#urban_heat_islands
#heat_waves

In 2018, Lei Zhao from Princeton University and coauthors analyzed the effect of heat waves on the magnitudes of urban heat island effect.

All the findings were based on Community Earth System Models over 65 cities in North America. The heat waves were defined using the rural land unit’s temperature. The analysis were conducted based on both 2m and land surface temperatures, and the results were consistent. However, note the convective term’s contribution to land surface temperature is to the opposite of the 2m temperature.

The major conclusions are as follows:

1. During heat waves, the urban heat island magnitude is either not changed, or exacerbated.

2. The exacerbation happens to daytime temperature in the humid eastern U.S. in the present-day climate, and is caused by the evaporative effect and the increased anthropogenic waste heat from air conditioning. The evaporative effect is as such: the ample water availability in this region allows increased evaporative cooling in the rural regions during a heat wave, but the urban region, with reduced vegetation, does not have this benefit.

In the future climate, precipitation increases in this region in CESM, such that the urban land unit becomes amply watered and no longer suffers from the lack of evaporative cooling.

3. The lack of change happens to daytime temperature in the arid southeastern U.S. in the present-day climate, where the evaporative cooling contrast does not exist.

In the future climate, precipitation increases in this region in CESM create evaporative cooling contrast, and makes the urban heat island effect stronger during heat waves.

4. The exacerbation of nighttime temperature happens to all regions in the present and future climate, and is mainly caused by the increased use of air conditioning.

https://iopscience.iop.org/article/10.1088/1748-9326/aa9f73/meta