Joint influence of anomalous medium- and small-scale circulations on short-term heavy rainfall events over Beijing

Dense auto-weather station data, variational doppler radar analysis system (fast updating radar four-dimensional variation analysis system) data, radiosonde data and ERA-Interim reanalysis data are used to investigate the joint influence of anomalous medium and small-scale circulations associated with short-term heavy rainfall events, which are influenced by the interaction between Pacific subtropical high and westerly trough (Type i) over Beijing between June and August 2007-2014. This Type i primarily comprises short-term heavy rainfall events over Beijing. The results show that short-term heavy rainfall events appear mainly over Northeastern and Southwestern Beijing, especially Northeastern Beijing, and generally reach a peak at dusk (19:00-21:00). Large-scale circulations (an anomalous high in the east and an anomalous low in the west over Eastern Asia) favour the southeastern and southwestern winds to transport water vapour northward to Beijing. An anomalous mesoscale cyclone moves from west to northeast via the north to Beijing at approximately 20:00. An abnormal air temperature structure (warm and wet air at whole levels), an abnormal mesoscale circulation structure (with lower-level convergence and mid-upper-level divergence) and the resulting abnormal vertical upward movement are well developed over Beijing. The upward branch over the southern outskirt of the anomalous mesoscale cyclone partly overlaps with that of the anomalous small-scale cyclonic circulations or convergence lines (anomalous thunderstorm highs), and a striking vertical upward motion is produced. One airstream of the upward branches moves into the upper divergence region (it "pumps" mid-lower level air into the mid-upper levels) and makes the upward motion more vigorous. Another airstream turns around and descends fiercely to the surface (downward branches of anomalous thunderstorm highs); that is, the anomalous thunderstorm highs are reinforced, and the frequency peak in the short-term heavy rainfall events occurs at dusk. The short-term heavy rainfall events weaken at night. The analyses of vertical velocity anomalies for anomalous medium-scale cyclone as well as turbulent vertical velocities and diagnostic radar echoes for anomalous small-scale circulations in the heavy rainfall event on July 21, 2012 further verify the statistical results.