Weather Front

MAY / JUNE 2023 WEATHERWISE People Leaving Urban Areas Can Reduce City Temperature In the latest study on the urban heat island effect, researchers at Nanjing University in Nanjing, China, found that during the week-long Chinese New Year, the massive outflow of residents of Chinese cities lowered the temperature in these cities significantly. During the Chinese New Year, also known as the Lunar New Year, work is suspended, businesses close, and nearly 3 million people leave cities to join their families in rural areas for traditional gatherings. The holiday is the largest short-term suspension in human activity on Earth. Urban infrastructure retains more heat than natural landscapes or farmland, making cities up to 7°F warmer than surrounding areas. Infrastructure such as roads and sidewalks absorbs more heat than areas covered with grass and trees. Meanwhile, cars, air-conditioning, heating, building operations, and industrial facilities are all examples of human activities that emit heat and also contribute to increased temperatures in cities. The researchers used satellite data and hourly air temperature data collected from ground-based meteorological observation networks from 2017–2019 to determine that during the New Year, the reduction in human activity was enough to lower the average urban heat island intensity in 31 cities by an average of 33%. This corresponded with an average drop in surface air temperature of 0.63°F in cities. The heat reduction was significantly less pronounced in the outskirts of the cities, suggesting that the more populated city centers experience larger swings in heat intensity variability when there are changes in human activities. Zihan Liu, an Earth system scientist at Nanjing University and corresponding author of the study, said that anthropogenic heat release is a major contributor to warming in urban areas, and this result implies that urban heat mitigation during warmer times of the year when people are at risk from extreme temperatures could benefit people living in cities throughout China. The study was published in AGU’s Geophysical Research Letters.

I n the latest study on the urban heat island effect, researchers at Nanjing University in Nanjing, China, found that during the week-long Chinese New Year, the massive outflow of residents of Chinese cities lowered the temperature in these cities significantly. During the Chinese New Year, also known as the Lunar New Year, work is suspended, businesses close, and nearly 3 million people leave cities to join their families in rural areas for traditional gatherings. The holiday is the largest short-term suspension in human activity on Earth.
Urban infrastructure retains more heat than natural landscapes or farmland, making cities up to 7°F warmer than surrounding areas. Infrastructure such as roads and sidewalks absorbs more heat than areas covered with grass and trees.
Meanwhile, cars, air-conditioning, heating, building operations, and industrial facilities are all examples of human activities that emit heat and also contribute to increased temperatures in cities.
The researchers used satellite data and hourly air temperature data collected from ground-based meteorological observation networks from 2017-2019 to determine that during the New Year, the reduction in human activity was enough to lower the average urban heat island intensity in 31 cities by an average of 33%. This corresponded with an average drop in surface air temperature of 0.63°F in cities.
The heat reduction was significantly less pronounced in the outskirts of the cities, suggesting that the more populated city centers experience larger swings in heat intensity variability when there are changes in human activities.
Zihan Liu, an Earth system scientist at Nanjing University and corresponding author of the study, said that anthropogenic heat release is a major contributor to warming in urban areas, and this result implies that urban heat mitigation during warmer times of the year when people are at risk from extreme temperatures could benefit people living in cities throughout China.
The study was published in AGU's Geophysical Research Letters. In addition to there being a flood event eight out of every 10 days, the analysts found that most floods are caused by weatherfront New National Record Low Wind Chill Set A new national low wind chill record was set on February 3, 2023, during a cold spell in the Northeast. The record was set at Mount Washington, New Hampshire, on a night when instruments recorded a wind chill of -108°F thanks to a temperature of -46°F and wind gusts of 127 mph.

Flooding Is Even More Common Than We Might Think
According to Brian Brettschneider, an Alaskan climate scientist, the number was likely the lowest wind chill ever recorded in the United States since meteorologists began calculating wind chills. Wind chill records are not historically tracked as closely as temperature records, but the mark would beat what most meteorologists believe to be the U.S. record (-105°F in Alaska). The prior record for Mount Washington was -102.7°F in 2004.
However, reconstructing wind chill based on historic records at Mount Washington shows the wind chill also could have been at -108°F or below on Jan. 22, 1885. The low that day-which still stands as the record low-was -50°F, and the 24-hour average wind speed was 89 mph, Brettschneider said. That combination would produce a wind chill below -108°F, he said.
The old weather observatory at Mount Washington, New Hampshire, shown here encased in ice, set a new national record for lowest recorded wind chill.

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WEATHERWISE weather events that are not hurricanerelated, even during hurricane season.
Since the year 2000, during hurricane season, there were 757 days with flooding associated with tropical cyclones and 3,416 days with floods not associated with tropical storms. Meanwhile, during hurricane season, only 52% percent of flooding occurs in coastal states, with the remaining 48% occurring in landlocked states.
According to Pew, only major flood events tend to make headlines, but the statistics show the large risk that communities in the United States faces from flooding even during minor events. Moreover, Pew says, the data show the need for improving flood resilience through legislation and policy at local, state, and federal levels.

Hurricane Hunters Fly Into Atmospheric Rivers
T he Hurricane Hunter airplanes are busy through the hurricane season in summer and into fall, gathering data on tropical storms to help researchers better understand hurricane dynamics and to provide actionable information on the ground for emergency managers. In the hurricane off-season, though, the planes do not sit in hangars gathering dust. They are put to work flying into atmospheric rivers that often inundate the West Coast of the United States to gather data to help water managers plan for excessive rainfall in California and nearby states.
Atmospheric rivers, which are moisture plumes that come from the Pacific Ocean, are crucial to the West Coast, providing half of the rain and snow the West gets in any given season. These rivers cross an area with very few observation sites, making them challenging to forecast. However, a project called Atmospheric River Reconnaissance (AR Recon) seeks to fill the information gap by flying into these rivers and collecting data. AR Recon is a Research And Operations Partnership (RAOP) among The Center for Western Weather and Water Extremes, NOAA's Hurricane Hunters, the Airforce, Naval Research Lab, and several academic institutions. The group uses dropsondes dropped into atmospheric rivers to relay pinpointed live weather data. The information is immediately put into weather forecast models, which improves the accuracy of the forecast dramatically.
The AR Recon Program originated as three missions carried out in 2016 and has been expanding in the number of observations and partners since 2016. AR Recon supports water management decisions and flood forecasting by collecting targeted airborne and buoy observations over the Northeast Pacific to improve forecasts of the landfall and impacts of atmospheric rivers on the U.S. West Coast.
"With the dropsondes released by the hurricane hunters, we get profiles of moisture, temperature, winds, in the lower part of the atmosphere, which are really critical for understanding the atmospheric river's structure," said Atmospheric River Reconnaissance Coordinator Anna Wilson.

Improved Roofing Material Could Lower Air Temperatures
U pdating all roofs in Chicago from traditional materials to white, green, or solar panelcovered roofs would lower air temperatures by as much as 2°C and reduce energy consumption more than 16% according to a new study.
"Previous studies have shown the advantages of implementation of cool roofs and green roofs in urban areas at a building scale or neighborhood scale," said Haochen Tan, an atmospheric scientist at Argonne National Laboratory. "However, such advantages cannot simply transfer to cityscale benefits because of the differences of each building and the local climate." Tan and colleagues wanted to scale up those previous studies and look at the effectiveness of heat-lowering roofs over an entire city.
The study focused on the Chicago metropolitan area during a 6-day heatwave in August 2021. The team used numerical weather prediction models to see what would happen if the roof of every building in the city were changed to either cool (white, reflective coating), green (vegetation), or solar panel-covered roofs.
"Results show that the deployment of the cool roof can reduce the air temperature by about 2 degrees [Celsius] over their most urban areas," said Tan. "Cool roofs can reduce the hours of high temperatures during a heat event day." With lower temperatures, there is less need for air conditioning. Tan says at the city-wide scale, cooling energy consumption can be reduced by 16.6% when cool roofs cover buildings. Green roofs and solar panel roofs were also effective at reducing energy consumption, creating drops of 14.0% and 7.6%, respectively.
There are trade-offs for each of these roof materials, said Tan. For example, large-scale deployment of cool roofs has the best potential for cooling, but the performance can be misleading because the reflective material can degrade over time because of weathering or dirt accumulation. On the other hand, solar panels have a lower reduction in energy consumption and can be expensive to install and maintain, but the roofing material is not passive. "If we assume that all electricity generated by the solar panel can be applied to the cooling demand, we can expect savings of almost half (46.7%), of cooling energy consumption," said Tan.

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Composite satellite photos of an atmospheric river connecting Asia to North America in October 2017.