Abstract
Pesticide applications during surface inversions can lead to spray drift causing severe damage up to several kilometers off-target. Current regulations in Australia prohibit spray application of certain agricultural chemicals when hazardous surface inversions exist. This severely limits spray opportunities. Surface inversions can be classified as weakly or strongly stable. In the weakly stable case, moderate to strong turbulent mixing is not supportive of long-distance concentrated drift. In the very stable case, weak turbulent mixing can support the transport of high concentrations of fine material over long distances. A review of the literature and our analyses indicate that if the turbulence, as measured by the standard deviation of the vertical wind speed σw, is greater than about 0.2 m s−1 then turbulence-driven mixing and dispersion is moderate to strong and conversely if σw is less than about 0.2 m s−1 then turbulence-driven mixing and dispersion is weaker (an order of magnitude). The concept of maximum downward heat flux as a natural division between the regimes is applied within Monin–Obukhov stability theory, and it is shown that the observed mean σw of 0.2 m s−1 aligns with the ridge line of maximum heat flux in stable conditions. The level of turbulence in the weakly stable regime is comparable to the turbulence typically observed in near-neutral conditions that are recommended under current guidelines as suitable for spraying and is therefore seen as an acceptable prerequisite to avoid nondispersive spraying conditions.
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