Browsing by Author "Deushi, Makoto"
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Item A machine learning examination of hydroxyl radical differences among model simulations for CCMI-1(EGU Publications, 2020-02-05) Nicely, Julie M.; Duncan, Bryan N.; Hanisco, Thomas F.; Wolfe, Glenn M.; Salawitch, Ross J.; Deushi, Makoto; Haslerud, Amund S.; Jöckel, Patrick; Josse, Béatrice; Kinnison, Douglas E.; Klekociuk, Andrew; Manyin, Michael E.; Marécal, Virginie; Morgenstern, Olaf; Murray, Lee T.; Myhre, Gunnar; Oman, Luke D.; Pitari, Giovanni; Pozzer, Andrea; Quaglia, Ilaria; Revell, Laura E.; Rozanov, Eugene; Stenke, Andrea; Stone, Kane; Strahan, Susan; Tilmes, Simone; Tost, Holger; Westervelt, Daniel M.; Zeng, GuangThe hydroxyl radical (OH) plays critical roles within the troposphere, such as determining the lifetime of methane (CH₄), yet is challenging to model due to its fast cycling and dependence on a multitude of sources and sinks. As a result, the reasons for variations in OH and the resulting methane lifetime (τCH₄), both between models and in time, are difficult to diagnose. We apply a neural network (NN) approach to address this issue within a group of models that participated in the Chemistry-Climate Model Initiative (CCMI). Analysis of the historical specified dynamics simulations performed for CCMI indicates that the primary drivers of τCH₄ differences among 10 models are the flux of UV light to the troposphere (indicated by the photolysis frequency JO¹D), the mixing ratio of tropospheric ozone (O₃), the abundance of nitrogen oxides (NOₓ≡NO+NO₂), and details of the various chemical mechanisms that drive OH. Water vapour, carbon monoxide (CO), the ratio of NO:NOₓ, and formaldehyde (HCHO) explain moderate differences in τCH₄, while isoprene, methane, the photolysis frequency of NO₂ by visible light (JNO₂), overhead ozone column, and temperature account for little to no model variation in τCH₄. We also apply the NNs to analysis of temporal trends in OH from 1980 to 2015. All models that participated in the specified dynamics historical simulation for CCMI demonstrate a decline in τCH₄ during the analysed timeframe. The significant contributors to this trend, in order of importance, are tropospheric O₃, JO¹D, NOₓ, and H₂O, with CO also causing substantial interannual variability in OH burden. Finally, the identified trends in τCH₄ are compared to calculated trends in the tropospheric mean OH concentration from previous work, based on analysis of observations. The comparison reveals a robust result for the effect of rising water vapour on OH and τCH₄, imparting an increasing and decreasing trend of about 0.5 % decade⁻¹, respectively. The responses due to NOₓ, ozone column, and temperature are also in reasonably good agreement between the two studies.