Above the interfacial layer, the gradient is typically constant and slightly stable. Although the

interfacial layer depth varies with time, for the purposes of determining the strength of the stable

stratification aloft, AERMET uses a fixed layer of 500 m to insure that a sufficient layer of the

morning sounding is sampled. A 500 m layer is also used by the CTDMPLUS model (Perry

1992) for this same calculation. This avoids strong gradients (unrealistic kinks) often present in

these data. For a typical mixed layer depth of 1000 m an interfacial layer depth of 500 m is

consistent with that indicated by Deardorff (1979). A constant value of 0.005 K m-1 above the

interfacial layer is used as suggested by Hanna and Chang (1991). Using the morning sounding

to compute the interfacial temperature gradient assumes that as the mixed layer grows

throughout the day, the temperature profile in the layer above zi changes little from that of the

morning sounding. Of course, this assumes that there is neither significant subsidence nor cold

or warm air advection occurring in that layer. Field measurements (e.g. Clarke et al. (1971)) of

observed profiles throughout the day lend support to this approach. These data point out the

relative invariance of upper level temperature profiles even during periods of intense surface

heating.

Below 100 m, in the SBL, AERMOD uses the definition of the potential temperature

gradient suggested by Dyer (1974) as well as Panofsky and Dutton (1984). That is,

 

Eq. (30) is similar to that of Businger et al. (1971). Above 100 m the form of the potential

temperature gradient, taken from Stull (1983) and van Ulden & Holtslag (1985) is

where zmx = 100 m, z MAX[z m] and the constant 0.44 within the exponential term of iθ im = ;100

eq. (31) is inferred from typical profiles taken during the Wangara experiment (Andre and Mahrt

1982). For all z, M2/Mz is limited to a minimum of 0.002 K m-1 (Paine and Kendall 1993).

In the SBL if d2/dz measurements are available below100 m and above zo, then 2

* is

calculated from eq. (30) using the value of M2/Mz at the lowest measurement level and zTref

replaced by the height of the M2/Mz measurements. The upper limit of 100 m for the vertical

temperature gradient measurements is consistent with that imposed by AERMET for wind speed

and temperature reference data used to determine similarity theory parameters such as the

friction velocity and the Monin-Obukhov length. Similarly, the lower limit of zo for the vertical

temperature gradient measurements is consistent with that imposed for reference temperature

 

 

 

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