REAL Lagrangian Visualization of Aerosols and Wind Field

2-component vector wind fields recovered by motion estimation algorithms (cross-correlations, Typhoon) from REAL data give us the unique ability to visualize and study aerosol distributions and wind fields from a Lagrangian point of view.

We consider the wind field u to be the sum of mean motion U and turbulent fluctuations u': u = U + u'. From a sequence of REAL backscatter data, corresponding wind fields u are recovered by motion estimation algorithms. Then,

1. a rectangular box is defined by the user on the first scan of the sequence;
2. the mean wind velocity U in the box is computed;
3. this mean velocity is subtracted from the full wind field u to exhibit turbulent fluctuations u';
4. at the next frame, the box center is moved by an amount corresponding to the previous mean velocity multiplied by the time-step between scans (integrated displacement), and the process is repeated from point 2.

Convective boundary layer, 23 August 2013

This example features data recorded by the REAL in Chico (CA) on August 23, 2013 around 14:00 PTZ (21:00 UTC), at 6° elevation (input data). Wind field was retrieved by Typhoon algorithm. The video displays the aerosol backscatter data (copper shades) corresponding to the box (left) as well as the full scan for reference (top right). Mean wind U speed and direction values, computed within the box, are shown bottom right. The box, moving according to this mean velocity, is represented top-right by a white rectangle along with the trajectory of the center point (white path). Turbulent fluctuations u' are shown left (white vectors). This motion field was decimated by a factor of 6 in both dimensions and scaled by a factor of 32 for the sake of visualization. Time speed-up is about x100.

These two videos represents the same sequence; the estimated wind field is visualized thanks to tracers instead of arrows. On the left, tracers follow the turbulent perturbations u' (within the reference frame) corresponding to the vector plot above. On the right, tracers follow the full motion field u.

The video below is a longer sequence, recorded the same day between 13:30 and 15:30 PTZ. 20 tracers are released simultaneously within a 100x100 m box located in the near range (white rectangle). Tracers are advected by the estimated wind field, and their trajectories shown in blue. As soon as one tracer reaches the boundaries of the displayed area, another set of tracers is released.

[Work in Progress] Next video is the same sequence as above. We use the flow field derived from the lidar imagery to advect tracer from some arbitrary point in the scan area. It allows one to estimate the dispersion of a constant release of hypothetical material (gas, pollutant, ...) by a fixed source located in the near range (100x100 m white box). The released material is advected by the estimated wind field and colored in blue-green. The color itself is not relevant as this quantity does not represent a concentration, it rather indicates where the material would go. We will add an animation of what is an approximation of the relative concentration of the effluent in the future.