AERMOD Tech Guide

Gaussian Plume Air Dispersion Model

8.7 Determining the Mixing Height when the Sounding is too Shallow

The left hand side of eq. (9) is determined from the morning temperature sounding and the right hand side from the daytime history of surface heat flux.

When the temperature sounding, obtained from the NWS, does not reach a height which is greater than the convective mixing height, we must assume a profile for the potential temperature gradient in order to estimate zic. This is accomplished as follows:

- Determine d/dz in the top 500m layer of the sounding. However, if part of the 500m layer is within the first 100m’s of the PBL the layer should be reduced (to a minimum thickness of 250m) to avoid using the portion of the sounding that is below 100m. If the above conditions can not be satisfied then zic is defined as missing.
- Extend the sounding by persisting d/dz up and recomputing zic.
- Provide warning messages which tell users
- The height of the actual sounding top,
- That d/dz has been extrapolated above the sounding zic, and
- That zic has been recomputed.
- Allow the user to reject the “fixed-up” value for zic by defining it as missing.