A New Perspective on Heat Waves in the United States

Abstract:

Investigations of changes in the nature of heat waves have traditionally focused exclusively on temperature, despite substantive evidence that moisture is a key determinant of impacts associated with extreme heat. Isobaric equivalent temperature, the temperature that an air parcel would have if all associated water vapor were condensed and the resulting latent heat used to increase the temperature of the parcel, is used here to investigate historical changes in extreme heat events in the United States. Unlike apparent temperature, which given sufficiently high air temperature, can be elevated in the absence of high humidity, apparent temperature only attains extreme high values when both temperature and humidity are high. Heat waves are identified using the excess heat factor (EHF), which includes terms for both acclimatization and departure from climatology. Analysis of EHF is conducted using both temperature and equivalent temperature derived from multiple reanalysis datasets including recent reanalysis that assimilate data from multiple platforms (e.g., North American Regional Reanalysis, NCEP-DOE Reanalysis) as well as long-term reanalyses that assimilate only sea-level pressure (e.g., NCEP and ECMWF 20^th Century Reanalysis products). The results will be discussed in the context of similarities and differences among the reanalysis products and time periods considered. Results based on equivalent temperature are also contrasted with those based on temperature alone.