Introducing new lightning schemes into the CHASER (MIROC) chemistry–climate model
Authors: He Yanfeng , Hoque Hossain Mohammed Syedul , Sudo Kengo 
Published: July 2022

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 dominant NOx source in the
middle to upper troposphere. Therefore, improving the prediction accuracy of
lightning and LNOx in chemical climate models is crucially important.
This study implemented three new lightning schemes with the CHASER (MIROC)
global chemical transport and climate model. The first lightning scheme is based
on upward cloud ice flux (ICEFLUX scheme). The second one (the original
ECMWF scheme), also adopted in the European Centre for Medium-Range Weather
Forecasts (ECMWF) forecasting system, calculates lightning flash rates as a
function of QR (a quantity intended to represent the charging rate of
collisions between graupel and other types of hydrometeors inside the charge
separation region), convective available potential energy (CAPE), and
convective cloud-base height. For the original ECMWF scheme, by tuning the
equations and adjustment factors for land and ocean, a new lightning scheme
called the ECMWF-McCAUL scheme was also tested in CHASER. The ECMWF-McCAUL scheme
calculates lightning flash rates as a function of CAPE and column
precipitating ice. 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 the
Lightning Imaging Sensor (LIS) and 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
ECMWF-McCAUL scheme showed the highest prediction accuracy for the global
distribution of lightning. Evaluation by atmospheric tomography (ATom)
aircraft observations (NO) and tropospheric monitoring instrument (TROPOMI)
satellite observations (NO2) shows that the newly implemented lightning
schemes partially facilitated the reduction of model biases (NO and
NO2), typically within the regions where LNOx is the major source
of NOx, when compared to using 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 changes in
oxidation capacity in different regions of the troposphere. Historical trend
analyses of flash and surface temperatures predicted using CHASER
(2001–2020) show that lightning schemes predicted increasing trends of
lightning or no significant trends, except for one case of 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 17.69 % ∘C−1 and 2.50 % ∘C−1, respectively, with
and without meteorological nudging. The un-nudged runs also included the
short-term surface warming but without the application of meteorological
nudging. Furthermore, the ECMWF schemes predicted a larger increasing trend
of lightning flash rates under the short-term surface warming by a factor of
4 (ECMWF-McCAUL scheme) and 5 (original ECMWF scheme) compared to the CTH
scheme without nudging. In conclusion, the three new lightning schemes
improved global lightning prediction in the CHASER model. However, further
research is needed to assess the reproducibility of trends of lightning over
longer periods.