Observational Insights into Humidity Evolution during Cold Pool Passages

Cold pools are formed by convective clouds as precipitation evaporates below the cloud base, generating cool and dense air. The air descends rapidly, creating downdrafts that extend outward across the surface. These events generate gust fronts that lift ambient air and trigger secondary convection. The temperature structure of cold pools has been studied more extensively than their moisture dynamics. Simulations often suggest “moisture rings”, which are regions with increased water vapor content at the cold pool edge. However, these are rarely confirmed by observations.

To address this gap, data from the FESSTVaL (Field Experiment on Sub-mesoscale Spatio-Temporal Variability in Lindenberg) campaign, conducted during the summer of 2021, were analyzed. A dense network of surface stations was strategically positioned over an area of 30 km in diameter to provide high-resolution measurements. In total, 99 stations were deployed, including 19 Vaisala WXTs capable of measuring relative humidity every 10 seconds.

High-resolution temperature time series were used to detect the timing of more than 300 cold pool passages at individual stations. All these events were compiled into one composite using a common time axis relative to the time of the passage. This analysis of the average humidity evolution reveals that the median specific humidity is about 1 g/kg higher after a cold pool passage compared to before, indicating a post-cold-pool moisture rise. When distinguishing between stronger and weaker cold pools, weaker cold pools exhibited a short decrease in specific humidity – a “dry dip” – shortly after a cold pool passage, followed by an increase in humidity. This pattern was not observed in stronger cold pools. However, there is a large spread in humidity evolution, and individual cold pool passages might deviate significantly – even in sign – from the aforementioned patterns.

In addition, the impact of measurement uncertainty in terms of calibration and inertia of sensors will also be discussed. All these findings contribute to the upcoming VITAL II (Vertical profiling of the troposphere: Innovation, opTimization, and AppLication) campaign in 2026, which will further expand the observational basis to describe the moisture structure of cold pools.