Non-Gaussian Temperature Distribution Tails in Observations and Models: Implications for Future Extreme Temperature Exceedances

在气候变暖的情况下，预计会出现更多热浪是合理的，但目前还不清楚，在给定的平均变暖条件下，应该会出现多少热浪。在特定地点有热浪的日子可以通过确定95%的记录温度低于该温度的温度来计算，这被称为95%的温度。然后，热浪日可以被定义为超过95%的温度，根据定义，热浪发生在最热的5%的日子。然后我们可以问，当气候变暖时，有多少天比今天95%的温度多温暖，预计答案将是超过5%的天数。PIS之前的工作表明，给定平均变暖的热浪天数的增加可能会因地区而异，例如，他们的结果表明，西部各州和中西部北部的热浪增加幅度大于美国东南部。这个项目试图确定这些结果在预测热浪频率增加方面的适用性，并了解导致热浪频率增加的基本过程。极端温度发生变化的估计通常假设温度分布为“正态”或高斯，其中平均温度与第50百分位(或中位数)温度相同，其中心温度在第95到第5百分位数之间。相反，如果中值温度向第95个百分位数移动，这样冷的过度温度比温暖的过度温度更极端，那么这种分布在温暖的一侧有一条“短尾”。PI指数显示，如果变暖相当于分布的整体移动，这意味着如果第5、50、95和其他百分位数的温度都增加了相同的量，那么短尾分布的热波发生的增加将比正态分布更大。因此，通过了解在当今气候中引起短温度尾的过程，有可能洞察由变暖导致的热浪日数的增加。PIS根据当今气候中长尾和短尾区域的大尺度空间格局假设，超标变化的空间变化主要是气象和大气环流的结果，而不是诸如土壤干燥等局部因素。通过观测和气候模型模拟验证了这一假设，并使用模型模拟中当前气候和预测未来气候之间的差异来评估对当前尾部的分析可以在多大程度上解释气候变暖引起的过度变化。鉴于极端高温对人类和自然系统构成的危害，极端温度的行为具有实用和科学意义。对农业、林业、卫生服务和城市基础设施等领域的决策者来说，更好地指导热浪发生的变化将是有价值的。这项工作通过与俄勒冈州立大学的Undergrauduates研究经验(REU)网站的联系产生了更广泛的教育影响，它还为研究生提供支持和培训。

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.