| 英文摘要: | It is reasonable to expect more heat waves in a warmer climate, but it is not clear how many more heat waves should occur for a given mean warming. Days with heat waves at a specific location can be counted by determining the temperature for which 95 percent of recorded temperatures are below it, referred to as the 95th percentile temperature. Heat wave days can then be defined as exceedances of the 95th percentile temperature, and they occur by definition on the warmest five percent of days. We can then ask how many additional days are warmer than that today's 95th percentile temperature when climate warms, with the expectation that the answer will be more than five percent of days. Previous work by the PIs suggests that the increase in heat wave days for a given mean warming can be highly variable from region to region, for example their results suggest a greater increase in heat waves in western states and the northern midwest than in the southeast US. This project seeks to determine how applicable these result are for anticipating increases in heat wave frequency, and to understand the fundamental processes which produce regional differences in the increases.
Estimates of change in extreme temperature occurrence often assume a "normal", or Gaussian, temperature distribution, in which the average temperature is the same as the 50th percentile (or median) temperature, which is centered between the 95th and the 5th percentile temperatures. If instead the median temperature is shifted toward the 95th percentile, so that cold exceedances are more extreme than warm exceedances, the distribution has a "short tail" on the warm side. The PIs show that if warming amounts to an overall shift in the distribution, meaning that if the 5th, 50th, 95th and other percentile temperatures all increase by the same amount, there will be a greater increase in heat wave occurrence for a short tailed distribution than for a normal distribution. Thus it is possible to gain insight into warming-induced increases in heat wave days by understanding the processes that give rise to short temperature tails in present-day climate. The PIs hypothesize, on the basis of the large-scale spatial patterns of regions of short and long tails in present-day climate, that the spatial variations in exceedance change are primarily the result of meteorology and atmospheric circulation rather than local factors such as the dryness of the soil. This hypothesis is tested using observations and climate model simulations, and differences between present-day and projected future climate in model simulations is used to assess the extent to which analysis of present-day tails can account for warming-induced exceedance change.
The behavior of temperature extremes is of practical as well as scientific interest given the hazards to human and natural systems posed by extreme heat. Better guidance on changes in heat wave occurrence would be valuable for decision makers in areas including agriculture, forestry, health services, and urban infrastructure. The work has educational broader impacts through its connection with a Research Experiences for Undergrauduates (REU) site at Oregon State University, and it also provides support and training for a graduate student. |