Effects of local-scale orography and urban heat island on the initiation of a record-breaking rainfall event

Numerically simulating the convection initiation (CI) was challenging due to insufficient model resolution and inaccurate boundary layer parameterization, this study therefore examined the pre-convective mesoscale processes based on their four-dimensional analyses obtained through data assimilation of radar and surface observations using the four-dimensional Variational Doppler Radar Analysis System (VDRAS).

Results suggested that the Huadu Mountain to the north of Guangzhou City played a crucial role in the CI through orographic blocking and nighttime cooling. Dynamically, the mountain blocked the upstream airflow causing an accumulation of water vapor in boundary layers; thermally, the large near-surface temperature gradients between the mountain and its southern foot area induced northerly downslope winds and low-level convergences and updrafts.