particularly in complex terrain, that the surface and upper air

winds are independent from each other. Hence, the use of the

relationship between the surface and upper air winds may yield

spurious results. Another factor to consider with respect to

the mixed depth average winds is the behavior of the mixed

depth, as illustrated in Figures 4a and 4b. In the northwest

corner of the 1200 LST MESOPUFF-II mixing height field (Figure

4a), the mixing depth jumps from a height of around 500 meters

to approximately 1600 meters in adjoining grid cells. Thus,

the mixed layer average wind will represent very different

quantities between those adjoining cells.

3-25

The ARM3 wind fields ostensibly meet the IWAQM criteria of

accounting for the effects of terrain on the wind fields. The

method the ARM3 uses to account for terrain effects warrants

some discussion. The wind field generating portion of the

model is called the Diagnostic Wind Model (DWM). The order of

analysis that the DWM uses is to first generate a horizontally

uniform "first-guess wind field." This first-guess field is

defined from the central-most sounding found in the modeling

domain. The effects of terrain, blocking and kinematic

effects, are applied to this first-guess, mean flow field. The

remaining surface and upper air observations are then applied

with the weighted interpolation scheme described earlier. The

strength of this approach is that it can yield a more

reasonable flow field in complex terrain where meteorological

observations are sparse. It does, however, introduce some

problems when generating a regional scale flow field.

If a wind field is to be generated over a relatively small

air basin, which includes complex terrain, where one may have

only one sounding within the domain, the aforementioned use of

a first-guess wind field is probably valid. When one is

generating a wind field over a larger domain, however, the

assumption of a first guess-field, based on one sounding, is

probably not appropriate. If, for example, a major topographic

barrier runs through the domain, it is quite likely that the

air flow on the opposite sides of the barrier may be very

different. Blocking, for example, will only occur on the

windward side of the barrier. If only one sounding is used,

this blocking and subsequent turning of the wind will only

occur on one side of the barrier, where in reality there may be

upslope flows on both sides of the barrier, with subsequent

terrain modifications to the flow. Thus, while one of the

strengths of the ARM3 wind generation model is its ability to

treat flows in complex terrain, its implementation may

ultimately lead to the generation of spurious winds on the

3-26

sides of barriers opposite the station used to generate the

first-guess field.

The generation of mixing depths by the two models is

dismal. The implementation of the MESOPUFF-II algorithms

yields large discontinuities in adjoining grid cells, while the

ARM3 implementation of the calculation of mixing depths did not

reproduce any expected diurnal variability. As noted earlier,

the reasons for the behavior of the two algorithms are

generally understood. The MESOPUFF-II algorithms only rely

upon data from the nearest sounding. There has been no attempt

 

 

 

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