Cloud-top entrainment during polar day: first results from in situ observations in Pallas, Finland

Turbulent entrainment at cloud top regulates exchange across the cloud–clear-air interface and shapes cloud microphysical structure, variability, and evolution. Yet the governing gradients and mixing events remain difficult to represent in models and to observe with sufficient vertical context, largely due to limited observational capabilities at small scales. Furthermore, quantifying turbulent transport requires high-frequency measurements of vertical velocity and scalar quantities.

Here, we present first results on cloud-top entrainment from the IMPACT field campaign (”In-situ Measurement of Particles, Atmosphere, Cloud and Turbulence”, May-June 2024) in Pallas, Finland, under polar-day conditions. During IMPACT, we deployed the Max Planck WinDarts, lightweight airborne in situ probes designed for vertically distributed measurements on the Max Planck CloudKite (a tethered kite-balloon system). Each WinDart provides high-resolution measurements of 3-D wind, temperature, relative humidity, and pressure, enabling the derivation of turbulent statistics and fluxes across a vertical column. By deploying four WinDarts spaced 50 m apart along the tether, we estimate vertical turbulent fluxes of heat and moisture in the cloud-top region. The vertically resolved measurements provide insights into entrainment processes, turbulence statistics and scalar variability near the cloud top. The findings also demonstrate the potential of tethered, vertically distributed in situ sampling to advance our understanding of entrainment processes.