14.2. Model Parameters¶
Land surface models use parameters to describe the surface.
14.2.1. Albedo¶
From the short-wave radiation (K), within the Eq.14.3 Radiation balance the albedo (\(\alpha\)) is calculated:
using the incoming (\(\downarrow\)) and outgoing (\(\uparrow\)) shortwave radiation (K) fluxes.
14.2.2. Roughness length (\(z_0\)) and displacement height (\(d\))¶
If the displacement height is known, or is negligible, the logarithmic law equation can be rearranged with observed \(z_0\) and mean wind speed to allow \(z_0\) to be determined. As this may vary we normally take the median of a minimum of 20 results for a wind direction sector. If you have a period with a lot of neutral conditions you may be able to get a lot of samples rapidly.
14.2.2.1. How does it vary with wind direction?¶
A rule of thumb for calculating d is to assume it is \(~0.7 h\) where \(h\) is the height of the canopy. As the heights may vary with direction you can determine how much this may vary. What are expected to be consistent sectors?
The wind profile can also be used to determine \(z_0\) and \(d\) if there are more than 2 levels in the profile. This requires fitting a straight line (linear regression) through the data to determine the intercept, which provides the \(z_0+d\) value. See equations 1-2 in Grimmond et al. (1998)
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