Aquifer-atmosphere interactions can be important in regions where the water table is shallow (<2 >m). A shallow water table provides moisture for the soil and vegetation and thus acts as a source term for evapotranspiration to the atmosphere. A coupled aquifer-land surface-atmosphere model has been developed to study aquifer-atmosphere interactions in watersheds, on decadal timescales. A single column vertically discretized atmospheric model is linked to a distributed soil-vegetation-aquifer model. This physically based model was able to reproduce monthly and yearly trends in precipitation, stream discharge, and evapotranspiration, for a catchment in northeastern Kansas. However, the calculated soil moisture tended to drop to levels lower than were observed in drier years. The evapotranspiration varies spatially and seasonally and was highest in cells situated in topographic depressions where the water table is in the root zone. Annually, simulation results indicate that from 5-20% of groundwater supported evapotranspiration is drawn from the aquifer. The groundwater supported fraction of evapotranspiration is higher in drier years, when evapotranspiration exceeds precipitation. A long-term (40 year) simulation of extended drought conditions indicated that water table position is a function of groundwater hydrodynamics and cannot be predicted solely on the basis of topography. The response time of the aquifer to drought conditions was on the order of 200 years indicating that feedbacks between these two water reservoirs act on disparate time scales. With recent advances in the computational power of massively parallel supercomputers, it may soon become possible to incorporate physically based representations of aquifer hydrodynamics into general circulation models (GCM) land surface parameterization schemes. RightsWorks produced by employees of the U.S. Government as part of their official duties are not copyrighted within the U.S. The content of this document is not copyrighted. AbstractAquifer-atmosphere interactions can be important in regions where the water table is shallow (<2 m). A shallow water table provides moisture for the soil and vegetation and thus acts as a source term for evapotranspiration to the atmosphere. A coupled aquifer-land surface-atmosphere model has been developed to study aquifer-atmosphere interactions in watersheds, on decadal timescales. A single column vertically discretized atmospheric model is linked to a distributed soil-vegetation-aquifer model. This physically based model was able to reproduce monthly and yearly trends in precipitation, stream discharge, and evapotranspiration, for a catchment in northeastern Kansas. However, the calculated soil moisture tended to drop to levels lower than were observed in drier years. The evapotranspiration varies spatially and seasonally and was highest in cells situated in topographic depressions where the water table is in the root zone. Annually, simulation results indicate that from 5-20% of groundwater supported evapotranspir...

[1] We present a model and application designed to study the coupled land-atmosphere hydrologic cycle, following water from its inflow into a region by horizontal atmospheric transport through surface-atmosphere exchange processes and aquifer recharge to outflow as runoff and river discharge. The model includes a two-way water flow among its major reservoirs (atmosphere, vadose zone, groundwater, surface water, river). A unique feature of the model is that phreatophytic interactions are included when the water table intersects the root zone. The model emulates a uniform grid box of an atmospheric general circulation model, but with finer horizontal resolution for the land processes, and forms a test bed for developing continental-scale simulation of the hydrologic cycle. The model is calibrated using the First International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment (FIFE) observations for 1987 and validated using FIFE observations for 1988 and 1989. Four physical factors emerge as important for simulating the FIFE water cycle: effective relative humidity for initiating stable (large scale) condensation, length of the growing season, amount of available soil water, and cloud cover parameterization. Further evaluation uses water table and river discharge measurements for years up to 1993. The model simulates multiyear behavior in the hydrologic cycle reasonably well. Average differences between FIFE observations and simulated fluxes during the calibration period are only a few percent, including fluxes not specifically calibrated. Model-observation differences in surface sensible and latent heat fluxes are larger during the 1988 drought but recover to relatively small values for 1989, suggesting some difficulty in simulating hydrologic extremes occurring outside the calibration conditions. A model sensitivity study using statistical disaggregation to allow precipitation to fall on only a portion of the landscape indicates that spatial disaggregation of precipitation can have strong impact on groundwater storage and surface discharge, potentially improving agreement between observed and simulated streamflow. Water redistributed through the model's aquifer-river network can at times raise the water table high enough for water to seep back to the vegetation root zone and increase evapotranspiration. During relatively dry periods, up to 33% of monthly evapotranspiration was derived from groundwater-supported evapotranspiration, emphasizing the need to quantify better aquifer-atmosphere interaction. The work also demonstrates the feasibility and utility of fully coupled water budgeting schemes.

In this paper, we will describe our analysis of a polar low event that occurred in the Labrador Sea during the winter of 1992. As there are unfortunately no in‐situ observations of this event, we will rely on satellite data as well as the high‐resolution objective analaysis from the ECMWF to document the environment in which the low developed and the structure of the low itself. We will show that the polar low developed during a cold air outbreak that was precipitated by the passage of an intense synoptic‐scale low. The polar low appears to have developed along a linear cloud feature as the result of an interaction between a low‐level diabatically induced potential vorticity anomaly and an upper‐level potential vorticity anomaly that propagated into the area from the Canadian Arctic. We will also show that with the TOMS and TOVS retrievals for total column ozone, we are able to identify a signature of the upper‐level potential vorticity anomaly. In its mature state, we will show that there were very strong winds, and as a result large fluxes of sensible and latent heat, associated with the polar low. In summary, the 1992 Labrador Sea polar low provides one with an excellent opportunity to study air‐sea interactions and the coupling between the troposphere and stratosphere. The realization that the strong heating of the atmosphere and the concomitant cooling of the ocean associated with these storms may be sufficient to initiate downwelling events in the ocean may represent a hitherto undocumented link between the fast and slow climate systems that deserves further attention.

In this paper, we will describe our analysis of a polar low event that occurred in the Labrador Sea during the winter of 1992. As there are unfortunately no in-situ observations of this event, we will rely on satellite data as well as the high-resolution objective analaysis from the ECMWF to document the environment in which the low developed and the structure of the low itself. We will show that the polar low developed during a cold air outbreak that was precipitated by the passage of an intense synoptic-scale low. The polar low appears to have developed along a linear cloud feature as the result of an interaction between a low-level diabatically induced potential vorticity anomaly and an upper-level potential vorticity anomaly that propagated into the area from the Canadian Arctic. We will also show that with the TOMS and TOYS retrievals for total column ozone, we are able to identify a signature of the upper-level potential vorticity anomaly. In its mature state, we will show that there were very strong winds, and as a result large fluxes of sensible and latent heat, associated with the polar low. In summary, the 1992 Labrador Sea polar low provides one with an excellent opportunity to study air-sea interactions and the coupling between the troposphere and stratosphere. The realization that the strong heating of the atmosphere and the concomitant cooling of the ocean associated with these storms may be sufficient to initiate downwelling events in the ocean may represent a hitherto undocumented link between the fast and slow climate systems that deserves further attention.