Urban lands and aerosols amplify dangerous weather and direct storms to cities

Violent convection is only one direction of urban land, and artificially generated aerosols can form dangerous weather. Longer lasting rainfall and larger hail are other potential by-products of the interaction between cityscapes and storms, according to a new study by PNNL scientists. Credits: 12019 |

Cityscapes and artificial aerosols (particles floating in the atmosphere) not only strengthen gusts and increase hail, but also have potential. They can even start storms earlier and even draw them to cities, according to a new study led by scientists at the US Department of Energy’s Pacific Northwest National Laboratory to investigate the impact of urban development on dangerous weather.

By modeling two thunderstorms near Houston, Texas and Kansas City, Missouri, atmospheric scientist Jiwenfan explained that urban landscapes and artificial aerosols can affect storms, rain, and hail. Brought out the synergistic effect of.

In the case of the Kansas City storm, urban lands and aerosols worked together to increase the frequency of large-scale hail by about 20 percent. In Houston, otherwise mild thunderstorms developed faster, amplified, longer-lasting rainfall, among other changes.

Fans shared their findings at the American Geophysical Union’s Fall 2020 conference on Tuesday, December 1, effectively answering questions on Tuesday, December 15.

“The novelty of our research is to consider both urban lands and aerosols together, not separate effects,” fans said.

In previous studies, researchers have shown that urban land shapes the weather through both its topographical properties and the heat it produces. Cities are often warmer than their surroundings because buildings not only absorb and retain the heat of the sun differently than trees and farmlands, but also block the flow of wind.

However, many studies focus solely on how cities and aerosols change precipitation and temperature, not their joint effects, but only the effects of these factors. ..

Simulated storms reveal dangerous weather changes

Fans have modeled two very different types of storms. The intense, spinning, hailstorms of Kansas City and the mild, sea breeze-induced thunderstorms of Houston. She isolated the effects of these two different factors by simulating multiple versions of the same storm with and without cities and aerosols.

In Houston, the afternoon breeze swelled as urban lands and aerosols worked synergistically to amplify rainfall. Compared to the cityless simulation, the rain wet Houston about 30 minutes earlier, increasing its total by an additional 1.5 millimeters. Due to the influence of the land in the city, the sea breeze was blowing strongly.

As cooler, denser air from the sea breeze flowed toward Houston, it brought moisture and collided with warmer, lighter urban air. The two mixed during the meeting to create stronger convection compared to the city’s landless simulation.

The thundercloud in Houston began as a warm cloud with only droplets, but as the sea breeze intensified, the transition to a multiphase cloud named after the simultaneous mixing of water vapor, ice particles, and supercooled water droplets was accelerated. .. Even after the sea breeze blew, the residual heat from the city continued to provide storm convection overnight, causing long-lasting rain, Huang said. In contrast, fan simulations with cities removed show that weak sea breeze and storms dissipated faster.

Aerosols played a greater role in increasing precipitation than urban land in Houston. As the mixed-phase clouds formed and the convection became stronger, many ultrafine particles turned into cloud droplets. This conversion facilitated the conversion of water vapor into a cloud condensate, which increased latent heat and further strengthened the storm.

In the case of the Kansas City storm, heat from the city was carried downwind, where it encountered a storm that had already formed on the northern urban-rural boundary. When the warm, dry air met the cold, moist country air, it strengthened convergence, creating turbulent mixes and more intense storms, moving towards urban lands.

In contrast to Houston’s thunderstorms, Kansas City aerosols did not affect the onset or propagation of storms, and by themselves did not significantly affect hail. However, when simulated with urban lands, two amplified hail synergistically occur, creating a more dangerous hail storm. Because of this relationship, fans say it is important to consider both the city’s land and aerosols when investigating the impact of the city on the weather and related hazards.

Hail alone can cost billions of dollars in the United States, and according to the US National Oceanic and Atmospheric Administration, particularly large hail can fall at speeds of over 100 miles per hour.

Description of aerosol

According to fans, urban lands and aerosols have different forms of weather, depending on other environmental conditions, such as whether the air is already polluted.

“The aerosol effect really depends on the background concentration,” Fan said. “If the environment is already polluted, adding aerosols doesn’t seem to have much effect, but it’s already in good condition and adding aerosols can have a big impact.”

Houston’s busy shipping channels and nearby oil refineries (three of which are in metropolitan areas) regularly release aerosols into the atmosphere, Huang said. Humidity can also amplify the aerosol effect, she added.

Fans hope her work will lead to more accurate predictions of dangerous weather, reducing death and damage from storms. She plans to explore more deeply how vast urbanization will shape severe storms in future climate scenarios.

Jeddah rains

For more information:
Jiwen Fan et al, Impact of land and aerosols caused by urbanization on sea breeze circulation and convective precipitation, Atmospheric chemistry and physics (2020). DOI: 10.5194 / acp-20-14163-2020

Provided by Pacific Northwest National Laboratory

Quote: Urban lands and aerosols amplify dangerous weather and steer storms towards the city (15 December 2020) 15 December 2020 Obtained from -urban-aerosols-amplify-hazardous-weather.html

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Urban lands and aerosols amplify dangerous weather and direct storms to cities

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