Prevalence of ice-supersaturated regions in the upper troposphere: Implications for optically thin ice cloud formation

In situ measurements of water vapor and temperature from recent aircraft ca mpaigns have provided evidence that the upper troposphere is frequently sup ersaturated with respect to ice. The peak relative humidities with respect, to ice (RHI) occasionally approached water saturation at temperatures rang ing from -40 degreesC to 70 degreesC in each of the campaigns. The occurren ce frequency of ice supersaturation ranged from about 20% to 45%. Even oil flight segments when no ice crystals were detected, ice supersaturation was measured about 5-20% of the time. A numerical cloud model is used to simul ate the formation of optically thin, low ice number density cirrus clouds i n these supersaturated regions. The potential for scavenging of ice nuclei (IN) by these clouds is evaluated. The simulations suggest that if less tha n about 5 x 10(-3) to 2 x 10(-2) cm(-3) ice nuclei are present when these s upersaturations are generated, then the cirrus formed should be subvisible. These low ice number density clouds scavenge the IN from the supersaturate d layer, but the crystals sediment out too rapidly to prevent buildup of hi gh supersaturations. If higher numbers of ice nuclei are present. then the clouds that form are visible and deposition growth of the ice crystals redu ces the RHI down to near 100%. Even if no ice clouds form. increasing the R HI from 100% to 150% between 10 and 10.5 km results in a decrease in outgoi ng lon-wave radiative flux at the top of the atmosphere of about 8 W m(-2). If 0.02-0.1 cm(-3) IN are present, the resulting cloud radiative forcing r educes the net radiative flux several watts per square meter further. Given the high frequency of supersaturated regions without optically thick cloud s in the upper troposphere. there is a potential for a climatically importa nt class of optically thin cirrus with relatively low ice crystal number de nsities. The optical properties of these clouds will depend very strongly o n the abundance of ice nuclei in the upper troposphere.