Tropical Subvisual Cirrus and Contrails at -85°C

The microphysical properties of aircraft contrails and subvisual cirrus in the region of the tropical tropopause have been measured with a cloud aerosol and precipitation spectrometer (CAPS) during the Costa Rica Aura Validation Experiment (CR-AVE) in February, 2006. The CAPS is an instrument that contains two particle sensors, the cloud aerosol spectrometer (CAS), that measures forward (4-12°) and backscattered (168-176°) light from individual particles having a size range from 0.5-45µm, and a cloud imaging probe (CIP) that measures crystal images from 25 μm to > 1600 μm. The refractive index is derived from spherical particles and a shape factor from those that are aspherical. The CAPS was mounted on the right wing of the NASA WB-57F aircraft and made measurements during 12 flights during the period January 9 to February 11, 2006.

On consecutive days, February 1 and 2, clouds with optical depths less than 0.03 were encountered in the tropopause region between 17 and 18 km, with an average temperature of -85°C and a relative humidity with respect to ice (RHI) of approximately 140%. On the first of these flights, the aircraft measured its own contrail within this same temperature and RHI regime. A comparison of the microphysical properties of the cirrus and contrails showed that the majority of particles were less than 40 μm and 50% of the particles were sphere-like. The principle difference between the subvisual cirrus and contrails was found in the spatial distribution of the particles. Those in the subvisual cirrus had randomly uniform spatial distributions whereas the particles in the contrail appeared more organized with non-random spacing that could indicate clustering. Of the particles that were classified as sphere-like, approximately 20% had refractive indices representative of ice, i.e. in the range 1.31-1.33. The other sphere-like particles had average refractive indices of 1.40 and 1.54. The frequency distribution of the refractive indices of particles in the cirrus and contrail were quite similar. In this presentation we discuss the processes that could lead to mixed-phase cirrus and contrail at the tropical tropopause and the implications for heterogeneous chemical reactions, climate impact and interpretation of satellite measurements.