ABSTRACT: Climate change can manifest in many ways, including impacts on the start, end, and duration of the frost-free season. We examined the climatology and variability of the first fall frost day (FFFD), last spring frost day (LSFD), and length of the frost-free season (LFFS) across Iran for the period 1978-2017. Trend analysis revealed that FFFD shifted later by 6.4 d over the study period while LSFD shifted earlier by slightly over 2 wk, and LFFS is now >3 wk longer than it was only 4 decades ago. Since land-use changes around meteorological stations may affect the temperature measured at these stations (especially the magnitudes of nocturnal cooling rates), atmospheric thickness changes, which reflect temperature changes and are independent of station-based measurements, were used as a secondary dataset to investigate minimum temperature trends. The analysis revealed a very strong relationship between frost-related indices and atmospheric thickness. Sequential Mann-Kendall statistical analysis revealed abrupt changes in the applied frost-related indices, minimum temperatures, and atmospheric thicknesses. The first abrupt changes in FFFD and LFFS occurred around 1996, which matched the timing of abrupt changes in atmospheric thickness over Iran. Interestingly, seasonal trend analyses of minimum temperature over the Northern Hemisphere using Era5 reanalysis data indicated consistent regional patterns of warming over the last 4 decades. The results suggest that the increase in LFFS is largely driven by regional-scale warming as opposed to local urbanization and/or land-use changes. Our results document an important and ongoing change of potentially considerable interest to agriculturalists in Iran and elsewhere.

中文翻译：

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伊朗无霜季开始、结束和长度的气候学和变异性

摘要：气候变化可以通过多种方式表现出来，包括对无霜季节开始、结束和持续时间的影响。我们研究了 1978 年至 2017 年伊朗各地的第一个秋季霜冻日 (FFFD)、去年春季霜冻日 (LSFD) 和无霜季长度 (LFFS) 的气候学和变异性。趋势分析显示，FFFD 在研究期间延迟了 6.4 天，而 LSFD 提前了 2 周多一点，而 LFFS 现在比 40 年前长了 3 周以上。由于气象站周围土地利用的变化可能会影响这些站测量的温度（尤其是夜间降温速率的幅度），大气厚度变化反映了温度变化，并且与基于站的测量无关，被用作研究最低温度趋势的辅助数据集。分析表明，霜冻相关指数与大气厚度之间存在非常强的关系。连续 Man​​n-Kendall 统计分析揭示了应用的霜冻相关指数、最低温度和大气厚度的突然变化。FFFD 和 LFFS 的第一次突然变化发生在 1996 年左右，这与伊朗大气厚度突然变化的时间相匹配。有趣的是，使用 Era5 再分析数据对北半球最低气温的季节性趋势分析表明，过去 40 年的区域变暖模式一致。结果表明，LFFS 的增加主要是由区域尺度变暖而不是当地城市化和/或土地利用变化驱动的。