Sub-Hourly Precipitation Extremes in Mainland Portugal and downburst events

Sub-hourly heavy precipitation events (SHHPs) frequently underlie major meteorological hazards. This study uses a 71-weather-station dataset for 2000–2020 to diagnose SHHPs corresponding to 10-minute precipitation events of at least 5.0 mm (above the 99th percentile of the precipitation days) and identify their associated synoptic-scale conditions, using a meridional pressure gradient (MPG index) defined on an hourly timescale. This index revealed two main synoptic-scale weather types: remote and regional low-pressure systems (RemL and RegL, respectively). RegL events show two pronounced maxima in spring and autumn, whereas RemL events have a single maximum in autumn. Moreover, RegL events are largely concentrated in the afternoon and evening, while RemL events are much more uniformly distributed during the day, despite some bias toward mid-day and early afternoon. These features suggest that RegL events tend to be thermally driven, whereas RemL events are associated with frontal systems.

Moreover, a preliminary linkage between the SHHPs and mesoscale convective systems is established by detecting sudden surface pressure surges, which are manifestations of mesohighs associated with downbursts. Finally, two case studies of downburst events are presented. The first event occurred on 23 December 2009 when a rapidly deepening extra-tropical cyclone crossed Mainland Portugal. During this event, several power towers were destroyed by downburst winds associated with a mesovortex, observed in a bow echo line triggered by an upper cold front.

The second event occurred on 17 June 2017 and contributed to the rapid spreading and intensification of unprecedented forest fires in Portugal, with the loss of many human lives. On 17 June, the distinctive markers of mesohighs, cold pools and the associated downbursts, were identified by weather stations and these signatures were also present in the AROME forecasts. These downbursts were caused by an MCS (visible in satellite images and radar reflectivity), which developed in an environment characterized by weak synoptic forcing, accompanied by moderate CAPE and an inverted V-profile, with a very warm and dry layer from the surface to 650 hPa, which is typical of hybrid downbursts.