Ensemble data assimilation of total column ozone using a coupled meteorology–chemistry model and its impact on the structure of Typhoon Nabi (2005)

摘要

Ozone (O) plays an important role in chemical reactions and is usuallyincorporated in chemical data assimilation (DA). In tropical cyclones (TCs),O usually shows a lower concentration inside the eyewall and anelevated concentration around the eye, impacting meteorological as well aschemical variables. To identify the impact of O observations on TCstructure, including meteorological and chemical information, we developed acoupled meteorology–chemistry DA system by employing the Weather Research andForecasting model coupled with Chemistry (WRF-Chem) and an ensemble-based DAalgorithm – the maximum likelihood ensemble filter (MLEF). For a TC casethat occurred over East Asia, Typhoon Nabi (2005), our results indicate thatthe ensemble forecast is reasonable, accompanied with larger background stateuncertainty over the TC, and also over eastern China. Similarly, theassimilation of O observations impacts meteorological and chemicalvariables near the TC and over eastern China. The strongest impact on airquality in the lower troposphere was over China, likely due to the pollutionadvection. In the vicinity of the TC, however, the strongest impact onchemical variables adjustment was at higher levels. The impact onmeteorological variables was similar in both over China and near the TC. Theanalysis results are verified using several measures that include the costfunction, root mean square (RMS) error with respect to observations, anddegrees of freedom for signal (DFS). All measures indicate a positive impactof DA on the analysis – the cost function and RMS error have decreased by16.9 and 8.87 %, respectively. In particular, the DFS indicates a strongpositive impact of observations in the TC area, with a weaker maximum overnortheastern China.