Temporal Variability of the 0°C Isotherm Height Over the Coast of the Biobío Region (36.8° S), Chile: Characterisation and Case Studies

ABSTRACT We analyse inter‐annual, monthly, synoptic and sub‐daily variability aspects of the 0°C isotherm height (H0), using ERA5 reanalysis, observations and estimates, over the coast of the Biobío Region, Chile (36.8° S, 73° W). As this region lacks radiosonde‐based H0 measurements, we use hourly H0 observations onboard aircraft taking off and landing at the Carriel Sur International Airport (SCIE, 36.77° S, 73.06° W) between 2017 and 2019. The validation of ERA5 using observations reveals an excellent performance of this reanalysis in resolving observations. H0 varies throughout the year, reaching a maximum in January and a minimum in July. Then, we estimate H0 for 2022 and 2023 based on Micro Rain Radar data acquired at the Atmospheric River Observatory in Concepción, which has high vertical and temporal resolution. We use vertical profiles of fall velocity and hydrometeor reflectivity during precipitation events. Fall velocity increases on average from ~2.3 to ~4.0 m/s within 300 m just below H0, indicating the presence of melting hydrometeors. Hence, for precipitation storms, it is possible to determine the height of the melting layer (also known as the bright band), which, in turn, enables the estimation of H0. These events show a fair agreement with H0 values from ERA5. Particularly in winter, this region is exposed to hydrometeorological events caused by mid‐latitude synoptic activity. Atmospheric rivers are responsible for up to ~50% of the total annual precipitation and can induce extreme precipitation in some cases. Therefore, three precipitation events are analysed to study their associated synoptic patterns, and the evolution of their respective H0 values. These events correspond to those days for which the highest H0, the lowest H0 and the maximum amount of precipitation were registered.