Trajectory calculation for giant mineral dust particles
This protocol is extracted from research article:
The mysterious long-range transport of giant mineral dust particles
Sci Adv, Dec 12, 2018; DOI: 10.1126/sciadv.aau2768

Five-day backward trajectories were calculated using LAGRANTO (46, 47) for the 10-year period between 2006 and 2015 to obtain a robust climatological estimate. LAGRANTO is a Lagrangian trajectory analysis tool, which solves the following trajectory equationEmbedded Image(1)

With Embedded Image being the position vector in geographical coordinates (longitude, latitude, height in pressure coordinates) and Embedded Image being the three-dimensional wind vector (zonal, meridional, and vertical). LAGRANTO is driven by 6-hourly ERA-Interim (36) three-dimensional wind fields. The trajectories were started every 6 hours at the locations of buoys M3 and M4 (see Fig. 2), resulting in a total of about 1200 trajectories per month. Trajectories that potentially carry dust are determined on the basis of their passage through a target region defined here as the area between 12°N and 35°N and between 18°W and 30°E (Fig. 2B).

Three types of experiments were conducted:

1) Transport within the boundary layer: In winter time, dust usually remains in the lowest 1.5 km (5, 23, 48). In this layer, the atmosphere is characterized by the near-surface northeasterly trade winds over the ocean. Fastest transport from Africa to M3 and M4 can be expected in this layer, as winds become increasingly westerly with height in winter (28). Therefore, to estimate the shortest possible traveling time (assuming no sedimentation), trajectories were started at 50 hPa above M3 and M4 and computed with the ERA-Interim three-dimensional wind fields. This was only done for February when wintertime dust reaches its westernmost extension over the ocean (49).

2) Transport within the SAL: In summer, fastest transport occurs within the SAL associated with the western extension of the African easterly jet with its core around 600 hPa. To test shortest possible travel times, we therefore computed trajectories starting from this level over M3 and M4 during August, when the jet reaches its northernmost position and is fully developed (50).

3) Transport affected by convective lifting: In summer, dust travels in the vicinity of the ITCZ and could therefore be lifted within convective updrafts. To estimate the effect of this on travel time, also taking into account sedimentation, a simple thought experiment was conducted. The vertical velocity (ω) field in ERA-Interim was modified to reflect the settling velocities of dust particles. A constant value of 400 mm s−1 (1.44 km hour−1), typical for particles of 100 μm in diameter (19), was added globally. This settling velocity was converted into pressure coordinates, as ERA-Interim defines vertical velocities in Pascal per second using Eq. 2Embedded Image(2)where ω is the vertical velocity in pressure coordinates, ρ is the density, g is the gravitational constant, and w is the vertical velocity in height coordinates. The density ρ was calculated using Eq. 3Embedded Image(3)where p is pressure, R the ideal gas constant for dry air (Rd = 287 J kg− 1K− 1), and T is the temperature from the U.S. Standard Atmosphere (51).

When calculating backward trajectories, air parcels ascended rapidly due to the effect of sedimentation. Upon crossing the tropopause (defined here as 150 hPa), parcels were immediately set down to 950 hPa, mimicking the (backward) effect of a convective updraft. The number of convective updrafts needed for the parcel to reach the target region was counted. The winter months did not yield any notable results due to the predominance of westerlies at upper levels.

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