Hemispheric comparison of cirrus cloud evolution using in situ measurements in HIAPER Pole-to-Pole Observations

The radiative forcing of cirrus clouds is highly sensitive to their microphysical properties as well as their evolution. However, there are very limited observations and analyses on the cirrus cloud formation and evolution with both microscale resolution and global scale coverage. The lack of such observations limits the overall comparison of cirrus cloud evolution between the two hemispheres. In fact, the previous hemispheric comparisons of cirrus cloud microphysics using in situ observations were only based on the sampling over two locations at 55°N and 55°S (Ovarlez et al., 2002; Gayet et al., 2004). Here we use a more comprehensive aircraft-based dataset (~200 m) from the NSF HIAPER Pole-to-Pole Observations (HIPPO) Global campaign at 87°N–67°S over North America and the central Pacific Ocean to analyze the hemispheric differences in cirrus cloud evolution.

In contrast to the previous work, we found that not only there are no significant differences in the initial condition of ice crystal formation – clear-sky ice supersaturation (ISS) – between the Northern and Southern Hemispheres (NH and SH), but also no significant differences are observed in the evolution of other microphysical properties, such as relative humidity with respect to ice (RHi), ice crystal number density and mean diameter. In addition, when separating the cirrus cloud lifecycle into five phases, i.e., clear-sky ISS, ice crystal nucleation, early growth, later growth, and sedimentation/sublimation, the relative timescale of each evolution phase between the NH and SH is also similar. These results suggest that despite the significant differences in aerosol loading between the hemispheres, the evolution of the microphysical properties of cirrus clouds follows similar trend in the HIPPO observations.