Diurnal variation of heavy rainfall over the Beijing-Tianjin-Hebei region : Role of 1 aerosol cloud effect and its sensitivity to moisture 2 3

Abstract. Our recent study found that, during 2002–2012, the diurnal variation of heavy rainfall over Beijing-Tianjin-Hebei (BTH) region exhibits different characteristics between clean and polluted environment. Here we use satellite cloud products together with meteorology and aerosol data to further examine the aerosol impact on the associated clouds focusing on its sensitivity to moisture. During the days with large aerosol loading, the characteristics of earlier starting time, earlier peak hour and the longer duration of heavy rainfall are usually accompanied by increased cloud fraction, reduced cloud top height and increased/reduced liquid/ice effective radius. However, the aerosol effects on the cloud top and liquid effective radius are distinct at lower and higher humidity. Different from the radiative effect that black carbon heats the lower troposphere and may generate the earlier start of heavy rainfall, the aerosol cloud effect enhances the efficiency of precipitation and advances the rainfall peak, which may be ascribed to increased cloud droplet number and cloud water, enhanced collision-coalescence and accelerated rainfall formation when the background moisture supply is sufficient. The speculation warrants further numerical experiment to verify.



Introduction 50
Aerosols modify the global hydrologic cycle through both radiative effect (direct effect) and cloud effect 51 (indirect effect) (IPCC, 2013).On the one hand, through absorbing or scattering solar radiation, aerosols can 52 lead to the air aloft heating (e.g.Jacobson 2001; Lau et al. 2006) or the surface cooling (Lelieveld and 53 Heintzenberg 1992; Guo et al. 2013;Yang et al., 2018), which changes the atmospheric vertical static stability 54 and modulates rainfall (e.g.Rosenfeld et al. 2008).On the other hand, water-soluble aerosols serving as cloud 55 condensation nuclei (CCN) could affect the warm-rain processes and cold-rain processes through influencing 56 the cloud droplet size distributions, cloud top heights and the depth of the mixed-phase cloud (Jiang et al.,57 2002; Givati and Rosenfeld 2004; Chen et al., 2011; Lim and Hong 2012; Tao et al., 2012).58 Beijing-Tianjin-Hebei (BTH) region is the heaviest aerosol polluted area in China and concerns have been 59 raised about the aerosol-radiation-cloud-precipitation interaction over this region.The impact of aerosols on 60 light rainfall or warm-rain processes over BTH region almost reaches consistent agreement (e.g., Qian et al., 61 2009), but aerosols impact on the heavy convective rainfall in this region still has large uncertainties (Wang et 62 al., 2009;Guo et al., 2014;Wang et al., 2016).63 The clouds that can generate the heavy convective rainfall in BTH region usually contain warm clouds, cold 64 clouds and mixed-phase clouds (e.g.Guo et al., 2015).Due to the complicacy of these clouds, aerosol indirect 65 effect on associated clouds of heavy rainfall is more complicated than its direct effect (Sassen et al., 1995;66 Sherwood, 2002;Jiang et al., 2008, Tao et al., 2012).For warm clouds, by serving as CCN for more cloud 67 droplets, aerosols can increase cloud albedo (Twomey, 1977), increase the cloud lifetime (Albrecht, 1989), 68 and enhance thin cloud thermal emissivity (Garrett and Zhao, 2006), which were collectively known as condition of moisture, the contrary results of aerosol impact on clouds have been also reported in observations.75 e.g., "Anti-Twomey" effect denotes that the cloud droplet effective radius increases with aerosol amount when 76 the environment has a plenty of moisture supply (Yuan et al., 2008;Bulgin et al., 2008;Panicker et al., 2010;77 Jung et al., 2013;Harikishan et al., 2016;Qiu et al., 2017).Besides, the influence of aerosols on ice clouds 78 also depends upon the moisture content (Jiang et al., 2008).Therefore, how the aerosols modify the clouds 79 associated with heavy convective rainfall does not reach a consensus, particularly if considering different 80 moisture conditions.81 Heavy convective rainfall usually occurs within one day.Several previous studies have found that the 82 aerosols can modify the rainfall diurnal variation in other regions of China (Fan et al., 2015;Guo et al., 2016;83 Lee et al., 2016).However, the above studies do not address the changes of associated cloud features and 84 don't include the different moisture conditions.Although our recent work over BTH region (Zhou et al. 2018) 85 attempted to remove the meteorological effect including moisture and circulation and found that the peak of 86 heavy rainfall diurnal variations shifts earlier under polluted condition, it only excluded the extreme moisture 87 conditions and focused on aerosol radiative effect on the rainfall diurnal variation.Therefore, this study aims 88 to deepen the previous study (Zhou et al., 2018) and extends the investigation into the following questions: (1) 89 how do aerosols modify different features of diurnal rainfall variation (starting time, peak time and duration)?90 (2) how do aerosols influence cloud characteristics with inclusion of moisture condition?(3) what distinct 91 roles do the aerosol radiative effect and cloud effect play on the different developing phase of heavy rainfall in 92 diurnal variation?To solve the questions, the paper is organized as following: The data and methodology are 93 introduced in Sect. 2. Section 3 presents the distinct characteristics of rainfall diurnal variation on 94 clean/polluted days.Section 4 addresses the aerosol effect on cloud with inclusion of moisture.Section 5 95 discusses the distinct roles of the radiative effect and cloud effect of aerosols that play on diurnal variation of 96 heavy rainfall.Conclusion will be given in Sect.6. 97 daily precipitation was rejected), the internal consistency check (wiping off the erroneous records caused by 108 incorrect units, reading, or coding) and spatial consistency check (comparing the time series of hourly 109 precipitation with nearby stations) [Shen et al., 2010].Here we chose 176 plain stations below the topography 110 of 100 meter in BTH region, which is similar with our previous work because we purposely removed the 111 orographic effect (Zhou et al., 2018).The record analyzed here is the period of 2002 to 2012.112

Aerosol data 113
Aerosol optical depth (AOD), which is a proxy for the amount of aerosol particles in a column of the 114 atmosphere and serves as an indicator for the division of the aerosol pollution condition in this study, was 115 obtained from MODIS (Moderate Resolution Imaging Spectroradiometer) Collection 6 L3 aerosol product 116 with the horizontal resolution of 1°x1° onboard the Terra satellite (Tao et al., 2015).The Collection 6 aerosol 117 dataset is created from three separate retrieval algorithms that operate over different surface types: the two 118 "Dark Target" (DT) algorithms for retrieving (1) over ocean (dark in visible and longer wavelengths) and (2) 119 over vegetated/dark-soiled land (dark in the visible), plus the "Deep Blue" (DB) algorithm developed 120 originally for retrieving (3) over desert/arid land (bright in the visible) (Levy et al., 2013).The merged data 121 combing DB and DT retrievals in Collection 6 product was used in this study.The quality assurance of 122 marginal or higher confidence was used in this study.The reported uncertainty in MODIS AOD data is on the 123 order of (-0.02-10%), (+0.04+10%) (Levy et al., 2013).The Terra satellite overpass time at the equator is 124 around 10:30 local solar time in the daytime, which we suppose is before the occurrence of most heavy 125 rainfall events since the starting time of heavy rainfall is mostly after 12:00 LST (Fig. 1).126

MACC-II (Monitoring Atmospheric Composition and Climate Interim Implementation) reanalysis product 127
provided by ECMWF (the European Centre for Medium-Range Weather Forecasts), which assimilates total 128 AOD retrieved by MODIS to correct for model departures from observed aerosols (Benedetti et al., 2009), 129 provided the two-dimensional AOD and three-dimensional aerosol mass concentration datasets for different 130 kinds of aerosols (BC, sulfate, organic matter, mineral dust and sea salt).MACC-II reanalysis products are 131 Airborne Simulator (MAS) instrument (https://modis-atmos.gsfc.nasa.gov/products/cloud).Likewise, the 145 quality assurance of marginal or higher confidence was used in this study.146 The three-dimensional cloud variables, such as CF, cloud liquid water and cloud ice water, were obtained 147 from MERRA2 (the second Modern-Era Retrospective analysis for Research and Applications) reanalysis 148 datasets.MERRA2 reanalysis data is undertaken by NASA for the satellite era using GEOS-5 (version 5 of 149 the Goddard Earth Observing System Data Assimilation System), which is the first long-term global 150 reanalysis to assimilate space-based observations of aerosols and represent their interactions with other 151 physical processes in the climate system.The horizontal resolution is 0.624°x0.5°and the vertical resolution is 152 42 levels with three-hour intervals (Rienecker et al., 2008).Since the clouds associated with heavy rainfall in 153 the BTH region during the early summer contain warm clouds, cold clouds and mixed-phase clouds (e.g.Guo 154 et al., 2015), we purposely selected the clouds with its top pressure above 600 hPa because the 0℃ isotherm 155 of BTH region is nearly located at this height.156

Other meteorological data 157
Other meteorological factors, including the wind, temperature, and relative humidity (RH), were obtained 158 from the ERA-Interim reanalysis datasets with 1°x1 °horizontal resolution and 37 vertical levels at six-hour 159 intervals.ERA-Interim is the global atmospheric reanalysis produced by ECMWF, which covers the period 160 from 1979 to near-real time (Dee et al., 2011).To unify the datasets, we interpolated the gridded datasets into 161 stations using the average value in a 1°×1° grid as the background condition of each rainfall station.162 163

Selection of sub-season and circulation 165
Consistent with our previous work (Zhou et al., 2018), we focused on the early summer period (1 June to 20 166 July), which is before the start of the large-scale rainy season over the BTH region, to better identify the effect 167 of aerosols on local convective precipitation.And to unify the background atmospheric circulation, we only 168 selected the rainfall days with southwesterly flow, which is the dominant circulation (around 40%) over the 169

Classification of the heavy rainfall and clean/polluted conditions 171
With the circulation of southwesterly, we selected heavy rainfall samples when the hourly precipitation 172 amount was more than 8.0 mm/hour (defined by Atmospheric Sciences Thesaurus, 1994).The 25 th and 75 th 173 AOD (the value is 0.98 and 2.00 respectively) were used as the thresholds of clean and pollution condition.It 174 shows that there are 514 cases of heavy rainfall on polluted days and 406 cases of that on clean days.175 Using the same percentile method, we chose cases of more BC/sulfate when the AOD of BC/sulfate is 176 larger than the 75 th AOD of itself in all rainy days with southwesterly, and cases of less BC/sulfate when that 177 is less than the 25 th AOD of itself in the same condition.Accordingly, we selected 459 cases of more BC and 178 274 cases of less BC with heavy rainfall.Similarly, 361 cases of more sulfate and 419 cases of less sulfate 179 with heavy rainfall were selected.180

Statistical analysis 181
We adopted the probability distribution function (PDF) to compare the features of heavy rainfall and cloud 182 variables on clean and pollution days or in different condition of aerosols, by which we can understand the 183 changes of rainfall/cloud properties more comprehensively than by the mean state.Student's t-test was used to 184 check the significance of all the differences of the variables between different conditions of pollution.185 186

Distinct characteristics of heavy rainfall diurnal variation associated with aerosol pollution 187
Our previous study (Zhou et al. 2018) has reported the distinct peak shifts of rainfall diurnal variation between 188 clean days and polluted days over the BTH region during early summer.The PDF of the heavy rainfall peak 189 time shows that the peak time is about two hours earlier on the polluted days (20:00 LST) than that on the 190 clean days (22:00 LST) (Fig. 1b).To comprehensively recognize the change of rainfall diurnal variation 191 associated with air qualities, here we examined the PDF of the starting time, the duration and the intensity 192

Characteristics of clouds on clean and polluted days 206
To understand the cloud effect of aerosols on heavy rainfall diurnal variation, we need to recognize the 207 associated cloud features on clean and polluted days.The differences of cloud diurnal features were examined 208 in both macroscopic properties (including CF, CTP, COT and CWP) and microscopic properties (including 209 CER) between the clean and polluted circumstances, as shown in Fig. 2. The PDF distribution of CF is 210 significantly different between clean and polluted conditions, which shows that the CF with maximum 211 occurrence frequency on the clean days is nearly 50% while reaches more than 90% on the polluted days.The 212 PDF of CTP on the polluted days shows a decrease at 200-300 hPa but an increase at around 400 hPa with a 213 mean increase of 24.3 hPa, which indicates the cloud top height is lower on the polluted days.214 The COT, CWP and CER were further analyzed for the liquid and ice portions of clouds as shown in Fig. 2. 215 Both liquid and ice COT on polluted days exhibit a significant increase compared with that on clean days.The 216 mean amount of liquid COT increases by 3.9 and ice COT increases by 6.3.Similar with COT, the amount of 217 liquid and ice CWP increase on polluted days.And the mean amount of liquid CWP increases by 40.3 g/m 2 218 and ice CWP increases by 94.4 g/m 2 .The PDF of liquid CER also shows shifts to the larger size and its mean 219 value increases by 0.6 μm on polluted days.In contrast with the CER of liquid clouds, the CER of ice clouds 220 shows a slight shift to the smaller size with an averaged decrease of 2.8 μm.Thus, except for the ice CER, the 221 other cloud variables consistently exhibit increases on the polluted days.222 Figure 3 shows the distinct variation of three-dimensional cloud liquid/ice water on clean and polluted days 223 as well as their differences.On clean days, the liquid clouds are mainly located between 300 hPa and 850 hPa, 224 with two maximum layers respectively at 350 hPa and 700 hPa (Fig. 3a).The major characteristics are that the 225 peak of liquid water occurs in the evening (at 20:00-23:00 LST) (Fig. 3a) while the ice water appears in the 226 mid-night (at 20:00-3:00 LST) (Fig. 3d).Compared with clean condition, the amount of the liquid and ice 227 water are both significantly increased on polluted days.Meanwhile, the peak value of liquid water appears 228 much earlier by almost 8 hours than that on clean days.i.e., the peak of the liquid water occurs at 14:00 LST 229 under pollution (Fig. 3b).The ice water exhibits the similar shift of its peak under pollution and its maximum 230 center appears in the afternoon (at 14:00-17:00 LST) rather than the mid-night (Fig. 3e).The difference of ice 231 water between polluted and clean condition also indicates that the cloud top on polluted days is lower than 232 that on clean days (Fig. 3f), which is consistent with the result in Fig. 2. 233 According to the above results, the increased aerosols correspond to the increase of CF, COT, CWP of both 234 while 60-90% RH dominates the polluted cases (Fig. 4b), which indicates that the above changes of cloud 247 properties on the polluted days in Sect.4.1 often occur in the condition of higher RH.To identify the effect of 248 aerosols on the properties of clouds, we purposely investigated the changes of cloud properties with inclusion 249 of moisture change respectively on the clean days and polluted days (Fig. 5).250 A common feature is that all examined variables of clouds exhibit increases along with the increase of 251 moisture on both clean and polluted days (Fig. 5).If fixing the moisture, the amounts of CF, COT (both liquid 252 and ice), CWP (both liquid and ice) become larger on the polluted days, which are consistent with the 253 above-mentioned results without removing the moisture effect in Sect.4.1.However, the aerosol effect on 254 CTP is evidently distinct between low and high RH conditions (Fig. 5f).When the RH is relatively low 255 (<70%), the amount of CTP on polluted days is larger than that on clean days.In contrast, the CTP becomes 256 smaller when the RH is relatively high (>70%).That is to say, aerosols reduce the cloud top at lower RH but 257 increase it at higher RH.258 As Fig. 4b has shown, usually the RH is lower (40-60%) on clean days and higher (60-90%) on polluted 259 days.The average of CTP on clean days at the RH of 40-60% is nearly 350 hPa but 420 hPa on polluted days 260 with the RH of 60-90% (Fig. 5d).Therefore, the cloud top on polluted days is normally lower than that in 261 clean cases, which is consistent with the result in Sect.4.1.In summary, although the aerosols can lift the 262 cloud top when RH is higher, the cloud top on polluted days is still lower than that on clean days due to their 263 different moisture conditions.264 To examine if the aerosol effect on cloud microphysical property is modified by moisture, we further 265 investigated the variation of the CER between clean and polluted condition along with different CWPs, as 266 shown in Fig. 6.The result exhibits that aerosol effect on liquid CER is modified by CWP.When the CWP is 267 smaller than 60 g/m 2 , increased aerosols reduce CER; When the CWP is larger than 60 g/m 2 , CER on polluted 268 days becomes larger.Different from the situation of liquid CER, ice CER on polluted days is always smaller 269 than that on clean days when fixing the ice CWP (Fig. 6).270

Possible effect of aerosols on cloud with inclusion of moisture 272
We attempt to understand the above results of aerosol effect on clouds with inclusion of moisture.The 273 aerosols serving as CCN nucleate a larger number of cloud droplets and accumulate more liquid water in the 274 cloud, so the CF, COT and CWP become increased.However, why the aerosol effect on cloud top and liquid 275 CER depends on different moisture conditions has not been clarified yet.276 In terms of cloud top, we speculate the following mechanisms in clean and polluted condition.On the clean 277 days with fewer moisture, the fewer cloud droplets cause the delayed precipitation due to relatively depressed 278 collision-coalescence process, thus the clouds tend to develop vertically to a higher altitude, which also 279 corresponds to the delayed formation of ice clouds (Fig. 3d).On the polluted days, the increased aerosols 280 (CCN) can increase the cloud droplet number (Squires and Twomey, 1966), which can enhance the 281 collision-coalescence process (Rosenfeld, 1999;Liu et al., 2003).When the moisture supply is sufficient, the 282 cloud drops can become larger via adequate collision-coalescence and easily convert to rain drops, which 283 facilitates the advance of rainfall start.After the rainfall started, the cloud top is restricted to grow higher.284 Therefore, the cloud top exhibits relatively lower in polluted cases over BTH region (Fig. 3f).285 For liquid CER, when moisture supply is fixed, aerosols serve as CCN nucleating larger number 286 concentrations of cloud drops but smaller size of droplets, which is Towmey effect (Albrecht 1989;Rosenfeld 287 et al. 2014).However, because the heavy pollution in BTH region is usually accompanied with high humidity 288 supply (Fig. 4), the aerosol effect on cloud exhibits "anti-Towmey" effect (Yuan et al., 2008;Jung et al., 2013;289 Qiu et al., 2017).i.e., the aerosols increase both the number and the size of cloud droplet via enhanced 290 collision-coalescence due to the plenty of moisture supply.291 However, the above mechanisms cannot work for the ice CER.The study has shown the ice CWP increases 292 but the ice CER decreases under pollution.We assume the aerosols increase the cloud droplets so that reduce 293 the vapor pressure inside clouds, thus decrease the supersaturation and weaken the process of transitions from 294 liquid droplet into ice crystal, which is known as Bergeron process (Squires, 1952).So far the detailed 295 physical processes of cold clouds and mixed-phase clouds are not clear, including the diffusional grow, 296 accretion, riming and melting process of ice precipitation (Cheng et al., 2010), which needs numerical model 297 simulations to further explore.298 299 5 Aerosol radiative effect and cloud effect on rainfall diurnal variation 300 Our previous study has indicated that the radiative effect of BC low-level warming may facilitate the 301 convective rainfall generation (Zhou et al., 2018).Based on the changes of cloud properties addressed in Sect.302 4, we further attempt to understand the different roles of aerosol radiative heating effect and cloud effect on 303 modifying the diurnal variation of heavy rainfall through the two aerosol types-BC and sulfate, which both influences the cloud and precipitation in the BTH region (Gunthe et al., 2011).We purposely selected the 306 cases with different BC/sulfate concentrations to compare the role of BC/sulfate on the diurnal variation of the 307 heavy rainfall.The methods have been described in Sect.2.2.2.308 The PDF of the starting time, peak time and duration of heavy rainfall were examined for the higher and 309 lower BC concentrations (Fig. 8a), respectively.The most striking result is that the starting time of heavy 310 rainfall in high BC concentrations evidently shifts earlier by 7 hours from 19:00 LST to 2:00 LST.Meanwhile, 311 compared with low BC cases, the peak time of heavy rainfall in high BC cases is more distinguishable and 312 shows an increase in the evening but a decrease at midnight to early morning.And the duration time of heavy 313 rainfall is slightly shorter in high BC cases.In contrast, when the sulfate has higher concentrations, the 314 starting time of heavy rainfall is evidently delayed while the duration time of heavy rainfall shows a 315 significant increase.The peak time of heavy rainfall occurrence also shows earlier and mainly locates at 316 around 21:00 LST but not as significant as that for high BC cases (Fig. 8b).317 We also compared the effect of BC/sulfate on the associated cloud to identify the cloud effect of the two 318 types of aerosols.We found more BC corresponds to a slight decrease of CF when CF is more than 90% (Fig. 319 9), which might be associated with semi-direct effect of BC (IPCC, 2013).By comparison, the CF increases 320 significantly with increased sulfate concentrations when CF is above 90%.The sharp increase of CF with 321 increased sulfate indicates that the CF is very sensitive to sulfate-like aerosols.Accordingly, the changes of 322 other cloud variables under pollution as above mentioned are also likely associated with this type of aerosols, 323 which can serve as CCN and influence the cloud properties.324 The earlier start of heavy rainfall and the decrease of CF in high BC cases denote that BC influences the 325 heavy rainfall through changing the thermodynamic condition of atmosphere (Zhou et al., 2018), which 326 increases upward motion and accelerates the formation of cloud and rainfall.Thus, BC heating effect should 327 play a dominant role in the beginning of rainfall.The delayed start and advanced peak of heavy rainfall with 328 higher sulfate concentrations indicate that the increased sulfate may accelerate the rainfall process from the 329 initial to the peak stage through enhancing the collision-coalescence and improving the efficiency of 330 precipitation in the condition of sufficient moisture.The longer duration in high sulfate cases corresponds to 331 that the sulfate as CCN increases the amount of cloud and lengthens the rainfall duration because of sufficient 332 moisture supply.Therefore, when the BTH pollution is relatively heavy, the moisture supply is usually 333 sufficient.In this situation, increased BC concentrations advance the beginning time and sharpen the peak 334 time via the radiative effect, while more sulfate aerosols accelerate the rainfall to the peak and remarkably 335 extend the duration time of heavy rainfall through the cloud effect.336 337

Conclusions 338
Using the gauge-based hourly rainfall records, aerosol and cloud satellite products and high temporal under pollution.We also investigated if the moisture influences the aerosol cloud effect, and found that the 344 aerosol effect on CF, COT (liquid and ice), CWP (liquid and ice) and ice CER does not depend on the 345 moisture condition.However, the aerosol effect on the cloud top height and liquid CER are opposite between 346 at lower and higher moisture conditions.The different roles of BC and sulfate on modifying the diurnal shift 347 were also examined.We found that higher BC concentrations correspond to the earlier start and peak of heavy 348 rainfall while higher sulfate concentrations correspond to earlier peak and longer duration of heavy rainfall.349 The two different types of aerosols play different roles on different stages of rainfall development.350 As a summary using a schematic diagram (Figure 10) to illustrate how aerosols modify the diurnal variation 351 of heavy rainfall over BTH region.On one hand, BC absorbs shortwave radiation during the daytime and 352 warms the lower troposphere, and then increases the instability of the lower to middle atmosphere so that 353 enhances the local upward motion and moisture convergence.As a result, the BC-induced thermodynamic 354 instability of the atmosphere triggers the occurrence of heavy rainfall in advance (Zhou et al. 2018).On the 355 other hand, the increased upward motion transports more sulfate-like particles into the clouds so that more 356 CCN and sufficient moisture increase the cloud droplet number and cloud water, thus enhancing the 357 collision-coalescence and accelerating the conversion of cloud droplets into rain droplets (Johnson, 1982;358 Cheng et al., 2007), which enhances the efficiency of rainfall and advances the arrival of rainfall peak.359 Additionally, the increased CCN nucleates more cloud droplets and accumulates more liquid water in clouds, 360 the duration of heavy rainfall is accordingly prolonged.361 Although this work has attempted to exclude the impacts from the meteorological background particularly 362 circulation and moisture, the observation study still has its limitation on studying aerosol effect on rainfall, 363 such as the noise and uncertainty of different observational data, the interaction of aerosol and meteorological 364 factors and the mixing of different types of aerosols.Numerical model simulations are needed to examine the 365 mechanism we proposed here.And the process of aerosols effect on the ice cloud precipitation formation also 366 needs further exploration in our future study.367 368

Data availability 369
We are grateful to the National Meteorological Information Centre (NMIC) of the China Meteorological 370 Administration (CMA) for providing hourly precipitation datasets.MODIS aerosol and cloud data were 371 obtained from http://ladsweb.modaps.eosdis.nasa.gov;MERRA2 reanalysis data were obtained from

*
Correspondence to: Jing Yang, State Key Laboratory of Earth Surface Process and Resource 32 Ecology/Academy of Disaster Reduction and Emergency Management, Faculty of Geographical Science, 33 Beijing Normal University, 19#Xinjiekouwai Street, Haidian District, Beijing 100875, China.E-mail: 34 yangjing@bnu.edu.cn Atmos.Chem.Phys.Discuss., https://doi.org/10.5194/acp-2018-1072Manuscript under review for journal Atmos.Chem.Phys.Discussion started: 16 November 2018 c Author(s) 2018.CC BY 4.0 License.Twomey effect.Twomey effect increases cloud microphysical stability and suppresses warm-rain processes 70 (Albrecht 1989; Rosenfeld et al. 2014).For cold clouds and mixed-phase clouds, many studies reported that 71 the cloud liquid accumulated by aerosols is converted to ice hydrometeors above the freezing level, which 72 invigorates deep convective clouds and intensifies heavy precipitation so called invigoration effect (Rosenfeld 73 and Woodley, 2000; Rosenfeld et al., 2008; Lee et al. 2009; Guo et al. 2014).However, due to the different 74 Four types of datasets from the year 2002 to 2012 (11 years) were used in this study, which include (1) 101 precipitation, (2) aerosol, (3) cloud, and (4) other meteorological fields.102 2.1.1Precipitation data 103 Atmos.Chem.Phys.Discuss., https://doi.org/10.5194/acp-2018-1072Manuscript under review for journal Atmos.Chem.Phys.Discussion started: 16 November 2018 c Author(s) 2018.CC BY 4.0 License.To study diurnal variation of rainfall, the gauge-based hourly precipitation datasets were used, which were 104 obtained from the National Meteorological Information Center (NMIC) of the China Meteorological 105 Administration (CMA) (Yu et al., 2007) at 2420 stations in China from 1951 to 2012.The quality control 106 made by CMA/NMIC includes the check for extreme values (the value exceeding the monthly maximum in 107 observationally-based within a model framework, which can offer a more complete temporal and spatial 132 coverage than observation and overcome the shortcoming of simulation that fail in simulating the complexity 133 of real aerosol distributions.The horizontal resolution of MACC-II is 1°×1° and the vertical resolution is 60 134 levels.MACC-II data covers the period of 2003 to 2012, of which the time interval is six-hour.135 2.1.3Cloud data 136 Daily cloud variables, including cloud fraction (CF), cloud top pressure (CTP), cloud optical thickness (COT, 137 Atmos.Chem.Phys.Discuss., https://doi.org/10.5194/acp-2018-1072Manuscript under review for journal Atmos.Chem.Phys.Discussion started: 16 November 2018 c Author(s) 2018.CC BY 4.0 License.liquid and ice), cloud water path (CWP, liquid and ice) and cloud effective radius (CER, liquid and ice), were 138 obtained from MODIS Collection 6 L3 cloud product onboard the Terra satellite.The MODIS cloud product 139 combines infrared emission and solar reflectance techniques to determine both physical and radiative cloud 140 properties (Platnick et al., 2017).The validation of cloud top properties in this product has been conducted 141 through comparisons with CALIOP (Cloud-Aerosol Lidar with Orthogonal Polarization) data and other lidar 142 estimates using aircraft observations, and the validation and quality control of cloud optical products is 143 performed primarily using in situ measurements obtained during field campaigns as well as the MODIS 144 Atmos.Chem.Phys.Discuss., https://doi.org/10.5194/acp-2018-1072Manuscript under review for journal Atmos.Chem.Phys.Discussion started: 16 November 2018 c Author(s) 2018.CC BY 4.0 License.

4
besides the peak time of heavyrainfall.193    In terms of the starting time for the heavy rainfall, a significant advance of the starting time is found as 194 shown in Fig.1b.The time for maximum frequency of heavy rainfall initiation is 6 hours earlier on the 195 polluted days, shifting from around 0:00 LST on the clean days to the 18:00 LST.Regarding the durations of 196 heavy rainfall, the persistence of heavy rainfall on polluted days is nearly 0.8 hours longer than that on clean 197 days.According to the PDF shown as in Fig.1c, the occurrence of short-term precipitation (≤6 hours, Yuan et 198 al., 2010) decreases while that of long-term precipitation (>6 hours, Yuan et al., 2010) increases.The intensity 199 of hourly rainfall on the polluted days exhibits a decrease on the polluted days.However, compared with the 200 other features, the change of intensity does not pass the 95% statistical confidence level.Therefore, the 201 following only focuses on investigating why the starting time, peak time and duration of heavy rainfall change 202 with pollution in diurnal time scale.203 Atmos.Chem.Phys.Discuss., https://doi.org/10.5194/acp-2018-1072Manuscript under review for journal Atmos.Chem.Phys.Discussion started: 16 November 2018 c Author(s) 2018.CC BY 4.0 License.204 Cloud effect of aerosols with inclusion of moisture 205 liquid and ice clouds, and liquid CER but the decrease of cloud top height and ice CER.Additionally, the 235 peaks of the liquid and ice water shift earlier on the polluted days.236 Atmos.Chem.Phys.Discuss., https://doi.org/10.5194/acp-2018-1072Manuscript under review for journal Atmos.Chem.Phys.Discussion started: 16 November 2018 c Author(s) 2018.CC BY 4.0 License.4.2 Changes of cloud properties affected by moisture on clean and polluted days 238 The different moisture condition can influence the effect of aerosols on cloud properties (Yuan et al., 2008; 239 Jiang et al., 2008; Jung et al., 2013; Qiu et al., 2017).It is hard to completely remove the moisture effect on 240 the above results in a pure observational study, although we have fixed the wind direction in this study.Since 241 the southwesterly circulation background cannot only transport pollutants but also moisture to the BTH region 242 (Wu et al., 2017), more pollution usually corresponds to more moisture.And Figure 4a does show that the 243 humidity increases accompanied with increased AOD over BTH region.Because the moisture supply for BTH 244 is mainly transported via low-level southwesterly circulation, we purposely use the RH at 850 hPa as the 245 indicator of moisture condition.The PDF of humidity shows that the 40-60% RH dominates the clean cases 246 Atmos.Chem.Phys.Discuss., https://doi.org/10.5194/acp-2018-1072Manuscript under review for journal Atmos.Chem.Phys.Discussion started: 16 November 2018 c Author(s) 2018.CC BY 4.0 License.