Observation of Phase Composition and Humidity in Oceanic Mesoscale Convective Systems

The in-situ data of cloud microphysical and thermodynamical parameters were collected during the High Ice Water Content project (French Guiana, 2015) from the Canadian National Research Council Convair-580 airplane. The Convair-580 was equipped with state of the art in-situ and remote sensing instrumentation for cloud measurements. The measurements were conducted in mature oceanic mesoscale convective storms in the -15C<T<-5C temperature range, which corresponds to altitudes 6km<Z<7.5km. It was found that the main bulk of the cloud systems was dominated by pure ice regions, whereas mixed phase was found mainly in updrafts. The spatial occurrence of mixed phase on average did not exceed a few percent. The humidity in mixed phase cloud regions was found to be close to saturation with respect to liquid. However, humidity in ice clouds turned out to be on average dependent on IWC. Thus, in cloud regions with IWC>0.5g/m3 the distribution of relative humidity over ice was well centered at 100%. Whereas the clouds with IWC<0.2g/m3 were frequently undersaturated with respect to ice. This can be explained by small time of phase relaxation in clouds with higher IWC, so that the population of ice particles brings the environment into equilibrium state faster, compared to the clouds with lower IWC and larger time of phase relaxation. The statistics of time of phase relaxation for mixed phase and ice cloud regions and its dependencies on IWC are presented. It was found that the time of phase relaxation ranges from 30s to 1 hour depending on IWC. The conditions required for maintenance of mixed phase in updrafts are discussed and compared with the theoretical estimates.