Comparison of short-duration heavy rainfall environmental conditions during two extreme torrential rainfall events over Beijing area

The short-duration heavy rainfall observations, the synoptic patterns, the terrain effects, and the environmental conditions for short-duration heavy rainfall occurred during the two extreme torrential rainfall events of 21 July 2012 (shortly for "7.21") and 20 July 2016 (shortly for "7.20") are analyzed. The extremity of environmental low level forcing, moisture content, and instability are studied from point and regional perspectives by comparing with long time series data. Results indicate the following. (1) Though remarkable difference in the short-duration heavy rainfall observations, both events happened under favorable synoptic patterns. (2) Comparison of environmental conditions with long time series reference at point and regional scale shows that the short-duration heavy rainfall happened under extremely strong low level forcing and strong precipitable water of the whole layer (PWAT) environments, but significant difference for the instability represented by lifted index (LI). The LI during "7.21" is much stronger while during "7.20" is weaker than that for the reference. (3) The standard anomalies (SD) of environmental conditions with long time series data show that the 850 hPa wind field and PWAT during both events are greater than 3σ, indicating low level forcing and moisture content are very strong. But for latent instability indicated by LI, the SD during "7.21" is -1σ, while no remarkable SD obtained during "7.20". The difference of latent instability is one of the important reasons causing the great difference on intensities of short-duration heavy rainfall during the two events.