Analysis of a Record-breaking Hybrid-scale Rainfall Event in New Zealand

On Friday, 27 January 2023 - the eve of a holiday weekend celebrating Auckland’s founding anniversary - New Zealand’s most populous city was struck by the worst flood in its history. Exceeding a 1-in-200 year return period, much of the city received more than a typical summer’s worth of rainfall in less than 12 hours, including 80mm recorded in a single hour at the event’s peak. It represents the country’s largest weather-related insurance cost, with claims over $NZ 2 billion (EUR 1 billion), and infrastructure and other costs likely doubling this.The event exemplified hybrid-scale rainfall, characterised by convective structures embedded within synoptic features. Such systems pose significant risk to populated areas due to their potential for sustained high-intensity rainfall even in the absence of significant orographic enhancement. This contrasts with New Zealand’s more typical heavy rain events which involve frontal systems and atmospheric rivers impacting mountainous areas such as the Southern Alps. As such, hybrid events are often poorly resolved by models, exhibit low predictability, and challenge existing forecasting and warning systems. Their potential for flash flooding is expected to increase under ongoing climate warming.While the possibility of heavy rainfall was indicated well in advance and warnings were issued approximately 18 hours ahead, the severity of the event far surpassed forecasts and expectations. In its aftermath, several investigations have been undertaken to better understand the driving mechanisms, forecast performance, predictability, the role of warm-rain microphysics and explore improved model representations and warning strategies. This presentation synthesises selected findings, focussing on the structure and mechanisms behind the event. Key features align with international studies of comparable systems: a downstream blocking anticyclone, low-level jet (LLJ), rapid overturning and moisture replenishment at low levels resulting in a moist absolutely unstable layer (MAUL), weak warm or occluded synoptic features, and an over-running dry intrusion. The resulting structure was that of a mesoscale convective system (MCS) embedded within stratiform rainbands. Moisture replacement was rapid courtesy of intense low-level vapour flux via a 50 knot LLJ within a moist airmass laden with maritime condensation nuclei, resulting in efficient warm-rain processes. Convective structures appeared triggered over modest terrain and subsequently self-organised within the LLJ, resulting in extended lines of vigorous convection in the absence of lightning.