The time scale, J , governs the rate of change in height of the layer and is taken to be

proportional to the ratio of the turbulent mixed layer depth and the surface friction velocity (i.e. J

= zim / $J u*). AERMOD uses a constant $J value of 2. For example, if u* is of order 0.2 m s-1,

and zim is of order 500 m, the time scale is of the order of 1250 s which is related to the time it

takes for the mechanical mixed layer height to approach its equilibrium value. Notice that when

zim < zie, the mechanical mixed layer height increases to approach its current equilibrium value;

conversely, when zim > zie, the mechanical mixed layer height decreases towards its equilibrium


Because the friction velocity changes with time, the current smoothed value of zim{t+)t} is

obtained by numerically integrating eq. (25) such that

where zim{t} is the previous hour’s smoothed value. For computing the time scale in eq. (26), zim

is taken from the previous hour’s estimate and u* from the current hour. In this way, the time

scale (and thus relaxation time) will be short if the equilibrium mixing height grows rapidly but

will be long if it decreases rapidly.


Although eqs. (24) and (26) are designed for application in the SBL, they are used in the

CBL to ensure a proper estimate of the PBL height during the short transitional period at the

beginning of the day when mechanical turbulence generally dominates. The procedure, used by

AERMET, guarantees the use of the convective mixing height once adequate convection has

been established even though the mechanical mixing height is calculated during all convective

conditions. Since AERMET uses eq. (26) to estimate the height of the mixed layer in the SBL,

discontinuities in zi from night to day are avoided.

In AERMOD, the mixing height zi, has an expanded role in comparison to how it is used in

ISC3. In AERMOD the mixing height is used as an elevated reflecting/penetrating surface, an

important scaling height, and enters in the w* determination found in eq. (9). The mixing height

zi for the convective and stable boundary layers is therefore defined as follows:




n951 - n952 - n953 - n954 - n955 - n956 - n957 - n958 - n959 - n960 - n961 - n962 - n963 - n964 - n965 - n966 - n967 - n968 - n969 - n970 - n971 - n972 - n973 - n974 - n975 - n976 - n977 - n978 - n979 - n980 - n881 - n982 - n983 - n984 - n985 - n986 - n987 - n988 - n989 - n990 - n991 - n992 - n993 - n994 - n995 - n996 - n997 - n998 - n999 - n1000


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