We investigate the impact on the surface energy budget simulated by the Noah LSm of three schemes used for calculation of hydraulic conductivities and soil water diffusivities within the soil profile. In addition, its sensitivity to the number of soil layers is addressed. The three schemes have been applied in combination with ten vertical discretizations with 3 to 20 layers to atmospheric forcings measured between 1 January and 5 September 2003 at the Cabauw meteorological site (The Netherlands). This period includes the 2003 European heat wave.
Synthetic simulations are performed using initial soil moisture set to saturation and parameterizations adopted from standard tables. The numerical experiments demonstrate that capillary rise is underestimated by the original scheme, which can be an explanation for the slow spin up of Noah found in previous studies (e.g. Cosgrove et al. 2003 and Rodell et al. 2005). Two alternative schemes are proposed. Further, we find that the differences between schemes are reduced as the number of layers is increased.
Using inversely modeled initial soil moisture conditions and soil parameters, the performance of the original and modified versions of Noah is evaluated. Through incorporation of the capillary rise mechanism the simulated root soil moisture increases by 0.08 [m3m-3] at the end of the simulation period. Comparison with heat flux measurements shows that this soil moisture difference improves the λE and H simulation by 12.9 -10.2 and 11.6-7.4 [W m-2] depending on the number of layers.
R van der Velde, Z Su, M Rodell, MB Ek, FC Bosveld. The impact of vertical discretization of soil water flow on surface energy budgets simulated by the Noah LSm
submitted, J. Hydrometeor., 2009