Coupled Stratospheric Chemistry–Meteorology Data Assimilation. Part I: Physical Background and Coupled Modeling Aspects

Abstract

A coupled stratospheric chemistry–meteorology model was developed by combining the Canadian operational weather prediction model Global Environmental Multiscale (GEM) with a comprehensive stratospheric photochemistry model from the Belgian Assimilation System for Chemical ObsErvations (BASCOE). The coupled model was called GEM-BACH for GEM-Belgian Atmospheric CHemistry. The coupling was made across a chemical interface that preserves time-splitting while being modular, allowing GEM to run with or without chemistry. An evaluation of the coupling was performed by comparing the coupled model, refreshed by meteorological analyses every 6 h, against the standard offline chemical transport model (CTM) approach. Results show that the dynamical meteorological consistency between meteorological analysis times far outweighs the error created by the jump resulting from the meteorological analysis increments at regular time intervals, irrespective of whether a 3D-Var or 4D-Var meteorological analysis is used. Arguments in favor of using the same horizontal resolution for chemistry, meteorology, and meteorological and chemical analysis increments are also presented. GEM-BACH forecasts refreshed by meteorological analyses every 6 h were compared against independent measurements of temperature, long-lived species, ozone and water vapor. The comparison showed a relatively good agreement throughout the stratosphere except for an upper-level warm temperature bias and an ozone deficit of nearly 15%. In particular, the coupled model simulation during an ozone hole event gives better ozone concentrations than a 4D-Var chemical assimilation at a lower resolution.

Stratospheric Fluorine as a Tracer of Circulation Changes: Comparison Between Infrared Remote‐Sensing Observations and Simulations With Five Modern Reanalyses

“…It includes 65 chemical species interacting through 174 gas‐phase reactions, 9 heterogeneous reactions, and 60 photolysis reactions. The expansion of the scheme was made to include HFC species and their corresponding reactions because they are not included in other work based on the BASCOE chemical scheme (Errera et al., 2019; Huijnen et al., 2016; Ménard et al., 2020). The chemistry schemes related to the transformation of organic fluorine sources into inorganic fluorine reservoirs are simplified in BASCOE CTM.…”

Global Scale Inversions from MOPITT CO and MODIS AOD

“…Global aerosol, chemical, and meteorological reanalyses are consistently produced within the same DA infrastructure [25][26][27][28] to better understand the role of aerosols and chemistry in the climate system. A common approach in gas-phase chemical data assimilation systems is to estimate atmospheric states and/or emissions using variational [29][30][31][32][33][34] and ensemble Kalman filter approaches [19,[35][36][37][38], often with offline CTMs. Notable exceptions include using coupled chemistry-meteorology DA for stratospheric application [39], for the upper atmosphere [40].…”

A 22 GHz Pseudo-correlation Water Vapor Radiometer.

“…It is characterized by high concentrations of ozone, therefore, heating by absorption of ultraviolet solar radiation due to ozone molecules is responsible for the temperature increase in the stratopause Brasseur (2013); since O 2 photodissociates, resulting in atomic oxygen and very rapidly recombines with molecular oxygen giving rise to ozone O 3 . These "recombination type" chemical reactions are exothermic and release so much heat that they transform the vertical stratification of the atmosphere into the stratosphere, here the content of water vapor in this layer is very low Ménard et al (2020).…”