Analysis of Urban Heat Island Intensity Through Air Mass Persistence

The bulk of synoptic weather type research related to urban climate focuses on human health impacts; however, recent studies have begun to quantify urban heat island (UHI) magnitudes by weather type, or air mass classification. This study presents an analysis of UHI intensity through synoptic-scale air mass persistence during the spring season for four UHI-prone United States cities. Historical daily weather types for Birmingham, Alabama; Charlotte, North Carolina; Louisville, Kentucky; and St. Louis, Missouri were extracted from the Spatial Synoptic Classification database for 40 years from 1980 through 2019. Daily minimum surface air temperature data were downloaded from the Global Historical Climate Network to compute UHI. The historical daily weather type data were converted into a record of persistence, or the length of consecutive days that a synoptic weather type was in place at each location. A descriptive climatology of SSC weather types and UHI at each location was constructed before UHI magnitudes were segregated by day of persistence and examined for differences in intensity. Climatologically, the four urban areas experienced an increase in warm weather types at the expense of cool weather types throughout the study period. Specifically, the persistence of moist tropical weather types increased at a statistically significant rate at Birmingham, Charlotte, and Louisville, consistent with the theorized northward migration of the mid-latitude jet stream. Also evident is a statistically significant increase in UHI frequency and intensity at Birmingham, Charlotte, and Louisville during the study period. Results show that the moisture character of a weather type is an important differentiating factor in UHI intensification, as the mean UHI was found to increase with the persistence of dry weather types and decrease with the persistence of moist weather types, presumably reflecting differences in radiational heating and cooling with atmospheric moisture content. The most intense UHIs and the largest UHI magnitude increase by day of persistence are associated with dry weather types, which have become more frequent since 1980. The findings suggest that larger magnitude UHIs may become more common in the future should dry weather type persistence continue to increase. Higher urban temperatures put human health at risk due to a well-linked relationship between heat and mortality and morbidity rates. The effects of heat are cumulative; the more common persistent, oppressive days become, the larger the impact.