Interaction of Atmospheric Cold Pools with Background Flow: A Laboratory Study

Atmospheric Cold Pools (ACPs) are large, mesoscale cold air masses that form beneath deep convective clouds due to rain evaporation. These dense air pockets sink into the warmer surroundings, spread out horizontally after reaching the ground, and propagate as gravity currents (GCs). It is hypothesized that these GCs can trigger secondary convection by mechanically lifting warm air to the level of free convection. However, a detailed understanding of ACP dynamics has been limited by the lack of high-resolution field data and numerical simulations, particularly in the presence of ambient mean flow. A 2019 aircraft observational campaign in the Bay of Bengal suggested the presence of an ACP-induced GC, which, under the right conditions, could lift the ambient flow above the GC like a ramp, provided the GC remains intact without local mixing and destruction. Laboratory experiments simulated this interaction using a descending buoyant plume in a recirculating water channel, where particle image velocimetry (PIV) and flow visualization were used to study the updraft flow and its characteristics. It was found that a ramp-like flow facilitating an updraft is viable when the Richardson number (Ri) exceeds a critical value of approximately 6. Empirical relations derived from the study were applied to field cases, showing good agreement and offering valuable insights for future ACP-related research and field studies.