Clouds strongly modulate global and regional radiation balance, and cloud properties and cloud radiative effects (CREs) are profoundly influenced by circulations. CREs and their effects on general circulations is currently a hot and frontier research field, and is also listed as one of great scientific challenges by World Climate Research Program.
The spring over southeastern China (SEC) is a key transition period for regional thermal contrast and circulations, when regional circulations and CREs also exhibit remarkable subseasonal features. Hence, how spring CREs evolve at the subseasonal time scale and relate to regional circulation are essential issues for studying regional climate processes over SEC.
Recently Dr. LI Jiandong from Institute of Atmospheric Physics at Chinese Academy of Sciences and his collaborators used satellite and reanalysis data together with regional model simulations to investigate the spring shortwave cloud radiative effect (SWCRE) and the associated circulations over SEC. The research revealed distinctive spring shortwave cloud radiative effect over SEC. The work was published in Journal of Climate.
Researchers pointed out that the strongest spring SWCRE among the same latitudes of the Northern Hemisphere, with the maxim up to -110 W m-2, occurs over SEC. Over SEC, the SWCRE exhibits pronounced subseasonal variation and is closely associated with persistent regional ascending motion and moisture convergence, which favor large amounts of cloud liquid water and resultant strong SWCRE.
They analyzed the phenomenon in time sequence. Around late February, SWCRE abruptly increases and then remains stable between 22°N and 32°N until late June. Afterward, the temporal evolution of SWCRE falls roughly into two stages The first stage (late February to early May) is a slow change period, when the thermal and dynamic effects of Tibetan Plateau and westerly jet provide appropriate settings for the maintenance of ascending motion, while water vapor, as cloud water supply, stably comes from the southern flank of the Tibetan Plateau and South China Sea. In the second stage (early May to late June), the change is rapid. SWCRE is further enhanced by the quickly increased water vapor transport.
"This is mainly caused by the march of Asian summer monsoon, particularly the onset of the South China Sea monsoon. After late June, these circulations quickly weaken and the SWCRE decreases accordingly." Dr. LI noted.
This work provides new insights for seasonal features of cloud characteristics and is a useful attempt in combining cloud-radiation processes and regional circulations.
Reference: Li, J. D., W. C. Wang, J. Y. Mao, Z. Q. Wang, G. Zeng and G. X. Chen, 2019: Persistent spring cloud shortwave radiative effect and the associated circulations over southeastern China. J. Climate, 32, 3069–3087. https://journals.ametsoc.org/doi/abs/10.1175/JCLI-D-18-0385.1
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