Abstract
Using observed daily temperatures in China, three independent types of heat waves (HWs), including daytime HWs, nighttime HWs, and compound HWs (with both extreme daily maxima and minima), were defined. Different types of HWs showed distinctive preferences in occurrence locations and timing. However, spatial patterns of accompanying relative humidity were generally independent of categorization, except for closer association of nighttime events with high humidity level. Compound HWs and nighttime HWs experienced significant increases in frequency, participating days, mean duration, intensity and areal extent. Conversely, significant decreasing trends of above indicators prevailed in daytime HWs, especially in central-eastern China. Tendency of relative humidity changes didn’t vary with HW types. Instead it caused an interesting phenomenon that dry HWs in the west became more humid and humid events in the east got dryer, as manifested most obviously in compound type. Thorough comparisons highlight the evolutionary dominance of HW types. Specifically, previously-dominating independent daytime HWs have been increasingly replaced by independent nighttime events in central-eastern China, and by compound HWs in southern China. That’s the very reason for negative trends of independent daytime HWs in eastern China, even in a warming climate.
Highlights
A heat waves (HWs) is generally regarded as an event that exceeds prescribed temperature thresholds over a few days
Most compound HWs appeared in eastern China, especially across mid-lower reaches of the Yangtze River
During 1961–2010, basic features and linear trends in multiple aspects of HWs varied among different HW types
Summary
A HW is generally regarded as an event that exceeds prescribed temperature thresholds over a few days. A synthesized matrix containing multiple indicators is desirable to systematically quantify HWs in China For both scientific literatures and operational practices, previous definition of heat waves in China only considered extreme Tmax, so identified events were more indicative of scorching conditions during daytime[13]. The categorization in these studies was performed by delimiting single temperature variable (Tmax or Tmin) Such single-variable based definition possibly fails to group HWs properly, because identified HWs may be interchangeable among diverse types. Linear trends of multiple aspects for different HW types would be estimated and compared This observational analysis serves as a basic yet imperative step towards further physical interpretation, attribution and projections of heat waves
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