Wind Values from WIND_COV mavlink message are all 0

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I’m using the posix SITL to simulate fixed wing flight. I need to calculate an air-relative velocity and I planned on using the WIND_COV mavlink message to get the wind vectors. I notice its only 2d, but right now all three wind vectors, x,y, and z, are all zero in the mavlink message. How can I get the simulated wind data or enable wind in posix SITL?