Historical Changes in the Moisture Content of Heat Waves in the United States

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 (TE), 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, TE 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 on 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 reanalysis that assimilate only sea-level pressure (e.g., NCEP and ECMWF 20th Century Reanalysis products). The results are discussed in the context of similarities and differences among the reanalysis products and time periods considered . Results based on TE are also contrasted with those based on temperature alone.