Opposite effects of aerosols and meteorological parameters on warm clouds in two contrasting regions over eastern China

The sensitivity (S) of cloud parameters to the influence of different aerosol and meteorological pa-
rameters has in most previous aerosol–cloud interaction (aci) studies been addressed using traditional statistical
methods. In the current study, relationships between cloud droplet effective radius (CER) and aerosol optical
depth (AOD, used as a proxy for cloud condensation nuclei, CCN), i.e., the sensitivity (S) of CER to AOD,
are investigated with different constraints of AOD and cloud liquid water path (LWP). In addition to traditional
statistical methods, the geographical detector method (GDM) is applied in this study to quantify the relative
importance of the effects of aerosol and meteorological parameters, as well as their interaction, on S. Moderate
Resolution Imaging Spectroradiometer (MODIS) C6 L3 data and European Centre for Medium-Range Weather
Forecasts (ECMWF) ERA-5 reanalysis data, for the period from 2008 to 2022, were used to investigate aci over
eastern China. Two contrasting areas were selected: the heavily polluted Yangtze River Delta (YRD) and a rela-
tively clean area over the East China Sea (ECS). Linear regression analysis shows that CER decreases with the
increase in AOD (negative S) in the moderately polluted atmosphere (0.1 < AOD < 0.3) over the ECS, whereas,
in contrast, CER increases with increasing AOD (positive S) in the polluted atmosphere (AOD > 0.3) over the
YRD. Evaluation of S as function of the LWP shows that in the moderately polluted atmosphere over the ECS,
S is negative in the LWP interval [40 g m−2, 200 g m−2], and the sensitivity of CER to AOD is substantially
stronger as LWP is larger. In contrast, in the polluted atmosphere over the YRD, S is positive in the LWP in-
terval [0 g m−2, 120 g m−2] and does not change notably as function of LWP in this interval. The study further
shows that over the ECS, the CER is larger for higher low tropospheric stability (LTS) and relative humidity
(RH) but lower for higher pressure vertical velocity (PVV). Over the YRD, there is no significant influence of
LTS on the relationship between CER and AOD. The GDM has been used as an independent method to ana-
lyze the sensitivity of cloud parameters to AOD and meteorological parameters (RH, LTS and PVV). The GDM
has also been used to analyze the effects of interactions between two parameters and thus obtain information

on confounding meteorological effects on the aci. Over the ECS, cloud parameters are sensitive to almost all parameters considered except for cloud top pressure (CTP), and the sensitivity to AOD is larger than that to any of the meteorological factors. Among the meteorological factors, the cloud parameters are most sensitive to PVV and least sensitive to RH. Over the YRD, the explanatory power of the sensitivity of cloud parameters to AOD and meteorological parameters is much smaller than over the ECS, except for RH, which has a statistically significant influence on CTP and can explain 74 % of the variation of CTP. The results from the GDM analysis show that cloud parameters are more sensitive to the combination of aerosol and a meteorological parameter than to each parameter alone, but confounding effects due to co-variation of both parameters cannot be excluded