| 19 Aug 2026
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.
The article of Ghaemi et al. is about the evolution of IWV, LWP and CBH before rain onset
observed in the southeastern Alpine forelands. The article is well written and contains almost all explanations and details.
There are a few related studies, however the article contains new methods of a regime-dependent analysis and new results. In particular the data from the X-band radar are important for recognition of the rain areas. For my taste, the article is a bit
long but the plus point is that the details of the analysis are described.
Generally I recommend a publication in ACP because of the detailed analysis which showed that the evolution of water vapor and cloud droplets before rain is different for convective and non-convective rain events. It would be interesting if the weather models also show such a sharp transition from cloud droplets to rain droplets as the observations indicate for the increases of LWP before rain onsets.
A minor revision is needed.
Minor comments:
1) I think one should mention the manufacturer of the measurement instruments
2) Please discuss LWP during transition from cloud droplets to rain droplets.
3) line 100 ... time steps which ....
maybe it is better to write ... time steps when .... ?
4) Figures 10 and 12 Because of the related colors it is difficult to distinguish the rain gauge from the radar measurements
5) Figure 10 and later : please explain meaning of the horizontal black line
6) Conclusion section is too long.
I would put some parts of the conclusion section into a new section discussion.
7) Just a possible extension idea: it would be great to see the sharp LWP increase before rain onset in a model such as WRF.