The ARM3 generated fields (Figures 5a and 5b) show

relatively smooth fields for both daytime and nighttime. The

actual heights, however, are much lower during the daytime than

those calculated for MESOPUFF-II, ranging from 125 meters to

596 meters. During the nighttime, the values range from 185 to

593. So while the ARM3 mixing height fields do not exhibit the

discontinuities from one cell to the next that the MESOPUFF-II

3-23

Figure 6 - Plot of hourly average mixing depths (m), plotted

every third hour.

daytime fields show, they do not show any appreciable diurnal

variation.

To further illustrate this, domain average mixing heights

were calculated for each model and plotted (Figure 6). The

 

MESOPUFF-II heights show diurnal variation over the entire

time-span plotted, while the ARM3 mixing heights show almost no

relationship to the diurnal cycle. This can be partially

explained by the method used to calculate the mechanical mixing

depth. The choice of the 3000 meter wind speed as an indicator

of the free stream wind will almost always force the wind to be

at least on the order of 5 m/s. If the term U in the ARM3

mechanical mixing equation is set to 5, then the minimum mixing

3-24

depths will generally be on the order of 265. In order for

reasonable nighttime mixing heights to be calculated, the 3000

meter wind would have to be more on the order of 1 m/s. At

that altitude, however, a wind speed of 1 m/s is not likely to

occur very often. The daytime mixing depth calculations,

generated by the ARM3, are generally too low. The reason for

this has not been investigated at this time.

3.3.2.3 Meteorological Field Discussion: The wind fields

generated by the two models are each based on distinctly

different approaches to interpolating surface and upper air

observations. Both methods have some significant problems.

For the stated purpose of IWAQM, the MESOPUFF-II approach lacks

the ability to treat the effects terrain will have on the mean

flow, except as much as local observations represent the mean

flow in the terrain. The mixed depth average wind speed and

direction used by MESOPUFF-II can be both a strength and a

weakness. The strength of the system is that it provides some

other information to the interpolation scheme between 12 hour

soundings by using the relationship between the surface and

upper air data at the time of the twelve hour soundings. One

problem with this, however, is that it is based upon the

assumption that the surface and upper air data are indeed

coupled. It is quite possible that under some circumstances,

 

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