(INVITED) Land-atmosphere interactions on North American basins estimated from North American Regional Reanalysis products

The pathways between soil moisture and precipitation over North American basins are investigated through a contemporaneous correlation analysis of the surface and atmospheric variables of the recently developed North American Regional Reanalysis (NARR). NARR is a 24-year (1979 through 2002) dataset of highly consistent analyses based on the National Centers for Environmental Prediction (NCEP) Eta model that uses many observed quantities in its data assimilation scheme. Observed precipitation is by far the most important data assimilated into NARR, thus making it a valuable tool for hydro-climate studies. The strength of land surface-atmosphere interactions depends on the regional characteristics of the basins. In regions of strong interaction, soil moisture relates to precipitation through changes in the latent and sensible heat fluxes, and consequently through the Bowen ratio. An increase of soil moisture is associated with a wetter and more unstable boundary layer, lower base clouds, and more cloud cover. All these features are also associated with enhanced precipitation. This description fits sub-basins of the western Mississippi basin, the core North American monsoon and central Mexico. The larger soil moisture available over the eastern Mississippi implies that it has only a weak relation to precipitation. In the Ohio and Upper Mississippi Basins, where soil moisture and green vegetation are plentiful, the soil moisture anomalies are not well correlated to monthly or seasonal precipitation anomalies, because the dry soil moisture anomalies are not usually large enough to notably stress the vegetation, and hence not large enough to notably reduce the surface evaporation; consequently, the lifting condensation level, and convective precipitation, are minimally influenced. No clear linkages are found in the western basins of the United States. The Columbia is a basin of abundant water with large orographic effects; the largest (solid) precipitation is produced during the cold season, and much of the summer features seem to result from advective processes. A relative lack of land surface-precipitation interactions was also found in the Colorado basin, in the fringes of the North American monsoon system, although the link improves for the top soil layer that controls the bare soil evaporation. These interactions do not explain what type of feedbacks are taking place, but they provide a solid diagnostic basis for modeling studies of soil moisture memory effects on predictability. Regions of strong interactions may have feedbacks of either sign, potentially contributing to improving the predictability. Conversely, regions of weak or no interactions cannot sustain feedbacks and thus the contribution of land surface states to the predictability of the region will be restricted.