Using the 0.1° high-resolution hourly observational precipitation data from CMA and the 0.25° hourly surface temperature from ERA5 reanalysis data provided by ECMWF, we investigate the weekend effect in summertime surface air temperature and precipitation over the YRD region for 2008–2019. The results show that lower surface air temperature and higher precipitation generally appear during weekends compared to weekdays over the YRD region. Moreover, strong spatial differences in the weekend effect in summertime surface air temperature and precipitation are clearly identified.
Further, we explain the possible physical processes explaining the weekend effect in surface climate over the YRD region. The results show that the aerosol-cloud-radiation interaction may make a critical contribution to the weekend effect over the YRD region. Under the increasing urbanization, more aerosol emissions associated with intense human activities tend to increase the cloud cover amount and decrease the net radiation reaching the surface (Fig. 5), which may subsequently lead to more precipitation and lower surface air temperature during weekends.
Some previous research demonstrated that the pollutant accumulation was less during weekend than weekdays due to some reasons such as motor vehicles and factories emitting fewer pollutants at weekends (e.g., Gong et al. 2007). However, as shown in this study and other work (Xia et al. 2008; Song et al. 2018), over the YRD region the AOD magnitudes are larger during weekend than weekdays. Here the question raises, what mechanisms are responsible for the increased aerosols during the weekend over the YRD region?
The East Asian summer monsoon is mainly the prevailing southeast wind driven by the land-sea thermal contrast. However, the rapid urbanization makes the regional weekend temperature generally lower than weekdays (Fig. 4a), which reduces the land-sea thermal difference and further weakens the monsoon, resulting in northerly wind anomalies (Fig. 6). Due to the different urbanization levels, the inhomogeneous distribution of thermal anomalies leads to convergence and divergence of northerly winds, which makes the AOD accumulation (Fig. 5 and 6). In addition, there are other factors linked to the surface wind over the coastal area of the YRD region. For example, the lake-air interaction may lead to strong local divergence anomalies.
Besides, the rapid urbanization and human activities also exert effects on land surface processes such as soil types (Thielen et al. 2000; Hu et al. 2017; Song and Zhang 2020), soil moisture (Douville et al. 2001; Koster et al. 2010; Sun and Wang 2012; Wei and Dirmeyer 2012), soil temperature (Zhang and Wu 2014), the vegetation (Lee et al. 2012; Wei and Dirmeyer 2012), leading to changes in aerosol-cloud-climate interaction and land-air interaction (Rosenfeld 2000; Jin et al. 2005; Menon et al. 2008; Rosenfeld et al. 2014). These changes can influence radiation, energy balance, moisture, atmospheric boundary layer and regional atmospheric circulations, and further modulate temperature and precipitation variations over the YRD region. It also needs to note that weekday-weekend differences in anthropogenic heat release may play an important role in highly urbanized areas (Simmonds and Keay 1997). These complex physical progresses involved should be explored by combined diagnosis methods and numerical simulations in the future.