Suggestions for Improving Ground-Based Turbulence

Official Microsoft Flight Simulator Wishlist
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In a Dev Q&A a few months ago, Seb mentioned that turbulence in the sim had been designed to respond to the underlying ground cover. For example, airflow over mountains and trees will be more turbulent than airflow over a body of water. For flights near ground level, this effect is excellent and very welcome. But I’ve noticed that the same effect continues at much higher altitudes – something that is very unrealistic. Also, the turbulence generated by irregular ground does not appear to be properly modulated by atmospheric stability.

Yesterday, I was flying in central Maine (USA) near Millinocket (KMLT). There are mountains and irregularly-shaped lakes in the area. Winds near the surface were reported at about 10 knots gusting to 20 knots. The gusty winds were occurring as high as 8,000 ft. I was flying the Just Flight Arrow III, which provides a readout of headwind/tailwind and crosswind on its EFB. The wind at 8,000 ft was gusting by at least 10 knots. In a warm, dry summer atmosphere, rotors and friction-induced turbulence might extend above 8,000 ft, but I was flying near sunset when solar-driven mixing would have been diminishing. Ground-based turbulence is strongly modulated by the stability of the atmosphere aloft. If it is early morning with a strong temperature inversion near the surface, ground-based turbulence will decay rapidly with altitude.

Currently in the sim, I’m not seeing much evidence that ground-based turbulence decays with height or depends on atmospheric stability at all. In fact, I’ve observed what appears to be ground-based turbulence when flying over 40,000 ft in the Citation CJ4 over the Appalachian mountains.

During my flight yesterday, I also noticed that the turbulence suddenly decreased when I flew over small lakes at 8,000 ft. At such a high altitude, the distinction between ground and water will be indistinguishable, especially when the lakes are so small.

As live weather continues to be improved and tweaked, I would suggest two changes to make turbulence more realistic:

  1. Modulate ground-based turbulence by atmospheric stability. Most atmospheric model data includes parameters for atmospheric stability, such as Richardson Number, temperature lapse rate, etc. I suspect MeteoBlue could provide this data quite easily. It would make a positive difference for realism.

  2. Introduce an altitude decay parameter for the water/land turbulence distinction. At higher altitudes, it probably would not be possible to feel the difference, especially over small inland lakes.

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The general rule of thumb I was taught in my 135 training on mountain ops was expect turbulence approximately 1,000 above a ridge for every 10 knots of wind speed at altitude. However I’ve definitely experienced moderate to severe turbulence higher than that rule. Another rule of thumb is to fly higher than half the mountain height above the peaks.

The FAA guidance can be found in AC 00-57. Which basically says there is no limit to where mountain generated turbulence can occur.

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Right now the turbulence over the mountain seems to be modeled with the wind at plane’s elevation rather than at the mountain elevation. It’s extreme at high winds and even if you are flying 20,000 feet above the mountain peak. I know severe turbulence is normal with unstable, high wind and mountainous terrain but the current MSFS 2020 seems incorrect for some of the conditions. I like your suggestions.

Also, right now the clouds produce almost no turbulence which is also not realistic! Hope they fix this.

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