Improving Numerical Wind Forecasts of Wind Ramps at 100m Height in the Stable Boundary Layer

As wind power is an important energy resource, there is an increasing demand for higher accuracy in site-specific wind forecasting, especially abrupt wind speed increases or decreases known as ramp events. This study is an effort to evaluate and modify the Mellor, Yamada, Nakanishi, and Niino (MYNN) planetary boundary layer (PBL) scheme to improve simulation of the onset of a low-level jet (LLJ), a mechanism that can often result in wind ramp events.

The MYNN PBL scheme approximates the turbulence covariance variables, which define turbulence momentum and heat flux as well as turbulent kinetic energy (TKE). These approximations are derived from the Reynolds-averaged Navier-Stokes equations and consist of a system of interdependent diagnostic expressions with terms involving gradients of the mean flow and turbulent fluxes. The influence of each term is affected by a set of weighting factors known as “closure parameters”. Parameter values as used in the existing MYNN scheme were empirically derived and remain the same regardless of the weather regime.

In this study an updated set of parameters has been formulated specifically for a stable boundary layer that exhibits a LLJ. A large-eddy simulation (LES) model with spatial resolution of 3-4m was used to simulate turbulence response and effect for such cases. These data provided the means to generate Reynolds-averaged values for explicit representation of covariance variables and TKE, which in turn, provided the basis for calculating new MYNN closure parameters for mesoscale numerical forecasts specifically for LLJ cases. Preliminary results have shown the capability for significant improvement in numerical wind forecasts of ramp events for certain cases involving nocturnal LLJs.