Evaluation of WRF 4.5.1 surface layer scheme representation of temperature inversions over boreal forests

Abstract. In this study, the Noah Land Surface Model used in conjunction with the Mellor-Janjić-Yamada surface layer scheme (hereafter, Noah-MYJ) and the Noah MultiPhysics scheme (Noah-MP) from the WRF 4.5.1 meso-scale model are evaluated with regards to their performance in reproducing positive temperature gradients over forested areas in the Arctic winter. First, simplified versions of the WRF schemes, recoded in Python, are compared with conceptual models of the surface layer in order to gain insight into the dependence of the temperature gradient on the wind speed at the top of the surface layer. It is shown that the WRF schemes place strong limits on the turbulent collapse, leading to lower surface temperature gradient at low wind speeds than in the conceptual models. We implemented modifications to the WRF schemes to correct this effect. The original and modified versions of Noah-MYJ and Noah-MP are then evaluated compared to long-term measurements at the Ameriflux Poker Flats Research Range, a forest site in Interior Alaska. Noah-MP is found to perform better than Noah-MYJ because the former is a 2-layer model which explicitly takes into account the effect of the forest canopy. Indeed a non-negligible temperature gradient is maintained below the canopy at high wind speeds, leading to overall larger gradients than in the absence of vegetation. Furthermore, the modified versions are found to perform better than the original versions of each scheme because they better reproduce strong temperature gradients at low wind speeds.