Measurements of the Growth of the Ice Budget in a Persisting Contrail

Abstract

An experiment was conducted with the NCAR Sabreliner to measure the ice water content (IWC) and the total ice budget within its contrail. Particle size distributions were measured with an optical-array particle size spectrormeter. The experiment was performed in a region of the atmosphere void of natural ice crystals (cirrus) but estimated to be above ice saturation and favorable to ice crystal growth. The IWC measured was found to be in excess of 0.1 gm m⁻³ near the contrail axis with average values of a few hundredths of a gram throughout the contrail. The measured water mass within the contrail was found to be four orders of magnitude greater than that computed as a combustion product. Average crystal sizes of nearly 0.5 mm allow for a transfer of moisture at generation level to much lower levels before re-evaporation. Because of the magnitude of the measured effect and its believed frequent occurrence, the overall effect of sub-tropopause jet traffic is likely to lower the water abundance at the most traveled levels. The total number of ice crystals produced is similar to the number of droplets expected in the initial liquid water contrail. All ice crystals appear to be produced through the freezing of droplets. The rate of diffusional spreading of the contrail corresponds to an eddy diffusivity of 1.5× 10⁵cmsec⁻¹. No turbulence was detected and the diffusion appears isotropic after the rapid dissipation of the initial vertically organized turbulent wake.