Microphysical Properties of TTL Cirrus

Pervasive cirrus clouds in the Tropical Tropopause Layer (TTL) play an important role in determining the composition of stratospheric air through dehydration of tropospheric air entering the stratosphere.  This dehydration affects Earth’s energy budget and climate, yet little is known regarding the microphysical processes that govern TTL cirrus.  To investigate the processes influencing the microphysics of these TTL cirrus, we present the variability in ice concentrations, size distributions and habit as functions of temperature, altitude, supersaturation with respect to ice, and time since convective influence from recently collected airborne in situ measurements made in the Western Pacific TTL.  A Fast Cloud Droplet Probe (FCDP), 2D-Stereo (2D-S) probe, and Cloud Particle Imager (CPI) were flown during the NASA ATTREX and POSIDON missions to provide particle sizing, concentration, extinction, and high resolution cloud particle images for habit identification.  Supporting measurements of water vapor from the NOAA instrument and pressure and temperature from MMS are used to derive estimates of supersaturation with respect to ice.  Observed ice particles were predominantly spheroidal in shape, with the percentage of spheroids increasing with decreasing temperature.  In comparison to the large population of spheroids, the faceted habits (columns, plates, rosettes, and budding rosettes) were found in generally low percentages, and also show correlation with temperature. The trend showing higher percentages of faceted crystals at warmer temperatures may be due to diffusional growth as particles descend through cloud, and/or the more rapid diffusional growth rate at warmer temperatures.  Evidence of particle sedimentation with increasing time since convective influence is also presented.