Impact of sub-grid variability of precipitation and canopy water storage on land and atmosphere hydrological processes

The sub-grid variability of precipitation and canopy water storage plays an important role in regulating the land-atmosphere interactions. This study develops a canopy hydrology scheme that considers the sub-grid variability based on a dynamic-statistical method. When implemented into the offline Community Land Model version 3 (CLM3), the scheme is fairly robust under different temporal resolutions. The model evaluation is conducted by comparing the simulated interception loss ratio (i.e., the interception loss as a fraction of precipitation) with field measurements at various sites over the globe. The comparisons demonstrate that the model with the new canopy hydrology scheme performs much better than the one with the scheme not considering the sub-grid variability of precipitation and canopy water storage. The improvement is more significant over the tropics where convective rain is dominant and vegetation is dense. By applying the new scheme into the offline CLM3 and the coupled Community Atmosphere Model version 3 (CAM3)-CLM3, the impact of sub-grid variability of precipitation and canopy water storage on the hydrological processes is investigated. The impact is much more significant over the tropics both in the offline CLM3 and the coupled CAM3-CLM3. However, hydrological processes in the offline CLM3 and the coupled CAM3-CLM3 response differently to the sub-grid variability of precipitation and canopy water storage. For example, the evapotranspiration decreases when the sub-grid variability is included in the offline CLM3, while it remains more or less the same in the coupled CAM3-CLM3. The differences in the impact of sub-grid variability result from the land-atmosphere feedbacks that modify the large-scale circulation and atmospheric instability. Cloud feedback plays an important role in propagating the impact from land surface to atmosphere.