SEAC4RS WRF Trajectory Plots Description
WRF Version 3.6 was used to make simulations over 3 domains (see figure) during the SEAC4RS period (01 August - 25 September). From outer to inner, the domains used 36 km, 12 km, and 4 km grid spacing, respectively.
Initial and boundary conditions came from NCEP/GFS final analyses. The model was reinitialized every 5 days to ensure that the simulations did not stray too far from reality.
Temperature, horizontal winds, and moisture in the outer domain was nudged to the GFS analyses every 6 h. This nudging only occurred in levels above the boundary layer and was used to keep the synoptic-scale features consistent with the GFS final analyses.
All domains used 51 vertical levels that spanned from the surface up to 10 hPa. Additionally, one-way nesting was used for these multi-domain simulations. At the SEAC4RS Science Team Meeting, there was interest in seeing the difference in trajectories when using the 36 km domain versus the 4 km domain. By using one-way nesting, we can perform such an analysis without the 4 km domain influencing the results of the 36 km domain.The outer two domains used the Kain-Fritsch cumulus scheme whereas the 4 km domain did not employ convective parameterization. Cloud microphysics were parameterized using the WRF Single-Moment 6-class scheme. Finally, we used the YSU PBL scheme and the NOAH land surface model with USGS land use data.
For each DC-8 and ER-2 flight, five-day back trajectories were released along the flight path using HYSPLIT. To do this, each flight was broken into "flight legs." Each flight leg is a 10-minute segment of the flight. Within that 10-minute segment, we create points every 30 seconds (DC-8) or 60 seconds (ER-2), giving us 20 (10) points along that leg. Then, back trajecctories are released from these points at the temporal midpoint of the flight leg. For example, let us assume there was a flight leg from 12:00 - 12:10 UTC. We would gather points every 30 seconds along that leg, and release back trajectories from these points at the temporal midpoint, 12:05 UTC. Whenever possible, the trajectories used the 4 km WRF data. However, if a trajectory exited the 4 km domain, it would then use the 12 km data, and if it exited the 12 km domain, it used the 36 km data.
In addition to the flight level trajectories, we also computed back trajectories for each flight leg from 4 constant pressure altitudes: 850, 500, 300, and 100 hPa. These trajectories are meant to help diagnose where the air above/below the aircraft came from.
We computed encounters with NLDN-derivied lightning following the same general procedure as Fuelberg et al. (2007). Briefly, a trajectory was considered to be influenced by lightning if there was at least one NLDN flash within 0.5 degrees latitude/longitude and within +-30 min of its position. We also tested a spatial threshold of 0.25 degrees latitude/longitude. The distances of 0.5 and 0.25 degrees are used to account for the uncertainties in trajectory locations. Further, we used only the 36 km WRF data for these calculations. Because we are mixing model data with observed lightning, we wanted the model-produced convection to have little influence on the trajectory. Thus, the 36 km data was a compromise between accurately representing the synoptic-scale flow without over-emphasizing model-produced convection.
For each DC-8 flight, the plots show the flight track color coded by "time since lightning influence." For example, the color red indicates that back trajectories released from that location encountered NLDN lightning within 0-12 hours. Black indicates that the trajectories never encountered NLDN lightning. Text data for the lightning influence product is available for each flight and has the following format:
Flight leg, Traj # within that flight leg, starting lat, starting lon, starting height (m), starting month, starting day, starting hour, starting minute, time since convective influence (hours), ending lat, ending lon, ending height, ending month, ending day, ending hour, ending minute
In the description above, "ending" refers to the value the trajectory had when it encountered the NLDN lightning. "0" in the data indicates that the trajectory was not influeced by lightning during the 120 h back track.
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