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Understanding the Evolution From Cloud to Rain

The cloud water path of mature tropical cyclones can be estimated by a notable sigmoid function of near-surface rain rate, a study reveals.

Clouds play a significant role in the climate system due to their modification effects on the global radiation balance and atmospheric water cycle. Understanding the evolution of cloud parameters can help with simulation of cloud-to-rain transitions in models, thereby improving the capability to predict the track and intensity of tropical cyclones.

In the process from formation of clouds to occurrence of precipitation, knowledge on the actual linkage between the two is limited because of the complicated microphysics inside the cloud. Most previous studies have compared precipitation data from ground-based observations with cloud parameter information from satellite inversions. However, these two types of data are not synchronized in time and space, which causes errors in the results.

Professor Yunfei Fu, a professor at University of Science and Technology of China, led a team to overcome this drawback by combining time- and space-synchronized precipitation and spectral data obtained by the Precipitation Radar as well as the Visible and Infrared Scanner onboard the TRMM satellite. The team obtained 25 collocated satellite overpasses of mature typhoon cases in the Northwest Pacific Ocean, an area of frequent tropical cyclones, from 1998 to 2012 (144,515 precipitation pixels in total).

The results show that the cloud water path exhibits an oblique S-type change with increasing near-surface rain rate and ultimately tends toward saturation. In addition, the cloud water path and near-surface rain rate of mature typhoon systems with different precipitation types, precipitation cloud phases, and vertical depths of precipitation can be fitted by a notable sigmoid function, which may be useful for estimating the cloud water path and parameterizing precipitation in models.

“These newly derived relations certainly provide a new way to estimate the cloud water path of mature typhoon systems in the Northwest Pacific Ocean,” Dr. Shuang Luo, first author of the study published in Advances in Atmospheric Sciences, believes. “To better capture information inside the clouds, we need to obtain not only the cloud microphysical properties near the cloud tops, but also the profiles of cloud parameters inside the clouds, which is essential to analysing the correlation between cloud and precipitation profiles.” [APBN]