Stratiform clouds are relatively uniform formations that are non-convective, or lack vertical development, like a cumulus cloud. 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. Meteorologists have noticed that this phenomenon is common in northern China, prompting further analysis, especially for weather modification studies.

Aircraft measurements have helped scientists better understand stratiform clouds, as they are relatively safe to fly through or around. However, few observations of embedded convection are available since strong rising air within updrafts can significantly affect aircraft and therefore human safety. Despite this, researchers understand how important further study is to find the microphysical characteristics within the embedded convective cells.

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

A recently published study in Advances in Atmospheric Sciences features Dr. HOU Tuanjie and her research team from the Institute of Atmospheric Physics at the Chinese Academy of Sciences. These scientists 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. Additionally, the team analyzed these differences within both convective and stratiform regions. Using aircraft measurements from an ordinary mixed phase stratiform cloud system on 22 May 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. The first of which is a 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 helped gain insights into the microphysical properties of stratiform clouds, especially those with embedded convection, or thunderstorm activity. This data provides scientists with helpful information about the dynamics of precipitation formation and allows assessments for potential weather modification.

Citation: Hou, T. J., H. C. Lei, Y. J. He, J. F. Yang, Z, Zhao, and Z. X. Hu, 2021: Aircraft Measurements of the Microphysical Properties of Stratiform Clouds with Embedded Convection. Adv. Atmos. Sci., https://doi.org/10.1007/s00376-021-0287-8.

Media contact: Ms. LIN Zheng, jennylin@mail.iap.ac.cn