Assimilating C-Band Radar Data for High-Resolution Simulations of Precipitation: Case Studies over Western Sumatra

Bojun Zhu,Agie Wandala Putra,Zhiqiu Gao,Zhaoxia Pu

doi:10.3390/rs14010042

Abstract

Accurate high-resolution precipitation forecasts are critical yet challenging for weather prediction under complex topography or severe synoptic forcing. Data fusion and assimilation aimed at improving model forecasts, as one possible approach, has gained increasing attention in past decades. This study investigates the influence of the observations from a C-band Doppler radar over the west coast of Sumatra on high-resolution numerical simulations of precipitation around its vicinity under the Madden–Julian oscillation (MJO) in January and February 2018. Cases during various MJO phases were selected for simulations with an advanced research version of the weather research and forecasting (WRF) model at a cloud-permitting scale (~3 km). A 3-dimensional variational (3DVAR) data assimilation method and a hybrid three-dimensional ensemble–variational data assimilation (3DEnVAR) method, based on the NCEP Gridpoint Statistical Interpolation (GSI) assimilation system, were used to assimilate the radar reflectivity and the radial velocity data. The WRF-simulated precipitation was validated with the Integrated Multi-satellitE Retrievals for GPM (IMERG) precipitation data, and the fractions skill score (FSS) was calculated in order to evaluate the radar data impacts objectively. The results show improvements in the simulated precipitation with hourly radar data assimilation 6 h prior to the simulations. The modifications with assimilation were validated through the observation departure and moist convection. It was found that forecast improvements are relatively significant when precipitation is more related to local-scale convection but rather small when the background westerly wind is strong under the MJO active phase. The additional simulation experiments, under a 1- or 2-day assimilation cycle, indicate better improvements in the precipitation simulation with 3DEnVAR radar assimilation than those with the 3DVAR method.

Highlights

Convective bands were available around the western coast in Case 1, and Madden–Julian oscillation (MJO) phase 2 (P2) occurred during the Maritime Continent (MC) preconditioning stage
The exact mechanism was not investigated in MJO P1 remainsand an interesting topic for future study
Integrated Multi-satellitE Retrievals for GPM (IMERG) precipitation products were utilized for the validation of the CTL, 3-dimensional variational (3DVAR), and hybrid 3DEnVAR simulations

Summary

Introduction

The Maritime Continent (MC), which includes islands of various sizes, forms, and terrain types [1], lies along the equator between the Indian Ocean and the Pacific Ocean, with a zonal width of more than 5000 km [2]. Many studies have shown the MC’s essential role in the global weather-climate continuum [3,4,5,6]. The Madden–Julian oscillation (MJO) [7,8], a significant fluctuation in tropical weather, can interact with the MC on weekly to monthly timescales. MJO’s large-scale tropical disturbance usually propagates eastward at a speed of approximately 5 m s−1 , with a typical zonal extent of roughly 12,000–20,000 km and a period of 30–90 days [9]. An MJO passing the MC or being blocked by Remote Sens.

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Conclusion