3.4.1 CONVECTIVE MIXING HEIGHT (zic)

The height of the CBL is needed to estimate the profiles of important PBL variables and to

calculate pollutant concentrations. If measurements of the convective boundary layer height are

available they are selected and used by the model. If measurements are not available, zic is

calculated with a simple one-dimensional energy balance model (Carson 1973) as modified by

Weil and Brower (1983). This model uses the early morning potential temperature sounding

(prior to sunrise), and the time varying surface heat flux to calculate the time evolution of the

convective boundary layer as

where 2 is the potential temperature, A is set equal to 0.2 from Deardorff (1980), and t is the

hour after sunrise. Weil and Brower found good agreement between predictions and

observations of zic, using this approach.

3.4.2 MECHANICAL MIXING HEIGHT (zim)

In the early morning when the convective mixed layer is small, the full depth of the PBL may

be controlled by mechanical turbulence. AERMET estimates the heights of the PBL during

convective conditions as the maximum of the estimated (or measured if available) convective

boundary layer height (zic) and the estimated (or measured) mechanical mixing height.

AERMET uses this procedure to insure that in the early morning, when zic is very small but

considerable mechanical mixing may exist, the height of the PBL is not underestimated. When

measurements of the mechanical mixed layer are not available, zim is calculated by assuming that

it approaches the equilibrium height given by Zilitinkevich (1972) as

where zie is the equilibrium mechanical mixing height and f is the Coriolis parameter.

Venkatram (1980) has shown that, in mid-latitudes, eq. (23) can be empirically represented

as

where zie (calculated from eq. (24)) is the unsmoothed mechanical mixed layer height. When

measurements of the mechanical mixed layer height are available they are used in lieu of zie.

To avoid estimating sudden and unrealistic drops in the depth of the shear-induced, turbulent

layer, the time evolution of the mechanical mixed layer height (whether measured or estimated)

is computed by relaxing the solution toward the equilibrium value appropriate for the current

hour. Following the approach of Venkatram (1982)

 

 

 

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