Global analysis of generation mechanism of ice microphysics inferred from space-borne active sensors and infrared sounder.

Abstract:

We examined relationship between ice microphysics and super-saturation by the synergy use of CloudSat cloud radar and CALIPSO lidar and infrared sounder AIRS on Aqua. The retrieval algorithm of ice microphysics was based on Okamoto et al., (2010) but the following improvement was implemented. Ice non-sphericity, orientation and laser pointing angle were considered to estimate scattering properties needed in the analysis of CloudSat and CALIPSO lidar data by the physical optics approximation (Borovoi et al., 2012) and the discrete dipole approximation (Okamoto 2002). Different look up tables were applied for the period of laser-pointing angle of 0.3 degrees off-nadir direction and for the one of 3 degrees off-nadir. Super-saturation was retrieved using 110 temperature and 46 humidity channels of AIRS (Ishimoto 2009 and Ishimoto et al., 2014). The algorithm relied on channel ranking approach to detect clouds by the AIRS information alone so that microwave sounder information was not needed contrary to the AIRS standard product. This achieved three times higher spatial resolution (13.5km) compared with the AIRS standard product so that it is suitable to study super-saturation. We analyzed ice microphysics and super-saturation in the vicinity of the ice clouds to analyze the relation between ice microphysics and super-saturation. AIRS showed relative humidity can reach 160% but majority was below 130%. Super-saturation generally occurred in correspondence with ice clouds. Concept of equivalent ice water content (IWCeq) was introduced as difference between water vapor density and saturation water vapor density. IWCeq was generally larger than IWC. Geophysical patterns between IWCeq and IWC were in good agreement in mid- and high-levels. Global analysis showed IWC increased as IWCeq increased. It was found such relation did not depend on latitudes. Ice formation seemed to occur at about 5 to 10% below relative humidity.