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What does it mean by vertical mixing in the stability plots? Does this represent thermal height? If so, how's that different than the boundary layer? Also, is instability simply a measurement of lapse rate? One of the reasons for asking is that I noticed that for one of the WMO stations I look at the vertical mixing spike was off the chart, but the same sounding was showing a so-so lapse rate. On another day the spike was lower while the sounding for the same day was indicating a much stronger lapse rate.

The vertical mixing coefficient (m2/sec - mixing length times velocity) is a measure of the turbulent mixing within the boundary layer used by our HYSPLIT transport and dispersion model to calculate the vertical movement of pollutants. The mixing coefficient will decrease above the boundary layer.

Lapse rate is one component of the atmospheric stability, also usually called static stability. The vertical mixing is also affected by how much turbulence is generated by the change in the wind speed with height. The static stability may either enhance or suppress the wind generated turbulence. The ratio of these two is the dynamic stability, which can be represented by a parameter such as the vertical mixing coefficient. Note that the stability is never constant in space or time. The heating of the various terrain types by the sun causes the surface to heat unevenly, leading to variations in wind velocity and temperature. As sunlight increases (into the afternoon on a non-cloudy day), the mixing increases. So, it's a combination of the change in temperature and wind with height that causes the vertical stability or instability. This explains why you see differences between the temperature sounding and the vertical mixing coefficient, which includes the effects of both temperature and wind.

Roland Draxler

Example of the READY stability time series plot
(from the stability plot on READY)
Modified: October 7, 2020
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