Numerical Analysis of a Hybrid Stochastic Turbulence Model for Stable Stratification
<p>We present results on the modeling of intermittent turbulence in the nocturnal boundary<br>layer using a data-driven approach. In high stratification and weak wind conditions, the<br>bulk shear can be too weak to sustain continuous turbulence and the sporadic submesoscale<br>motions trigger the turbulence production.<br>The main idea is to extend a TKE-based, 1.5 order turbulence closure model by in-<br>troducing a stochastic differential equation (SDE) for the nondimensional correction of the<br>mixing length. Such a nonstationary SDE model is built upon the traditional surface-layer<br>scaling functions, which model the effect of the static stability on the surface-layer profiles<br>using scaling with the Richardson (Ri) number . The nonstationary parameters of the SDE<br>equation are determined from data with a model-based clustering approach. Furthermore, it<br>is found that parameters scale with the local gradient Ri number, resulting in a closed-form<br>nonstationary stability correction depending only on this local gradient Ri number. Benefi-<br>cial is the interpretation of the noise term of the SDE. This term is interpreted as an effect<br>of the submesoscale motions on turbulent mixing. Furthermore, the SDE model provides<br>a conceptual view on intermittent turbulence, whereby in the noise-free limit, the steady-<br>state solution converges to the traditional functional scaling. Per construction, the SDE<br>is readily incorporated in a turbulence closure by modifying the definition of the stability<br>correction. Details will be provided.<br>We will present a numerical analysis of such a hybrid model for quasi-steady-state so-<br>lutions with different model settings. Furthermore, we investigate the regime transitions<br>between weakly and strongly stable flows under intermittent mixing based on the temper-<br>ature inversion characteristics.</p>