The Critical Richardson Number and Its Implications for Forecast Problems

Abstract

It is shown that, in the lowest few hundred meters, nighttime inversions tend to break down due to onset of turbulence when the Richardson number falls between 0.2 and 0.5. Since the Richardson number is statistically related to the wind speed at one to a few hundred meters, it is found that the dew-point depression in the morning, as well as visibility, is related to the wind speed above the surface. It follows, that objective forecast techniques for the dew-point depression and visibility can be improved by including the wind speed above the surface as a parameter.