Impact of Asian Dust on Cirrus Formation Over the Central Pacific: CALIOP- and CloudSat-Observation-Based Case Studies

Abstract

Cirrus clouds are of great importance to the global climate, with their net radiative forcing strongly dependent on the microphysical properties that are related to the ice-nucleating regime. However, the influence of long-range transport of dust on primary ice formation in cirrus clouds is limitedly understood, specifically over the clean remote ocean regions. Here, two case studies show that transpacific Asian dust can impact the ice formation of cirrus clouds over the central Pacific based on Cloud-Aerosol Lidar with Orthogonal Polarization and Cloud Profiling Radar (CPR, CloudSat) observations. One case shows a well-developed horizontally extended cirrus embedded in a pure dust layer, with an average dust-related ice-nucleating particle concentration (INPC) of 7 L⁻¹ and 96 L⁻¹ for an ice saturation ratio S_i of 1.15 and 1.25, respectively; ice crystal number concentration (ICNC) with diameters >25 and 100 μm (denoted as n_{ice,25 μm} and n_{ice,100 μm}) are 64 L⁻¹ and 7 L⁻¹, respectively. Another case shows that cirrus clouds with a much smaller horizontal extent appeared in the vicinity of polluted dust, with an average INPC of 42–310 L⁻¹ for the typical higher S_i of 1.25–1.35 by considering a tenfold reduction of the ice nucleation efficiency of ice crystals; n_{ice,25 μm} and n_{ice,100 μm} are 168 L⁻¹ and 20 L⁻¹, respectively. The estimated INPC and ICNC values suggest the dominance of ice formation by dust-induced heterogeneous nucleation, proving that the long-range transport of dust toward the upper troposphere and the potential influence on cirrus formation over the central Pacific should be well considered in atmospheric models.