Use of a Water-Tracer Modeling Methodology to Characterize the Atmospheric Water Budget along North-Central Chile

The region in the north and central parts of Chile presents a marked latitudinal gradient in ambient humidity, ranging from the driest climate of the Atacama Desert to the Mediterranean climate of the central part of the country. A better understanding of the variability of the physical processes affecting the atmospheric water budget is important for the prediction and adequate management of the water resources in this region.

In this work we use a methodology designed to characterize the relative contributions of different sources in the atmospheric water budget at various time and spatial scales. Our modeling system is based on the Penn State/NCAR Mesoscale Model (MM5). We add to it evolution equations for a water tracer that follows the water substance introduced (through evapo-transpiration processes) in limited regions of the domain and in particular periods during the simulation. This methodology permits the visualization of the time history of water substance originating in particular areas. It also allows us to estimate the relative importance of different source areas in the water budget of a given basin. For the region of interest, for example, it is important to assess the relative contribution of water originating in sea-surface evaporation, local evaporation and import of water from neighboring basins.

In order to improve the treatment of transport of water substance in the atmospheric boundary layer, MM5 is run with the PSU 1.5-order turbulent kinetic energy predicting (TKE) sub-model. This scheme includes an improved treatment of mixing in cloudy layers, which is important in this case in which low-level stratus are a quasi-permanent feature along the coast of northern and central Chile.

Results will be shown for the simulation of a representative multi-day period. The applicability of the model results to short-term water budgets will be discussed, as well as the applicability of conclusions for short-term cases to long-term water budgets.