Abstract
The Brazilian Northeast (BNE) is located in the tropical region of Brazil. It is bounded by the Atlantic Ocean, and its climate and vegetation are strongly affected by continental plateaus. The plateaus keep the humid air masses to the east and are responsible for the rain episodes, and at the west side the northeastern hinterland and dry air masses are observed. This work is a case study that aims to evaluate the impact of updating the model initial condition using the 3DEnVar (Three-Dimensional Ensemble Variational) system in heavy rain episodes associated with Mesoscale Convective Systems (MCS). The results were compared to 3DVar (Three-Dimensional Variational) and EnSRF (Ensemble Square Root Filter) systems and with no data assimilation. The study enclosed two MCS cases occurring on 14 and 24 January 2017. For that purpose, the RMS (Regional Modeling System) version 3.0.0, maintained by the Center for Weather Forecasting and Climate Studies (CPTEC), used two components: the Weather Research and Forecasting (WRF) mesoscale model and the GSI (Gridpoint Statistical Interpolation) data assimilation system. Currently, the RMS provides the WRF initial conditions by using 3DVar data assimilation methodology. The 3DVar uses a climatological covariance matrix to minimize model errors. In this work, the 3DEnVar updates the RMS climatological covariance matrix through the forecast members based on the errors of the day. This work evaluated the improvements in the detection and estimation of 24 h accumulated precipitation in MCS events. The statistic index RMSE (Root Mean Square Error) showed that the hybrid data assimilation system (3DEnVar) performed better in reproducing the precipitation in the MCS occurred on 14 January 2017. On 24 January 2017, the EnSRF was the best system for improving the WRF forecast. In general, the BIAS showed that the WRF initialized with different initial conditions overestimated the 24 h accumulated precipitation. Therefore, the viability of using a hybrid system may depend on the hybrid algorithm that can modify the weights attributed to the EnSRF and 3DVar matrix in the GSI over the assimilation cycles.
Highlights
Introduction iationsAccurate weather forecasts of severe weather phenomena that affects the economy across the globe are essential for short to long-term planning of various human activities in order to avoid socio-economic damage caused by natural disasters
This work evaluated the 3DVar, the Ensemble Square Root Filter (EnSRF), and the 3DEnVar DA impact in the generation of analyses in order to verify their contributions in the heavy rain forecasts in the Brazilian Northeast (BNE)
We concluded that the inclusion of the errors of the day in the 3DVar backgound covariance matrix, i.e, by using the EnSRF and the 3DEnVar, the analysis was improved and it led to a better precipitation forecast
Summary
Accurate weather forecasts of severe weather phenomena that affects the economy across the globe are essential for short to long-term planning of various human activities in order to avoid socio-economic damage caused by natural disasters (e.g., landslides and floods). For the Brazilian Northeast (BNE), which is the region of study of this work, according to data from the portal of the National Confederation of Cities [1] for a long dry period, BNE had losses totaled in the order of BRL 52 billion between 2012 and 2015. Intense rain events have caused deaths, floods, and rupture of dams, causing many social and economic losses in only one day [2]. It is essential to have a good representation of the initial conditions of the atmosphere in order to obtain an accurate weather forecast.
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