Evaluation of the Inertial Dissipation Methodover Land

Abstract:

The focus was to evaluate the Inertial Dissipation Method (IDM) over land duringunstable conditions. This was done by comparing the friction velocity, u_', from theeddy-correlation method (ECM) with u_' from IDM. The result can be used to see ifIDM can rely on its assumptions, since the surface-layer theory is more fulfilled overland, where we for example do not have wave influence. The measurements weretaken from the flat agricultural site Marsta, 8 km north of Uppsala, Sweden.

The result shows that IDM works well over land (relative standard deviation of about 10 %). For weakly unstable stratification, it is enough to use an assumption of neutralconditions in the IDM calculations. If it is more unstable, one should include theinfluence of stability and also include an imbalance term. The imbalance term isintroduced implicitly by varying the effective Kolmogorov’s constant with stability.The effective Kolmogorov’s constant used here, varied from 0.50 up to above 0.80.

To calculate u_' using IDM, a first estimation of u_' was calculated from aparameterised drag coefficient C_D. Also, to imitate the measuring setup on a movingplatform on the sea, the stability parameter, z/L, was calculated using a bulkestimatedheat flux. The large scatter showed that it is important that theparameterisations of C_D and the heat flux are good.

One can conclude that the IDM as a method to determine turbulent fluxes over landworks satisfactory. The larger scatter over sea is probably an effect of sea waveinfluence, even though the sea surface is considered more homogeneous and theconditions more stationary.