Toward the implementation of an Ensemble based Data Assimilation System over Southern South America.

DILLON, MARIA EUGENIA; GARCIA SKABAR, YANINA; KALNAY, EUGENIA; RUIZ, JUAN; COLLINI, ESTELA

Buenos Aires

Workshop; Big Data & Environment; 2015

One of the big challenges in numerical weather prediction is to reduce the uncertainty in the initial conditions. At the National Meteorological Service (SMN) of Argentina, many efforts have been carried out to address this issue: a regional data assimilation system is beingdeveloped using the Local Ensemble Transform Kalman Filter (LETKF) coupled with the Weather Research and Forecasting Modeling System (WRF). The selection of this method is motivated mainly because it generates an ensemble of initial conditions that can be used forensemble forecasting and also by the favorable results obtained by many authors and its computational efficiency. In this manner, two important objectives are achieved: the improvement of the initial conditions and the explicit estimation of its uncertainty that can be propagated forward in time using an ensemble of forecasts.In this work, the LETKF-WRF data assimilation system is evaluated over Southern South America during a two month period including November and December 2012. A horizontal resolution of 40 km and 40 ensemble members are used, and the analyses are obtained with a 3-hour frequency, assimilating the observations available in the PREPBUFR files from the Global Data Assimilation System (GDAS-NCEP).The performance of this regional data assimilation system is evaluated considering different configurations. We explore the sensitivity of the analysis accuracy to: the use of a multischeme structure combining cumulus and planetary boundary layer parameterizations (Dillon et al., inpress); the inclusion of the vertical profiles retrieved from the Atmospheric Infrared Sounder (AIRS) in the assimilation cycles; and the use of perturbed boundary conditions.The impact of each experiment upon the assimilation system is evaluated through the verification of the 6-hour forecasts. In addition, an intense precipitation case study is selected in order to evaluate the performance of a 48-hour forecast. The results are encouraging and indicate that various improvements are achieved with the particular configurations, suggesting that all of them should be considered for the operational implementation of this system.Therefore, future work would focus on the implementation of a complete configuration taking advantage of a multischeme ensemble, the assimilation of the AIRS retrievals and the perturbed boundary conditions altogether. In addition, the evaluation of the hybrid methodproposed by Penny (2014) that combines Ensemble Kalman Filter and 3D-Var would be carried out.