally is due to particles having circled the globe.
Footprint residence time Same as above, but averaged over the lowest 150 m instead of vertically integrated. As anthropogenic emissions are mostly located at the surface, this gives an indication where emissions were likely taken up. The unit shown is nanoseconds divided by kilograms.
CO, NO2, SO2 source contributions This is the product between the "residence time" (or response function, or source-receptor-relationship; there are different names in the literature) and the anthropogenic emission flux (in kilograms per square meter and second) taken from the inventories. The result is an emission contribution in ppb per square meter. If the emission contribution is integrated over the earth's surface, a "tracer" mixing ratio at the sampling location is obtained. It is also reported on the plot and, furthermore, Asian, American and European contributions are listed separately. These mixing ratios are quantitatively comparable to the measurements under the assumption that the species is conserved (no chemistry, no deposition).
Using the ECMWF data, the North American and total tracer reported for the global and North Antlantic domains is different from that shown in the North American and east coast domains. This occurs because a 1 degree residence time output is used for the former domains, whereas for the latter a 0.25x0.333 degree domain is used. If emissions within a 1 degree grid cell are inhomogeneous, substantial differences can occur between the two resolutions, even though the residence times are exactly the same when averaged over the coarse grid cell.
Biomass burning CO source contributions Using a self-made inventory of daily emissions from biomass burning in North America and assuming an injection height of 0-3000 m (i.e., a footprint of 3000 m), CO source contributions from fires burning in North America are calculated, similarly to the anthropogenic pollution source contributions. As the inventory involves some smoothing (both spatially and temporally) of the available fire information, peaks in fire contributions may be underestimated and a more spread-out "background" of fire contributions than observed may be simulated. Fires can sometimes inject emissions at very high altitudes. The time series (see below) contain a sensitivity analysis to the assumed injection height.
Emission tracer time series These plots show the above tracers constructed from the backward simulations along the entire flight/cruise as time series, displayed seperately for total anthropogenic, Asian, North American, and European pollution. Time traces of the biomass burning CO are also shown, for different assumptions on the injection altitude: below 150 m, below 1000 m, below 3000 m, and below 10000 m.