On the Relation between Ice Saturation Ratio and Microphysical Properties in Cirrus Clouds

This study aims to investigate the interaction between the ice saturation ratio (Si, defined as relative humidity over ice divided by 100) and the microphysical properties of cirrus clouds. To this end, high quality in-situ aircraft measurements of relative humidity, ice crystal number concentration and ice water content as well as vertical velocity from field campaigns in mid-latitudes, the Arctic and the tropics are evaluated.

It is known from the theory of ice formation, that Si is the variable thatalready determines the point of formation of cirrus clouds. Ice particles form below -38C at the so-called freezing thresholds, which are specific Si's at which ice formation is triggered. These thresholds are larger than 1, i.e. cirrus clouds form in supersaturated conditions. The freezing threshold is generally higher for homogeneous ice nucleation of supercooled solution particles than for heterogeneous ice formation on ice nucleating particles (INPs). The number of ice crystals that form homogeneously is determined by the vertical velocity of the ascending air parcel with higher ice concentrations at faster updrafts, while heterogeneous freezing gives at most the number of the rare INPs. Since the number of ice crystals correlates with the ice water content (IWC), cirrus formed heterogeneously and homogeneously in slow updrafts tends to be physically and optically thin, while cirrus formed homogeneously in fast updrafts tends to be thick. Ice clouds that form over these pathways are called in-situ origin cirrus. The second type of ice cloud is called liquid origin cirrus, which forms as a liquid cloud at temperatures above -38C and then completely glaciates in rising air parcels. Since the air has a higher water content at warmer temperatures, cirrus of liquid origin are thick, i.e. they have higher IWCs.

From theoretical considerations it will be shown how the furtherdevelopment of the cirrus cloud microphysical properties depends on the evolution of Si inside of the clouds, which in turn is correlated with the vertical velocity. In the case of updrafts, the uptake of water molecules on the surfaces of the ice crystals is not sufficient to completely reduce Si to saturation, so the ice particles continue to grow in the supersaturated environment (Si > 1). The updrafts can keep the supersaturation high enough to grow the ice particles sufficiently large to fall out, thus reducing the total ice surface. As a result, Si can increase again up to the homogeneous freezing threshold and new ice particles appear. Such Si-driven formation-sedimentation-formation cycles are not uncommon in cirrus clouds. In downdrafts, i.e. subsaturated conditions (Si < 1), the ice crystals will evaporate on a timescale dependent on the strenght of the downdraft and the size of the ice crystals. Ice saturation (Si = 1), i.e. thermodynamic equilibrium without any change in ice crystal size, occurs only when the vertical velocity is zero.

We will further present that we have found from the observations in the fieldexperiments, in agreement with the theory, that high ice saturation ratios, i.e. supersaturated cirrus clouds, are associated with either to very low ice crystal numbers (thin cirrus), corresponding to low vertical velocities, or to very high ice crystal numbers (thick cirrus), corresponding to high vertical velocities. The highest supersaturations are found in the tropics at cold temperatures below 200K and at low as well as high ice concentrations (~ low/high vertical velocities). This is because the the water vapour transport to the ice surface slows down with decreasing temperature.

In addition, it is found that cirrus in the evaporation stage, which is characterized by low ice saturation ratios, i.e. subsaturation, are frequently associated to low ice concentrations. This is because the more abundant small ice crystals evaporate faster than the rarer larger ones.

Special conditions are found in Artic observations: simultaneously to high in-cloud supersaturations, more frequent and higher supersaturations are observed in clear sky in comparison to the other regions. Higher supersaturations in clear skies indicate a lack of INPs, which would otherwise initiate ice formation at the low heterogeneous freezing threshold. Instead, the homogeneous freezing threshold had to be reached in order to trigger ice formation. However, due to the prevailing low vertical velocity, only few ice crystals formed. As a result, the relative humidity continued to be supersaturated within the clouds.