Glaciation temperatures of convective clouds ingesting desert dust, air pollution and smoke from forest fires

Abstract

[1] Heavy aerosol loads have been observed to suppress warm rain by reducing cloud drop size and slowing drop coalescence. The ice forming nuclei (IFN) activity of the same aerosols glaciate the clouds and create ice precipitation instead of the suppressed warm rain. Satellite observations show that desert dust and heavy air pollution over East Asia have similar ability to glaciate the tops of growing convective clouds at glaciation temperature of Tg < ∼ −20°C, whereas similarly heavy smoke from forest fires in Siberia without dust or industrial pollution glaciated clouds at Tg ≤ −33°C. The observation that both smoke and air pollution have same effect on reducing cloud drop size implies that the difference in Tg is due to the IFN activity. This dependence of Tg on aerosol types appears only for clouds with r e-5 < 12 mm (r e-5 is the cloud drop effective radius at the −5°C isotherm, above which ice rarely forms in cloud tops). For the rest of the clouds the glaciation temperature increases strongly with r e-5 with little relation to the aerosol types, reaching Tg> ∼ −15°C for the largest r e-5 , which are typical to marine clouds in pristine atmosphere. Citation:

Effects of cloud condensation nuclei and ice nucleating particles on precipitation processes and supercooled liquid in mixed-phase orographic clouds

“…A recent observational study corroborated that increasing CCN decreases the cloud glaciation temperature and thus increases the abundance of the mixed-phase regime (Zipori et al, 2015). With abundant INPs such as dust particles, clouds glaciate at a much warmer temperature (Rosenfeld et al, 2011;Zipori et al, 2015). It was found that commonly occurring supercooled water in the clouds near the coastal regions of the western US is associated with low-CCN and limited-INP conditions .…”

The importance of interstitial particle scavenging by cloud droplets in shaping the remote aerosol size distribution and global aerosol-climate effects

“…As glaciation often occurs at warmer temperatures, we perform a sensitivity simulation to this temperature cutoff, described below. We justify this temperature threshold since super-cooled liquid clouds can exist at temperatures as cold as 238 K, although the onset of glaciation can occur at temperatures as warm as 258-263 K (Rosenfeld and Woodley, 2000;Rosenfeld et al, 2011). Recent studies indicate that low-level liquid clouds are ubiquitous in all seasons in remote regions such as the Arctic (e.g., Cesana et al, 2012).…”

Improving our fundamental understanding of the role of aerosol−cloud interactions in the climate system

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