The evaluation of mixing methods in HYSPLIT using measurements from controlled tracer experiments

The HYSPLIT dispersion model has different options to estimate the turbulent mixing depending on the availability of stability and turbulent parameters in the meteorological data. Dispersion simulations using different mixing options were conducted to simulate two controlled tracer experiments - the Project Sagebrush phase 1 (PSB1) for the sub-kilometer transport and the Cross Appalachian Tracer Experiment (CAPTEX) for the long-range transport. Through the comparisons of velocity variance and the evaluations of tracer concentrations, we evaluated different estimations of the turbulent velocity variance affecting the dispersion results. The mixing options in HYSPLIT are the Belijaars-Holtslag (BH) method, the Kantha-Clayson (KC) method, the turbulent kinetic energy (TKED) option, and the turbulent exchange coefficient (EXCH) option. The KC and EXCH method produced a larger maximum of the vertical velocity variance and at a higher altitude than other mixing options did. The vertical velocity variance profile of the BH scheme had a sharp increase from the surface to the height of the maximum values. The TKED option generated a flat profile with the smallest variation in its value with height. The plumes generated by the BH and TKED method (weaker mixing) had higher concentrations near the surface than those driven by the KC and EXCH option (stronger mixing). The statistical rank for the dispersion result using the TKED option was slightly better than others while the BH mixing generated results with a roughly worse rank. No mixing option always outperformed the other options. HYSPLIT users can select a mixing option according to the scenario and availability of meteorological fields, and use different options to generate dispersion ensembles.