Diffusion in the Convective Boundary Layer

Langevin equation model of dispersion in the convective boundary layer

“…Lamb's simulations inspired Deardorff (1978, 1981) key to explaining CBL vertical dispersion phenomena. As discussed by Lamb (1982), neutrally buoyant material emitted from an elevated source has a higher probability of encountering a downdraft because of the positively skewed vertical velocity distribution. Lamb (1978a) found that in the middle of the mixed layer the fraction of the horizontal area covered by downdrafts was approximately 60%, with the remaining 40% covered by stronger updrafts.…”

“…Much of the foundation for the current understanding of CBL turbulence was laid by Deardorff (1970aDeardorff ( , 1970bDeardorff ( , 1972Deardorff ( , 1974aDeardorff ( , 1974b, who studied the CBL using 3-D numerical large-eddy simulations (LES). Much of our current understanding of tracer dispersion in the CBL comes from Willis and Deardorff's (1976a, 1976b& 1981 water tank experiments, and from Lamb's (1978aLamb's ( , 1978bLamb's ( , 1982 numerical experiments using a Lagrangian dispersion model and Deardorff's LES data. In Section 2.1, turbulent structures in the CBL are reviewed.…”

“…2.4a shows that the height of the locus or "line" of the maximum ensemble-average cross-windintegrated concentration of tracer from a near-surface source stays near the surface initially and then quickly begins to increase with downwind distance starting near X = 0.5. Lamb (1978aLamb ( , 1978bLamb ( , 1982 studied CBL dispersion using a Lagrangian numerical model, and velocity fields from Deardorff's (1974a) Lamb also simulated elevated sources and found that the downwind dispersion was significantly different from that from a near-surface source. Lamb's simulations inspired Deardorff (1978, 1981) key to explaining CBL vertical dispersion phenomena.…”

Observar e Pensar antes de Modelar.

“…Na CLA convectiva as características da dispersão de poluentes dependem da localização da fonte e não podem ser descritas em termos da teoria de difusão padrão [84]; [150]; [114]). Dessa forma, o modelo LES tornou-se uma importante ferramenta no auxílio ao desenvolvimento de novas teorias, por exemplo, bottom-up/top-down [104].…”

“…liberação do poluente dentro dos updrafts e downdrafts. De acordo com Lamb [84], o fenômeno que causa o movimento descendente da pluma próximo à origem para fontes de emissão localizadas em alturas elevadas (z s > 0, 1z i ) é a assimetria da função densidade de probabilidade da velocidade vertical da escala resolvida w no plano horizontal. No caso de fontes de emissão localizadas próximas à superfície z s ≤ 0, 1z i (não mostrado) o comportamento da pluma para condições de estabilidade similares, apresenta um máximo de concentração ascendendo após percorrer uma curta distância longitudinal.…”