UC Berkeley

Previous observational and modeling studies have demonstrated the sensitivity of atmospheric processes to land surface and subsurface conditions. The extent of the connection between these processes, however, is not yet fully understood. A sufficient understanding is needed of the circumstances under which these coupled processes might play a more significant role and when they might be simplified into the decoupled systems so frequently modeled in practice. This work focuses on the effects of terrain and soil moisture heterogeneity in changing water table depth and energy fluxes at the land surface, and how this might impact the development and structure of the atmospheric boundary layer. A three]dimensional, variably saturated groundwater model coupled to a three dimensional mesoscale atmospheric model (PF.ARPS) is used here to study the two]way feedback between the subsurface, land]surface, and atmosphere for both idealized cases and a real watershed. This is done by addressing the following key questions: How do terrain, soil moisture heterogeneity, and subsurface properties affect the planetary boundary layer? What are the effects of water table depth on land surface fluxes and boundary layer development and depth? What times of the diurnal cycle and which locations within a watershed demonstrate stronger feedbacks between the subsurface and the atmosphere? These questions are first addressed for idealized simulations designed to illustrate subsurface-surface feedbacks on one hand, and land]atmosphere feedbacks on the other hand. The coupled hydrologic model is then used to simulate real conditions over the Little Washita watershed in Oklahoma with the goal of addressing the above questions for a real watershed, and exploring the two-way feedback between the atmospheric boundary layer and the water table. The coupled simulations are compared to non-coupled atmospheric simulations initialized with simplified and realistic soil moisture profiles. Effects of a storm system on the coupling between subsurface, land surface, and atmosphere are also demonstrated. Results demonstrate the connection between water table dynamics and land surface energy fluxes. This connection has a clear signature on the structure of the atmospheric boundary layer and becomes most significant within transitional zones of a watershed which lie between fully saturated regions and dry regions with deep water table. The effects of realistic soil moisture forcing which reflects subsurface conditions on boundary layer development can be equal to or greater than the effects from heterogeneous land-cover (soil and vegetation types), thus pointing to the need for improved soil moisture representations in current mesoscale atmospheric models.