meteorology: A perspective from the Yangtze River Delta region

With the rapid advance in urbanization, land-surface forcing related to the urban 10 expansion and anthropogenic heat (AH) release from human activities significantly affect the urban 11 climate and in turn the air quality. Focusing on the Yangtze River Delta (YRD) region, a highly 12 urbanized place with sever ozone (O3) pollution and complex geography, we estimate the impacts 13 of land-surface forcing and AH on meteorology (meteorological factors and local circulations) and 14 O3 using the WRF-chem model, which can enhance our understanding about the formation of O3 15 pollution in those rapidly developing regions with unique geographical features as most of our 16 results can be supported by previous studies conducted in other regions in the world. Regional O3 17 pollution episodes occur frequently (26 times per year) in the YRD in recent years. These O3 18 pollution episodes are usually under calm conditions characterized by high temperature (over 20 °C), 19 low relative humidity (less than 80%), light wind (less than 3 m s-1) and shallow cloud cover (less 20 than 5). In this case, high O3 mainly appears during the daytime influenced by the local circulations 21 (the sea and the lake breezes). The change in land-surface forcing can cause an increase in 2-m 22 temperature (T2) by maximum 3 °C, an increase in planetary boundary layer height (PBLH) by 23 maximum 500 m and a decrease in 10-m wind speed (WS10) by maximum 1.5 m s-1, and surface O3 24 can increase by maximum 20 g m-3 eventually. Furthermore, the expansion of coastal cities 25 enhances the sea-breeze below 500 m. During the advance of the sea-breeze front inland, the upward 26 air flow induced by the front makes well vertical mixing of O3. However, once the sea-breeze is 27 fully formed, further progression inland is stalled, thus the O3 removal by the low sea-breeze will 28 be weakened and surface O3 can be 10 g m-3 higher in the case with cities than no-cities. The 29 https://doi.org/10.5194/acp-2021-619 Preprint. Discussion started: 17 August 2021 c © Author(s) 2021. CC BY 4.0 License.


Introduction 42
Ozone (O3) is a key constituent in the atmosphere, and is deeply relevant to climate (Worden 43 et al., 2008), biosphere (Van Dingenen et al., 2009) and human health (Jerrett et al., 2009). O3 acts 44 quite differently in different parts of the atmosphere, often described as being "good up high and 45 bad nearby". O3 in the stratosphere helps protect life on earth from strong ultraviolet radiation. 46 However, high O3 in the troposphere is harmful to human respiratory system and the growth of 47  Fast-J photolysis (Fast et al., 2006) MADE/SORGAM (Schell et al., 2001) 209 As shown in Table 2, three numerical experiments are performed to study the effects of land-210 surface forcing and AH on meteorology and O3 in the YRD. The MODIS_noAH experiment is a 211 control simulation with commonly used settings. Compared with MODIS_noAH, USGS_noAH 212 selects the USGS data at run-time through the geogrid program. Thus, the difference between the 213 modeling results of MODIS_noAH and USGS_noAH can illustrate the changes caused by land 214 cover. As for the impact of AH, it can be identified by comparing the modeling results of 215 MODIS_withAH and MODIS_noAH. All three simulations run from 00:00 on 21 May to 00:00 on 216 4 June in 2017 with the first 88 h as spin-up time, using the same physical and chemical 217 parameterization schemes (Table 1). 218 219 where Si and Oi are the simulations and observations, respectively. N is the total amount of valid 231 data, and S and O represent the average of simulations and observations, respectively. Generally, 232 the model performance is acceptable if the values of MB and RMSE are close to 0, and that of COR 233

Regional O3 pollution episodes in the YRD 237
Under adverse weather conditions, O3 pollution episodes occur frequently in the YRD (Gao et 238 al., 2020;Zhan et al., 2021). Sometimes, O3 pollution can spread throughout the YRD and cause 239 regional O3 pollution, affecting an area of up to 3.5 million square kilometers and harming more 240 than 200 million people. Based on the surface O3 observations, we define the regional O3 pollution 241 in the YRD as when more than half of the 26 typical cities in the YRD fail to meet the national O3 242 standard (In China, the national ambient air quality standard for MDA8 O3 is 160 µg m -3 ), and then 243 sort out all regional O3 pollution episodes and the corresponding weather patterns from 2015 to 244 2019 (Table S1). There were 20, 19, 34, 28 and 30 regional O3 pollution cases in the YRD from 245 2015 to 2019, respectively. These cases mainly occurred in April to October of each year, and were 246 usually related to high pressure, uniform pressure field and typhoon activity. (from 8:00 to 20:00 local time) when regional O3 pollution occurs in the YRD. All the variables 249 show significant monthly variations. The highest (lowest) temperature is found in July (April), and 250 the relative humidity is highest in June. This may be related to the Meiyu in June, and the hot weather 251 in July as the YRD is usually dominated by the western Pacific subtropical high after Meiyu. As for 252 the cloud cover, the sky is covered with fewer clouds in October than other months. In addition, 253 southeast wind prevails in the YRD from April to October under the influence of monsoon climate.

13
For simplicity but without loss of generality, the longest-lasting regional O3 pollution case in 270 Table S1 is selected to investigate the impacts of land-surface forcing and AH on meteorology and 271 O3 pollution in the YRD. This 10-day regional O3 pollution episode occurred from 25 May to 3 June high pressure/uniform pressure field ( Figure S1). This case meets the requirements of calm weather 276 and high O3 concentration. And the relatively long duration also provide a representative result. 277

Evaluation of model performance 278
In this study, three numerical experiments are conducted using WRF-Chem (Sect.  Additionally, a high mean MB is found to correspond to a high mean RMSE (1.9, 1.8 and 1.7 m s -303 1 ) in our simulations. In terms of WD10, the model captures well the shift in wind direction during 304 the study period ( Figure 5d). Thus, our modeling results of wind speed and direction basically reflect 305 the characteristics of wind fields. In summary, both the statistical metrics in Table 3 and time series 306 in Figure 5 illustrate that all the numerical experiments can reflect the major characteristics of 307 meteorological conditions during this O3 pollution episode. Nevertheless, using new land-use data 308 and adding AH can reduce the underestimation of T2 and the overestimation of RH and WS10 to 309 some extent.     Above all, the WRF-Chem model using our configuration has a good capability in simulating 336 the meteorology and O3 air quality over the studied region in this study. It is still noteworthy that 337 the object of inter-comparison between the three numerical experiments is not to determine which 338 setting is most skillful in reproducing the observations. Rather, it is to diagnose and understand the 339 differences induced by land-surface forcing and AH, and then to provide valuable insight into the 340 formation of the O3 pollution episodes. 341 342

Overall behaviors of O3 and local circulations 343
Based on the results of the control simulation (MODIS_noAH), we first give an overall 344 behavior of O3 and local circulations during the study period. And then the differences induced by

Sea and lake breezes 378
As shown in Figure 7a and b, in the areas where the local circulations meet the background 379 dominant winds (the southeast wind), the converging airflows make O3 concentrations higher than 380 those in the surrounding areas. Furthermore, the typical local circulations in the central YRD are the 381 sea and the lake breezes around the Tai Lake. In this study, the sea-breeze usually affected a wide 382 area and lasted a long time, which may be related to the local background field since they are mostly 383 in same direction, and it is difficult to separate the sea-breeze from the southeast wind. The sea-384 breeze was obvious around 14:00 LT and matured around 17:00 LT, and continuously transported 385 high O3 from coastal to the inland areas during this period (Figure 7b-d). Compared with the sea-386 breeze, the lake-breeze had a much smaller influencing area and a shorter duration. Around 11:00 387 LT, the lake-breeze was established. It reached its maximum intensity around 14:00 LT, and then 388 disappeared sharply due to the predominant sea-breeze (Figure 7c). Both the sea and the lake breezes 389 played important roles in the horizontal distributions of O3 in the central YRD. given since the lake is usually inside the land so that the lake breezes can have different directions. 404 The lake-breeze was established when the surface wind was weak by 11:00 LT (Figure 9d

The changes in vertical direction 451
As shown in Figure 11a-c, the sea-breeze below 500 m increased by 1-2 m s -1 due to the 452 existence of the cities which enhanced the temperature contrast between the land and the sea. Strong 453 turbulent mixing and updraft induced by the sea-breeze front promote the development of the urban 454 boundary layer, contributing to elevated O3 levels at surface in the city during the advance of the 455 sea-breeze front inland (Figure 11a and b). When the sea-breeze matured around 17:00 LT, its 456 transport effect reduced the surface O3 concentration of the coastal cities (Figure 9c) As for the lake-breeze, it was also enhanced by 1-2 m s -1 after the establishment because of the 461 larger temperature contrast resulting from the cities, just like the sea-breeze (Figure 11e  The phenomenon that cities are almost always warmer than their surroundings is known as the 527 urban heat island (UHI), and the difference between the urban and the rural surface energy balance 528 can further initiate the UHI circulation. It is clearly seen that an enhanced UHI circulation driven 529 by AH appeared in the megacity Shanghai around 14:00 LT (Figure 14b Land-surface forcing related to the urban expansion and AH release from human activities can 563 change the meteorology (meteorological factors and local circulations) and thereby affect O3 air 564 quality in and around cities. In this study, the YRD region, a highly urbanized place with sever O3 565 pollution and complex geography, is selected to discuss this issue. Firstly, we briefly describe the 566 general characteristics of O3 pollution in the YRD based on the surface observations. Secondly, we 567 simulate a representative case using WRF-chem and evaluate the model performance by comparing 568 with the observational data. Finally, the response of meteorology as well as O3 to land-surface 569 forcing and AH are investigated from the model results. The main findings are listed as below: 570 (1) Regional O3 pollution occurs frequently in the YRD (~ 26 times per year). Like other 571 regions, these O3 pollution episodes mainly occur in warm season (April to October) under calm 572 conditions characterized by high temperature (over 20 ℃), low relative humidity (less than 80%), 573 light wind (less than 3 m s -1 ) and shallow cloud cover (less than 5). In this case, the local circulations 574

31
(2) By updating the land-use data from USGS to MODIS, we find an increase in T2 by 577 maximum 3 ℃, an increase in PBLH by maximum 500 m and a decrease in WS10 by maximum 1.5 578 m s -1 in the YRD, which is comparable to those in the BTH region (Yu et al., 2012), the PRD region 579 inland is stalled on account of the rough urban surface. The transport of high O3 from coastal to the 586 inland areas is weakened and thereby O3 can be 10 g m -3 higher in the case with cities than without. 587 The similar results have been also reported in the Paulo (Freitas et al., 2007) and the PRD region 588 (You et al., 2019). With respect to the lake breezes, its lifetime will be extended from the noon to 589 the afternoon because of the urban expansion. Since the net effect of the lake-breeze is to accelerate 590 the vertical mixing in the boundary layer, the surface O3 can increase as much as 30 g m -3 591 influenced by the lake-breeze. Similar phenomenon also be observed in the Greater Toronto Area 592