Articles | Volume 9, issue 8
Development and technical paper
10 Aug 2016
Development and technical paper |  | 10 Aug 2016

Background error covariance with balance constraints for aerosol species and applications in variational data assimilation

Zengliang Zang, Zilong Hao, Yi Li, Xiaobin Pan, Wei You, Zhijin Li, and Dan Chen

Abstract. Balance constraints are important for background error covariance (BEC) in data assimilation to spread information between different variables and produce balance analysis fields. Using statistical regression, we develop a balance constraint for the BEC of aerosol variables and apply it to a three-dimensional variational data assimilation system in the WRF/Chem model; 1-month forecasts from the WRF/Chem model are employed for BEC statistics. The cross-correlations between the different species are generally high. The largest correlation occurs between elemental carbon and organic carbon with as large as 0.9. After using the balance constraints, the correlations between the unbalanced variables reduce to less than 0.2. A set of data assimilation and forecasting experiments is performed. In these experiments, surface PM2.5 concentrations and speciated concentrations along aircraft flight tracks are assimilated. The analysis increments with the balance constraints show spatial distributions more complex than those without the balance constraints, which is a consequence of the spreading of observation information across variables due to the balance constraints. The forecast skills with the balance constraints show substantial and durable improvements from the 2nd hour to the 16th hour compared with the forecast skills without the balance constraints. The results suggest that the developed balance constraints are important for the aerosol assimilation and forecasting.

Short summary
The aerosol data assimilation and forecasts can be improved by adopting balance constraints that spread observation information across variables, thus producing balanced initial distributions. Surface and aircraft aerosol observations were assimilated to demonstrate the impact of the balance constraints. The results showed that the forecasting experiment with balance constraints is more skillful and durable than the experiment without balance constraints.