A study by Dr. HOU Tuanjie's team from the Institute of Atmospheric Physics (IAP) of the Chinese Academy of Sciences provides more insights into the microphysical properties of stratiform clouds, especially those with embedded convection, or thunderstorm activity.

The study was published in Advances in Atmospheric Sciences on May 7.

Stratiform clouds are relatively uniform formations that are non-convective. They often follow behind convective lines of thunderstorms, producing light to moderate rain. However, sometimes convective cells are embedded within regions of stratiform clouds, under which precipitation rates are higher than in the surrounding stratiform cloud ceiling.

Aircraft measurements have helped scientists better understand stratiform clouds. However, few observations of embedded convection are available since strong rising air within updrafts can significantly affect aircraft and therefore human safety.

Photo of cumulus humilis taken from the aircraft. (Image by HOU Tuanjie)

The researchers set out to learn more about how supercooled liquid water, or liquid water that exists below 0°C,  and particle size distributions vary from upper to lower levels of the atmosphere.

They also analyzed these differences within both convective and stratiform regions. Using aircraft measurements from an ordinary mixed phase stratiform cloud system on May 22, 2017, they found several key insights on the internal structure of embedded convective clouds.

Most of the observed supercooled liquid water droplets existed above -6°C, with a substantial decrease of remaining liquid water below that threshold. Embedded convective cells featured much more supercooled liquid water than their stratiform counterparts.

The team also found two distinct differences in particle size distributions between convective and stratiform regions. One is the significant shift toward larger particles when temperatures reach -10°C in the convective region, where the maximum particle dimensions are greater than 1 cm. These particles are composed of dendrites, or branching snowflakes and other similar ice structures.

That said, between temperatures of -3 to -5°C, a large concentration of small particles existed in the convective region, where rimed ice particles and needles coexist. Needle shaped particles occurred in both stratiform and convective regions, but high concentrations of these ice crystals produced by the Hallet-Mossop, or ice riming process existed only in convective clouds.

The study helps gain insights into the microphysical properties of stratiform clouds, especially those with embedded convection, or thunderstorm activity, and allows assessments for potential weather modification.