As the planet and its cities heat up, our team is studying microweather to look at how planning at the street level can lower the temperature and reduce urban heat island effects to protect city residents.
Heat kills. In the United States, heat is the leading cause of death for weather-related fatalities, surpassing natural disasters like floods, tornados, and hurricanes. In 2022, more than 15,000 people died from the heat wave in Europe, according to the World Health Organization. The problem will only worsen as global temperatures increase by as much as 10°F by the end of this century.
When considering heat stress, it’s more useful to look at the heat index, which takes both temperature and humidity into consideration and is a more accurate measure than temperature alone of how hot it really feels. For example, in New York City’s South Bronx region, a “typical” July afternoon heat index today is about 88° F (31° C). Factoring in current climate change projections, in 2050 the average heat index could reach up to 101° F (38° C) without any changes to buildings or land use.
Present Day vs. 2050 Heat Indices in South Bronx, New York
These heat maps, created by the MITRE microweather research team, show the predicted heat indices in the South Bronx, New York, for July in present day and in 2050. Note that, on a street-level scale, the temperatures in different neighborhoods can vary by as much as seven degrees Fahrenheit at the same time of day.
Recognizing the need to mitigate the effects rising temperatures are having on urban areas around the world, a team within MITRE’s independent research and development program began exploring the value of modeling specific cities and reviewing options for future urbanization.
The effect of heat in a city varies greatly from one neighborhood or city block to another. The MITRE team knew we needed a microscale modeling approach to help urban planners, public health officials, and city climate resiliency leaders better understand the effect of heat—and urban planning—at the street and neighborhood level. This novel urban microscale modeling approach shows, for example, how positioning buildings and greenspaces to amplify ventilation corridors can reduce excessive heat exposure and possibly save lives.
Urban Planning to Mitigate Heat
In cities, increasing temperatures due to climate change are compounded by urban heat islands. Buildings, roads, and other infrastructure absorb heat. This means denser, more developed areas of cities experience higher temperatures (from one to seven degrees higher) than outlying areas.
The goal here isn’t to stop urbanization, but to provide tools to do it more safely and make cities more resilient to climate change.
As cities grow and more buildings are built, the urban heat island effect can intensify, resulting in more heat stress fatalities. This disproportionally affects lower-income areas, where people are less likely to have access to greenspaces or public infrastructure to help keep cool.
“When building a new development near a river, for instance, that neighborhood may not see significant changes to its heat index,” says Mike Robinson, principal investigator on the microweather project. “However, a few neighborhoods inland, you begin to see higher heat indices due to airflow blockage and altered heat distribution resulting from that upstream development.”
Planning may mitigate some of these effects, but only if planners understand the city’s weather on a street or neighborhood scale, accounting for the impacts of a building, park, or facility.
“We hypothesized that by modeling cities at this finer scale, we could predict the effects a new urbanization project may have on surrounding neighborhoods and streets,” Robinson says. “We wanted to see if strategic placement of new buildings and greenspaces could help minimize these increases in heat indices in a more equitable manner across broader areas of a city— while still achieving urban development objectives.”
Big Data Models for the Big City
To date, the team has modeled two different cities, New York City and Raleigh, North Carolina. They want to ensure the modeling approach could work for a city that’s already urbanized as well as one with substantial room for growth.
To simulate the effects of climate change and urbanization on temperature, moisture, and winds within cities, MITRE teamed up with Aeris LLC to use its Joint Outdoor-indoor Large Eddy Simulation (JOULES). The team used JOULES—a well-validated, high-resolution atmospheric physics model—to simulate microscale weather conditions at meters-scale resolution within each city.
“To appreciate the scale of the enterprise, it helps to know that most atmospheric models are considered high fidelity when they can measure weather conditions on the scale of a couple kilometers,” Robinson says. “Those models provide one temperature estimate for large swaths of cities and miss the vast differences that can occur from one city block to the next.”
“To effectively predict microweather with street-level resolution, we modeled the cities at three-meter resolution.”
That’s the difference between a data point the size of Disneyland versus data points at each individual ride in the park.
The team ran about 40 simulations per city with the model estimating how temperature, humidity, and air flow could change based on the time of day, larger scale wind direction, changing climate, and different urbanization scenarios.
In one scenario, areas near the river, which acts as a cooling source, were built out with additional taller buildings with greater space between them to promote airflow and ventilation inland. This decreased the heat index in 2050 by one to two degrees in some downstream areas when compared to present-day buildings in the same climate.
This means there could be ways for cities to increase their size and density while also decreasing potential heat stress risks, even with rising temperatures fueled by climate change.
In this 3D model, created by the MITRE microweather research team, the orange buildings represent potential future development in the South Bronx region of New York City. Buildings are much taller than present-day, but spaced apart to promote increased inland ventilation.
Today, this research could provide opportunities to improve heat stress alerting and resource allocation to better protect at-risk populations. Hospitals could learn when to expect an influx of heat-related admissions, and cooling centers could be set up in frequently impacted areas. Additionally, education on avoiding heat stress could be reinforced to specific at-risk neighborhoods.
“The goal here isn’t to stop urbanization, but to provide tools and data-driven evidence for how to do it more safely and make cities more resilient to climate change,” Robinson says. “The model and its derived analytics can help inform city officials on multiple ways they can help to decrease heat in critical areas, helping to protect some of their more vulnerable populations.”
Moving from Analysis to Action
Robinson’s team is expanding its research scope to investigate hyperlocal air quality concerns and the effects of heat on building energy efficiency within cities.
“We’re also engaging with additional stakeholders who would like to test how our methods, analytics, and toolkits may be leveraged to help reduce the effects of heat in their cities,” Robinson says. “We’re excited to work with Miami Dade County and the City of Miami to evaluate alternative heat stress outcomes given different urbanization scenarios under consideration in south Florida.”
Robinson has started working with the country’s first Chief Heat Officer, Jane Gilbert, and her team in Miami.
"MITRE's microweather modeling and research will help us better understand the factors contributing towards increasing heat in certain areas of the city,” Gilbert says. “The findings will be used to inform updates to land use policies, large scale development plans, and heat mitigation investments.”
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