The amount of rain caused by tropical storms may be decreasing.

Each year, Earth experiences numerous tropical cyclones, hurricanes, and typhoons. These powerful storms, distinguished by their location, can bring about incredibly high levels of precipitation. For example, when Storm Daniel hit the Mediterranean region last month, it caused devastating floods and dam failures that led to thousands of fatalities. However, recent research has uncovered data indicating that the amount of rainfall delivered by these storms has actually decreased over time when compared to regular rainfall not caused by storms. This unexpected finding should be taken into consideration when creating climate models to enhance projections for the future.

The Churning Effect of Wind

When a large storm moves over a body of water, strong winds cause the water to become turbulent. According to Zhanhong Ma, a researcher of tropical cyclones at the National University of Defense Technology in Nanjing, China, this causes the water in the upper column to become mixed. This mixing has the effect of stirring up the surface. As the ocean is divided into layers based on temperature, with warmer water near the surface and colder water at deeper levels, this mixing leads to a decrease in temperature at the surface near where the storm is located. This phenomenon, known as a “cold wake,” has been well-documented and can last for several weeks.

Recent research by Ma and his team demonstrated that cold wakes from tropical cyclones can have significant effects on climate. After studying over 1,300 storms, they discovered that cloud coverage and rainfall were often diminished in the wake of these storms. The team attributed this phenomenon to the drop in ocean surface temperature, which leads to less evaporation and subsequently, less moisture in the atmosphere. Their findings, published in a 2020 study, suggest that the colder-than-usual ocean water strongly hinders precipitation and cloud formation.

A worldwide survey of storms.

Ma and her team recently explored the impact of cold wakes on the rate of rainfall within a storm. By analyzing precipitation data from 2,100 tropical storms that took place globally between 1998 and 2019, the researchers used measurements from microwave wavelength sensors on satellites such as the Global Precipitation Measurement Mission and Tropical Rainfall Measuring Mission. They collected over 50 data points for each storm and tracked them over both water and land. Their findings showed that the rate of rainfall within a 500 kilometer radius of each storm tended to increase over time, with an average annual increase of 0.8% in rain per hour.

However, the amount of background rainfall also showed an increase. The scientists observed that when they compared their storm data to the rainfall from the previous year at the same location, there was a decrease in the amount of rain per hour within 500 kilometers of each storm over time. Specifically, the inner cores of the storms (within 200 kilometers of the center) were delivering approximately 1.5% less rain per hour each year on average.

Is there a decrease in precipitation?

The results indicate that storms are contributing less to the overall global precipitation rates. This trend is likely due to the fact that cold wakes are growing colder over time, as suggested by the researchers. The data collected by the team supports this claim.

According to Ma, this trend is logical as warmer temperatures in the atmosphere cause the water near the surface of the sea to heat up faster than deeper water. This leads to a greater difference in temperature between the surface and subsurface waters, making it more likely for a storm to create a colder wake. Ma explains that a larger temperature difference will result from the same storm force.

According to Ma, these models should take into account certain patterns. He also stated that “cold wakes” are often not considered in most climate models. These findings were recently published in the journal Climate and Atmospheric Science.

The role of cold wakes in storm precipitation is certain, however, the team’s findings on rainfall rates within the inner cores of the storms may be underestimated. Kerry Emanuel, an atmospheric scientist at the Massachusetts Institute of Technology who was not part of the study, noted that this could be due to the limited spatial resolution of the team’s satellite observations, which is approximately 25 kilometers. This may not be enough to accurately capture the rainfall occurring in the center of a storm.

“I believe the data is not suitable for this type of analysis,” he stated. According to a theoretical research conducted by Emanuel and his team, it has been proposed that the intensity of rainfall within a storm’s core may potentially increase by over 30% by the end of the 21st century. However, Emanuel emphasized the significance of comprehending precipitation patterns in storms, stating that “rain causes more destruction and fatalities than wind.”

Ma and his team aim to further develop their model of cold wakes in order to gain a deeper understanding of how precipitation is affected. They suggest that by considering potential asymmetry, researchers could improve the accuracy of predicting future changes in rainfall.

, Eos

“Katherine Kornei (@KatherineKornei), a contributing writer for Eos, shared this message.”

Reference: Kornei, K. (2023). Decrease in Tropical Storm Rainfall, Eos, 104, https://doi.org/10.1029/2023EO230400. Published on October 20, 2023.

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