High resolution joint atmospheric-terrestrial water balance estimations in a pre-alpine environment: comparison between stand alone, 1-way and fully 2-way coupled WRF-NDHMS simulations

Increased predictability of regional hydrometeorological variables requires the consideration of feedbacks between subsurface, surface and atmospheric water and energy fluxes on different spatial and temporal scales. Thereby, the relative importance of lateral water re-distribution for regional atmospheric processes has received increased attention in both atmospheric and hydrological sciences. Feedbacks between the land and atmosphere components can only be considered using comprehensive regional modeling systems that represent the joint atmospheric and terrestrial surface and subsurface water balance in a full, two-way coupled mode.

Our study aims to investigate possible atmosphere-land surface feedbacks in a mid-latitude region of complex terrain. For that purpose, the NCAR Distributed Hydrological Modeling System (NDHMS) (Gochis and Chen, 2003) and WRF (Skamarock et al., 2008) were applied to a pre-alpine region in southern Bavaria/Germany with a focus on the Ammer catchment (around 600 km2), located 60 kilometers southwest of Munich. Three modeling approaches were adopted and compared: 1) a NDHMS application in stand alone mode (i.e. driven by interpolated observed meteorological station data), 2) a one-way coupled WRF and NDHMS simulation (i.e. a stand alone NDHMS-simulation driven by WRF simulated meteorological fields), and 3) a fully coupled WRF-NDHMS simulation. The study focuses on the period spring and summer 2005 and is motivated by the 100 yr flood event that occurred in August 2005. The finest resolved computational domain with an extent of 100 by 120 kilometers (W-E/S-N) had a grid resolution of 1 km for both, the atmospheric and the land surface part, while the lateral overland flow, subsurface and channel routing functions are executed on a subgrid with 100 meter grid-spacing.

We show the impact and feedbacks of lateral water fluxes (overland flow and subsurface flow) on soil water distribution and consequently on simulated nears surface meteorological fields, particularly precipitation. Channel routing within NDHMS and comparison to observed streamflow in the Ammer catchment allows us to evaluate the performance of the three different three model approaches.