Regime-dependent evolution of water vapor, cloud liquid, and cloud base before rain onset in the southeastern Alpine forelands

Abstract. Liquid water path (LWP) and integrated water vapor (IWV) provide essential information on the moisture supply and condensation available for precipitation formation, while cloud base height (CBH) indicates where saturation and cloud formation start. The main goal of this study is to quantify how these parameters evolve before rain occurs.

We analyzed approximately 4 years of collocated observations from the WegenerNet 3D Open-Air Laboratory in southeast Austria, including an X-band dual-polarization radar, microwave and infrared radiometers, a GNSS station, and rain gauges. We separated convective and non-convective rain with a multi-criteria scheme based on radar polarimetric variables and the radiometer temperature profile, and warm, cold, and mixed rain types by comparing the echo top height with the -5 °C isotherm. During the six hours before onset, LWP stays low and then increases sharply within the last hour, with the steepest rise before convective events (about 300 g m^-2), while cold-season events show a more gradual increase of about 150 g m^-2. IWV changes earlier than LWP and is lowest in the cold rain type in both regimes. The joint evolution shows that convective events first move towards higher IWV and then towards a strong LWP increase near onset, while both parameters increase together in non-convective events. CBH decreases in all categories, most sharply before convective events (1000 m) and more modestly in the cold season (300 m). Overall, these signatures can serve as a reference to evaluate how models represent the transition from moisture to cloud and rain.