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


In order to calculate u* from eq. (15) an estimate of 2

* is needed. If representative cloud

cover observations are available the temperature scale in the SBL is taken from the empirical

form of van Ulden and Holtslag (1985) as:

where n is 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 at

one 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 and k (= 0.4) is the von Karman constant. Applying eq. (17) to the two levels of

temperature measurements and rearranging terms yields

Since both u* (eq. (12)) and 2* (eq. (18)) depend on L, and L (eq. (11)) in turn depends on u*

and 2

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

* are found by first assuming an

initial value for L and iterating among the expressions for u*, 2* (eq. (18)) and L (eq. (11)) until

convergence is reached. The expression used for u*, in the iteration, is taken from (Holtslag



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