Some Considerations for Developing an Ingredients-Based 1-3 KM Turbulence Algorithm
The 1-3 km layer of the atmosphere provides a particular challenge for forecasters. These challenges arise because of the layer's proximity to both the planetary boundary layer as well as mid level flows. A variety of meteorological features are responsible for turbulent flows in this layer, including wind speeds and shears, terrain flows, and thermals. Currently, the Knapp-Ellrod index is used to forecast turbulence in the layer at the Air Force Weather Agency (AFWA), but this index does not account for stronger thermodynamic and mechanical turbulence.
An additional challenge to forecasting turbulence lies within the constraints of operational numerical models. The needs for large domain coverage mixed with limited computer time force limitations within horizontal and vertical resolutions of WRF-ARW runs at AFWA. A 15-km horizontal resolution WRF-ARW run currently contains 56 vertical levels; 4-km WRF-ARW members of the Mesoscale Ensemble Prediction Systems contain 27 vertical levels. The goal of this presentation is to show how various turbulent features are represented within AFWA's operational numerical models for turbulence cases from August 2013 through January 2014. The hope is that in the future, an ingredients-based methodology for forecasting 1-3 km turbulence can be developed that encompasses all turbulence in the layer based on how these features are represented in WRF-ARW.