Upper‐tropospheric relative humidity observations and implications for cirrus ice nucleation

Abstract

Relative humidity (RH) measurements acquired in orographic wave cloud and cirrus environments are used to investigate the temperature‐dependent RH required to nucleate ice crystals in the upper troposphere, RH nuc (T). High ice‐supersaturations in clear air—conducive to the maintenance of aircraft contrails yet below RH nuc and therefore insufficient for cirrus formation—are not uncommon. Earlier findings are supported that RH nuc in mid‐latitude, continental environments decreases from water‐saturation at temperatures above −39°C to 75% RH at −55°C. Uncertainty in determining RH nuc below −55°C results in part from size detection limitations of the microphysical instrumentation, but analysis of data from the SUCCESS experiment indicates that RH nuc below −55°C is between 70 and 88%. A small amount of data acquired off‐shore suggests the possibility that RH nuc may also depend on properties of the aerosols.