Interaction of turbulence, ice crystals, and water vapour in cirrus clouds

The EMERALD-1 airborne campaign to investigate cirrus clouds was conducted at Adelaide Australia during September 2001. This involved the Egrett aircraft flying within cirrus clouds for in situ measurements of ice crystal properties, water vapour, wind, temperature and pressure. A second aircraft, a King Air, flew directly below the Egrett with an upward viewing lidar that mapped the structure of the clouds. Results are presented here that show the interaction between turbulence, humidity, ice crystal nucleation, growth, and sublimation. The maximum intensity of turbulence was measured at the top and bottom edges of the cloud, where ice crystals were either forming or sublimating. At the cloud top the largest ice crystal concentrations were associated with the greatest turbulence intensity. The width of the statistical distribution of relative humidity over ice was inversely proportional to the crystal concentration. The RHi distributions were approximately Gaussian in form and did not exhibit skewing toward supersaturation.