Analysis of dynamic and thermodynamic drivers linked to heavy sub-hourly precipitation events associated with regional low-pressure systems in mainland Portugal

Sub-hourly heavy precipitation events (SHHPs) associated with regional low-pressure (RegL) systems represent a significant natural hazard, frequently causing significant losses over a wide range of natural systems and socioeconomic sectors in mainland Portugal, such as in agriculture (e.g., viticulture). This study provides a preliminary identification of the main dynamic and thermodynamic drivers of the SHHP events associated with RegL systems, also providing a systematised and comprehensive assessment of the atmospheric conditions that are at their genesis. This study uses observations from operational automated surface weather stations (WSs), maintained by the Portuguese Weather Service (Instituto Português do Mar e da Atmosfera, IPMA) for the period 2000–2022 (23 years), with a 10 min temporal resolution, with special emphasis on three weather stations that are representative of the different climatic regions of mainland Portugal. The southern region of Portugal exhibits higher precipitation variability, characterized by a greater occurrence of extreme events on fewer rainy days. The research establishes a connection between SHHPs and low-pressure systems situated to the west of the Iberian Peninsula. These systems display a cold core, especially pronounced at mid-levels, and a positive vorticity anomaly extending from the upper troposphere to lower levels. These conditions induce differential positive vorticity advection in the upper and middle levels (increasing with height), thereby favouring rising motion to the east of the low-pressure systems (over western Iberia). Additionally, these systems facilitate moisture advection over western Iberia at lower levels, highlighted by the positive anomalies of 2-m dew point temperatures and promote instability conditions, diagnosed by different instability indices (Convective available potential energy, Total-Totals index, and K-index). Lastly, the total column cloud ice water revealed higher values for the heavier precipitation events, relative to the values of total column cloud liquid water, suggesting that it may be a useful predictor of such events. The combination of analysed conditions, suggests that some of these SHHPs may be associated with cut-off lows, however, this hypothesis should be validated in a future study.