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Abstract:
Lagrangian particle dispersion models (LPDM) have gained widespread acceptance
and usage in different applications. Currently, LPDM simulations over
regional to global scales are commonly driven using gridded, 3-D output from
numerical weather prediction models or mesoscale models. The turbulence profiles
contained in these output fields often differ from profiles assumed in idealized
simulations: they are discretized and are complex, often with sharp gradients near
interfaces such as the ground surface and the top of the planetary boundary layer. A
common problem when running LPDMs using such nonidealized turbulence profiles
is deviation from the well-mixed criterion; that is, particle distributions
initially distributed according to air density unmix over time.We outline a practical
method, based on a reflection/transmission algorithm, for adhering to the wellmixed
criterion in Gaussian turbulence. The algorithm represents turbulence properties
by way of stepped vertical profiles, such that turbulence statistics are constant
within layers and change abruptly at layer boundaries. Tests were carried out to
examine possible deviation from well mixedness under different turbulence conditions.
We compared runs adopting the reflection/transmission algorithm versus two
previous commonly adopted approaches. Unlike the two previous algorithms, the
reflection/transmission algorithm was robust under all of the turbulence conditions
tested, preserving well mixedness with no need for additional refinement of time
steps. These results point to the value of adopting the reflection/transmission
algorithm especially for (1) LPDMs simulating regional- to global-scale atmospheric
transport, in which Lagrangian particles encounter a wide variety of
turbulence, and (2) cases with sharp discontinuities in turbulence strength, such as in the vicinity of obstacles or near the top of the planetary boundary layer.
