Equation (15) produces real-valued solutions only when the wind speed is greater than or equal

to the critical value For the wind speed less than the critical value,

u* u[z g T C]cr m ref ref D= 4

*are parameterized using the following linear expression:These expressions approximate the

u*verses 2

*dependence found by van Ulden and Holtslag(1983).

In order to calculate

u*from eq. (15) an estimate of 2

*is needed. If representative cloudcover observations are available the temperature scale in the SBL is taken from the empirical

form of van Ulden and Holtslag (1985) as:

where

nis the fractional cloud cover. However, if cloud cover measurements are not available,AERMET can estimate 2

*from measurements of temperature at two levels and wind speed atone level. This technique, know as the Bulk Richardson approach, starts with the similarity

expression for potential temperature (Panofsky and Dutton 1984), that is,

where $

m. 5 andk(= 0.4) is the von Karman constant. Applying eq. (17) to the two levels oftemperature measurements and rearranging terms yields

Since both

u*(eq. (12)) and 2*(eq. (18)) depend onL, andL(eq. (11)) in turn depends onu*and 2

*, an iterative approach is needed to estimateu*.u*and 2

*are found by first assuming aninitial value for

Land iterating among the expressions foru*, 2*(eq. (18)) andL(eq. (11)) untilconvergence is reached. The expression used for

u*, in the iteration, is taken from (Holtslag

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castellano: DISPER CUSTIC DESCAR RADIA italiano:

castellano: DIS CUS DES RAD english: DIS CUS DES RAD

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