The Intra-Seasonal Variability of Land-Atmosphere Coupling over North America in CRCM5

Abstract:

The intra-seasonal variability of land-atmosphere coupling for the mid- to high-latitude regions of North America is investigated using carefully designed experiments performed with the fifth generation Canadian Regional Climate Model (CRCM5). We ran a coupled CRCM5 simulation, where the land and the atmosphere interact freely, and an uncoupled CRCM5 simulation, where the soil moisture is prescribed, to evaluate the coupling strength under current climatic conditions. Both simulations were driven by ERA-Interim reanalysis for the 1981 – 2010 period at 0.44˚ horizontal resolution, over a North American domain.

Analysis of the coupled and uncoupled simulations reveals that soil moisture/temperature coupling is strong over the Canadian Prairies during the summer, spring and fall seasons. The underlying mechanisms, however, vary with season. During the summer months, soil moisture affects the partitioning of net surface radiation into latent and sensible heat fluxes, which then influences the two-meter temperature through evaporative cooling. For the transition months (spring and fall), on the other hand, soil moisture variation - particularly moisture phase, i.e. the fractional liquid and frozen water contents, play a role in altering the surface albedo, which in turn modifies the absorbed shortwave and the emitted longwave radiations. In particular, the outgoing longwave radiation influences the two-meter temperature through radiative cooling. Surface temperature variability during the transition months is also reflected in the snow depth variability, which in turn affects surface albedo, emissivity and soil moisture again through a feedback loop. We also assessed the impact of coupling on temperature extremes during the transition seasons and found a strong relationship between coupling strength and the number of hot days during spring.