Improvement of Aviation Turbulence Forecast in the NOAA’s Rapid Refresh Model Version 4 (RAPv4) with Hybrid Vertical Coordinate System

Spurious mountain-wave features have been reported in the operational turbulence forecast systems such as the Graphical Turbulence Guidance (GTG) based on NOAA’s Rapid Refresh (RAP) model. These cause false alarms of light-or-stronger turbulence especially over the western mountainous region of North America. To reduce predicted wave amplitudes and associated mountain wave turbulence, a hybrid sigma-pressure vertical coordinate system has been implemented in the RAP model version 4 (RAPv4), which has been running in parallel with the old RAP version 3 (RAPv3), for comparison purposes. In this study, direct comparison of the vertical velocity (|w|) fields from the RAPv4 and RAPv3 models shows that the new RAPv4 model substantially reduces small-scale artificial wave motions induced by the conventional terrain-following coordinate system in RAPv3. |w| and |w|/Richardson number (|w|/Ri) derived from both the RAPv4 and RAPv3 models are converted into the Eddy Dissipation Rate (EDR)-scale for turbulence forecasts, which are evaluated using more than 1.2 million of in situ EDR reports from commercial aircraft for 4-months (2017.09 – 2017.12). Performance skill based on the area under a Receiver Operating Characteristic curves for the |w|and |w|/Ri–based EDR forecasts from the RAPv4 are 0.69 and 0.83, compared to 0.63 and 0.77, respectively for the RAPv3. Bias for the |w|and |w|/Ri-based null turbulence forecasts also decreases significantly from 0.316 and 0.149 in the RAPv3 to 0.238 and 0.127 in the RAPv4. The new RAPv4 is now officially operational at NOAA from July 12 2018, which will provide better guidance for users of operational turbulence forecast products.