The effect of groundwater interaction in North American regional climate simulations with WRF/Noah-MP

The fluxes of water and energy between the land surface and atmosphere involve many complex non-linear processes. In this study, the Noah and Noah-MP land surface models with multiple groundwater sub-models are used to assess how the treatment of canopy processes and interactions with deep groundwater affect 6 month regional climate simulations in two contrasting years, 2002 and 2010. Unlike the free drainage models, the models with groundwater capability have upward flux from the aquifer at different periods in the simulation. The inter-model Noah-MP soil moisture and latent heat flux results are consistent with recharge differences: the stronger upward flux capability with interactive groundwater results in the highest soil moisture and latent heat flux of the Noah-MP models. The increased latent heat effect on increased precipitation is small, which may result from negligible differences in convective precipitation. The Noah-MP model, independent of groundwater option, improves upon a cold and dry bias in the spring Noah simulations both during the day and night. The results for summer are region dependent and also differ between year and time of day. For a majority of the simulation period, there is little groundwater effect on the Noah-MP near-surface diagnostic fields. However, when the Noah-MP model produces large warm/dry biases in the 2010 summer, the aquifer interactions in Noah-MP improve the air temperature bias by 1-2 A degrees C and dew point temperature bias by 1 A degrees C.