Development and evaluation of a new convective index

Deep convective clouds, such as towering cumulus and Cumulonimbus (Cb), can endanger lives and property, being also a significant hazard to aviation. This study presents the convective index (IndexCON) used operationally at the Portuguese Meteorological Watch Office. IndexCON forecasts (for steps ranging from 12 to 35 hours) were evaluated against lightning and precipitation observations. Reports of hail and tornadoes were also used to define a convective event. This evaluation was conducted over two years (January 2022 to December 2023) across mainland Portugal and its surrounding regions.

IndexCON integrates several prognostic variables from the European Centre for Medium-Range Weather Forecasts (ECMWF), including stability indices, cloud water content, relative humidity, and vertical velocity, using a fuzzy-logic approach. The index performs well during the warm season (May–October), achieving a probability of detection (POD) of 70%, a false alarm ratio (FAR) of 30%, and a probability of false detection (POFD) below 5%, resulting in a critical success index (CSI) above 0.55. However, IndexCON tends to overestimate convective activity during the cold season (November–April), leading to lower performance scores. This study demonstrates that optimising the membership functions can partially mitigate this overestimation.

In addition, the study evaluates other convective predictors, such as convective available potential energy (CAPE), Total-Totals (TT), K index (KI), lifted index (LIs), and Jefferson Index. Finally, the performance of IndexCON is illustrated in detail for two thunderstorm episodes occurring in different seasons, using satellite products, synoptic and radiosonde observations, lightning, and precipitation data. For these events, the Cb cloud tops derived from IndexCON were also compared with the NWC SAF cloud top product.