Comment on gmd-2022-39

Abstract

The formation of nitrogen oxides (NOx) associated with lightning activities (hereinafter designated as LNOx) is a major source of NOx. In fact, it is regarded as the most dominant NOx source in the upper troposphere. Therefore, improve the prediction accuracy of lightning and LNOx in chemical climate models is crucially important. This study implemented two new lightning schemes with the CHASER (MIROC) global chemical transport/climate model. The first lightning scheme is based on upward cloud ice flux (ICEFLUX scheme), whereas the second, also adopted in the European Centre for Medium-Range Weather Forecasts (ECMWF) forecasting system (original ECMWF scheme). In the case of the original ECMWF scheme, by tuning the equations and adjustment factors for land and ocean, a modified ECMWF scheme was also tested in CHASER. In the original version of CHASER (MIROC), lightning is initially parameterized with the widely used cloud top height scheme (CTH scheme). Model evaluations with lightning observations conducted using an optical transient detector (OTD) indicate that both the ICEFLUX and ECMWF schemes simulate the spatial distribution of lightning more accurately on a global scale than the CTH scheme does. The modified ECMWF scheme showed the highest prediction accuracy for the global distribution of lightning. Validation by atmospheric tomography (ATom) aircraft observations and tropospheric monitoring instrument (TROPOMI) satellite observations shows that the ICEFLUX scheme reduced the model biases to a greater extent than the ECMWF schemes when compared using the CTH scheme. The effects of the newly introduced lightning schemes on the tropospheric chemical fields were evaluated by comparison with the CTH scheme. Although the newly implemented lightning schemes have a minor effect on the tropospheric mean oxidation capacity compared to the CTH scheme, they led to marked change of oxidation capacity in different regions of the troposphere. Long-term trend analyses of flash and surface temperatures predicted using CHASER (2001–2020) show that lightning schemes predicted an increasing trend of lightning, except for the ICEFLUX scheme, which predicted a decreasing trend of lightning. The global lightning rates of increase during 2001–2020 predicted by the CTH scheme were 17.86 %/°C and 2.60 %/°C, respectively, with and without nudging, which are slightly beyond the range of an earlier study (5 %/°C–16 %/°C). Furthermore, the ECMWF schemes predicted a larger increasing trend of lightning flash rates under global warming by a factor of 3 (modified ECMWF scheme) and 5 (original ECMWF scheme) compared to the CTH scheme without nudging. In conclusion, the two new lightning schemes improved global lightning prediction in the CHASER model. However, further research is needed to assess the reproductivity of long-term trends of lightning.