The regional water cycle and surface energy balances with the uncoupled NOAH LSM.

MARIO N. NUÑEZ; ALFREDO ROLLA; ERNESTO H. BERBERY

Toledo

Taller; CLARIS LPB M 36 Meeting Toledo; 2011

CLARIS LPB Project

The water cycle is a key component of the Climate System, and the quality of its represen-tation is intimately linked to the adequate simulation of seasonal and interannual climate variability. For this reason it is important in climate change studies and scenarios, and con-sequently it can also be used to evaluate a model’s performance. Not many estimates of the La Plata water cycle and energy balance have been presented in the literature, and we reference, among others, the paper of Berbery and Barros (2002). These authors were investigated the main components of the hydrologic cycle of the La Plata Basin using a combination of observations, satellite products, and National Centers for Environmental Prediction (NCEP) – National center for Atmospheric Research (NCAR) global reanalysis. Fengge Su and Lettenmair (2009) use the Variable Infiltration Capacity land surface hydrology model (VIC) forced by gridded observed precipitation and tempera-ture for the period 1979–99 to simulate the land surface water balance of the La Plata basin (LPB). The modeled water balance is evaluated with streamflow observations from the ma-jor tributaries of the LPB. The spatiotemporal variability of the water balance terms of the LPB were evaluated using offline VIC model simulations, the 40-yr European Centre for Medium-Range Weather Forecasts Re-Analysis (ERA-40), and inferences obtained from a combination of these two. The seasonality and interannual variability of the water balance terms vary across the basin. The results presented in the paper by Saurral (2010) show that GCMs are not capable of reproducing the hydrological cycle of LPB adequately, resulting in the need for the application of corrective, unbiasing schemes on the meteorological fields. This lack of skill in representing the water cycle of LPB is a result of the climate models having several deficiencies in capturing the actual circulation patterns, which then leads to errors in both the temperature and precipitation fields. Of particular interest is the study of the hydrological cycle components and its extremes. The Plata Basin is the largest water system in South America after the Amazon watershed and the fifth largest in the world. The basin includes important territories belonging to the central and northern Argentina, southern Brazil, southern and eastern Bolivia, most of Uru-guay, and the whole territory of Paraguay, and is home to about 50% of their combined population, generating about 70% of their total GNP. The principal sub-basins are those of the Paraná, Paraguay and Uruguay rivers and the average flow of the basin is 23,000 m³ / s. The surface energy balance is closely related to the water cycle and is an integral part of the interactions between the atmosphere and land surface (soil, vegetation, snowpack). In its most basic form and dismissing some minor magnitude terms, the surface energy responds to a simple balance between the net radiation energy gained at (evapotranspiration) heat fluxes. On an annual basis over La Plata basin, minor components of the balance are the ground heat flux and the energy consumed during snowmelt. Soil moisture is known to have a strong control on the partition between the sensible and latent heat fluxes, known as the Bowen ratio. In addition, the link between surface states and the atmospheric hydro-logic cycle intrinsically involves the atmospheric boundary layer. We performed numerical experiments using the HRLDS (High Resolution Land Data As-similation System) 3.2 model in a uncoupled version. The regional water cycle and surface energy processes of the La Plata basin as estimated from numerical experiments using the HRLDAS 3.2 model were discussed in this paper. The terrestrial water cycle and all energy related computations were analyzed here using a 26-year long (1980 to 2006) data set. 2. Model and experiment design. The High Resolution Land Data Assimilation System (HRLDAS) runs the Noah Land Sur-face Model (Noah LSM) in an uncoupled mode (i.e., not coupled with any atmospheric model) to evolve land surface and soil state variables over some time period. The time pe-riod for which the Noah LSM was run is 26 years. From initial conditions of soil tempera-ture, soil moisture, and other state variables, HRLDAS applies the Noah LSM, forced by analyses of atmospheric conditions, shortwave and long wave radiation, and precipitation, to update the land state. The applications of HRLDAS are often intended to address the issue of the soil state spinup, that is, evolving the soil state variables from low resolution or otherwise uncertain or inadequate initial conditions, through a long integration of Noah LSM forced by rela-tively well observed variables, to a state which is well balanced with respect to the Noah LSM physics, and represents high resolution soil and vegetation conditions appropriate for the model grid. The applications of HRLDAS are often intended to address the issue of the soil state spinup, that is, evolving the soil state variables from low resolution or otherwise uncertain or inadequate initial conditions, through a long integration of Noah LSM forced by rela-tively well observed variables, to a state which is well balanced with respect to the Noah LSM physics, and represents high resolution soil and vegetation conditions appropriate for the model grid. The use of Noah LSM in HRLDAS differs from the implementation of Noah LSM coupled within the Weather Research and Forecasting (WRF) model, in which WRF model results of atmospheric conditions, shortwave and longwave radiation, and precipitation are used to update the land state, which in turn influences the WRF simulation of atmospheric condi-tions. In a coupled implementation (e.g., Noah LSM in WRF), there is a two way feedback of information: atmospheric computations influence the soil results, and soil computations influence atmospheric results. This is in contrast to the uncoupled implementation (e.g., HRLDAS), in which there is no feedback from the Noah LSM to influence the atmospheric forcing conditions. HRLDAS is typically run on the horizontal grid of a mesoscale model, but the motivated user should be able to adapt it for running at individual points or sets of stations. For use with nested model configurations, HRLDAS must be run independently for each nest, and there is no communication of information among nests.