Case study of a humidity layer above Arctic stratocumulus using balloon-borne turbulence and radiation measurements and large eddy simulations

Abstract. Specific humidity inversions occur frequently in the Arctic. The formation of these inversions is often associated with large scale advection of humid air. However, small-scale boundary layer processes interacting with the humidity inversions are not fully understood yet. In this study, we analyze a three-day period of a persistent layer of increased specific humidity above a stratocumulus cloud observed during an Arctic field campaign in June 2017. The tethered balloon system BELUGA (Balloon-bornE moduLar Utility for profilinG the lower Atmosphere) recorded high-resolution vertical profile measurements of turbulence and radiation in the atmospheric boundary layer. We find that the humidity inversion and the cloud layer are coupled by eddy dissipation, extending above the cloud boundary and linking both layers through turbulent mixing. One case reveals a strong negative virtual sensible heat flux at cloud top (eddy covariance estimate of −15 W/m²), indicating entrainment of humid air from above into the cloud layer. Large Eddy Simulations (LES) based on field campaign data are conducted to supplement the flux measurements. Independent experiments for two days confirm the observed entrainment of humid air, reproducing the observed negative turbulent fluxes of heat and moisture at cloud top. The LES realizations suggest that in the presence of a humidity layer the cloud layer remains thicker and the inversion height is slightly raised, reproducing results from previous idealized LES studies. While this acts to prevent cloud collapse, it remains unclear how the additional moisture is processed in the cloud and how exactly it contributes to the longevity of Arctic cloud layers.