Sea-land breezes in the Guangdong- Hong Kong- Macau Greater Bay Area coastal zone from 2013 to 2022

Sea-land breezes (SLBs1) are mesoscale circulation phenomena that significantly affect weather and air quality in coastal regions. The Guangdong–Hong Kong–Macau Greater Bay Area (GBA), one of the most densely populated and economically developed regions in China, experiences frequent SLB intrusion. In this study, we examined the SLB features in the GBA from 2013 to 2022 based on observations, focusing particularly on the influence of synoptic conditions on SLB behaviour and their interplay. The annual average daily sea and land wind speeds ranged from 2.8 to 3.2 m/s and 1.9 to 2.2 m/s, and from 1.8 to 2.2 m/s and 1.6 to 1.9 m/s in the eastern and western GBA, respectively. The start time of sea (land) winds first appeared at 0000 (1100) UTC and finally occurred at 0900 (2000) UTC. Climatologically, SLBs were observed more frequently when southerly synoptic winds prevailed in the GBA. SLBs were stronger under synoptic northerly winds, but weaker when synoptic southerly winds prevailed. The diurnal evolution amplitude of SLB strength was much more significant under the control of synoptic northerly winds. In addition, SLBs modified synoptic winds, leading to the distortion of the observed diurnal wind patterns. A southerly increment in synoptic winds was observed from noon to early night on SLB days, whereas a northerly increment was observed from midnight to morning. Northerly synoptic winds weakened or even transformed into southerly winds in the afternoon, whereas southerly synoptic winds were found to decrease or even turn northerly at night. Our study provides an integrated understanding of SLBs in the GBA, especially regarding the behaviour of SLBs under various synoptic conditions and their modification on synoptic winds that reshape the observed wind patterns. These results provide useful information for weather simulations in coastal areas.