Ice supersaturation and cloud microphysical properties in cold cirrus formed near the tropical tropopause

We will use numerical simulations along with recent high-altitude aircraft measurements to investigate the occurrence of large supersaturations with respect to ice in cold tropical tropopause layer (TTL) cirrus. The Airborne Tropical TRopopause EXperiment (ATTREX) includes accurate and precise measurements of water vapor from the Diode Laser Hygrometer (DLH) and a new NOAA instrument that measures both vapor and total water (ice plus vapor). Cirrus ice crystal concentrations, sizes, and habits were measured with the Spec Hawkeye probe (a combination of a spectrometer, an imaging array probe, and a high-resolution imager) and a stand-alone Fast Cloud Droplet Probe (FCDP). A number of TTL cirrus systems were sampled with Global Hawk flights to the tropics from southern California in 2011 and 2013, and with flights in the western Pacific from Guam in 2014.

We will begin by surveying the relationships between ice supersaturation and cirrus microphysical properties using the entire ATTREX dataset. Next, we will use temperature histories from meteorological analyses along with a detailed cloud model to simulate specific ATTREX TTL cirrus cases. The model uses Lagrangian tracking of thousands of individual ice crystals. We will use the simulations to explore the time dependence of supersaturation and cloud microphysical properties. We will also investigate the impacts of assumptions about ice nucleation mechanism (heterogeneous versus homogeneous) and small-scale waves on cirrus microphysics and supersaturation.