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CR prepress abstract   -  DOI: https://doi.org/10.3354/cr01659

Climatology and variability of the start, end, and length of frost-free season across Iran

Mohammad Sadegh Keikhosravi-Kiany*, Seyed Abolfazl Masoodian, Robert C. Balling Jr

*Corresponding author:

ABSTRACT: Climate change can reveal itself in a multitude of different ways, including impacts on the start, end, and duration of the frost-free season. The goal of the current study is to examine the climatology and variability of the first fall frost day (FFFD), last spring frost day (LSFD), and the length of frost-free season (LFFS) across Iran for the period 1978-2017. The trend analysis revealed that FFFD have moved later by 6.4 days over the study period while the LSFD have moved earlier by slightly over two weeks. The analysis for LFFS shows that it is now over three weeks longer than it was only four decades ago. Since land use changes around meteorological stations may affect the temperature measured at the station (especially the magnitudes of nocturnal cooling rates), atmospheric thickness changes, which actually 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 these two parameters. Next, we used sequential Mann-Kendall statistical analysis to detect abrupt changes of the applied frost-related indices, minimum temperatures and atmospheric thicknesses. The analysis revealed that the first abrupt changes of FFFD and LFFS have occurred near 1996 which is matched with the timing of abrupt changes of atmospheric thickness over Iran. Interestingly, seasonal trend analyses of minimum temperature over Northern Hemisphere using Era5 reanalysis data indicate consistent regional patterns of warming over the last four decades. The results suggest that the increase in the length of the frost-free season is largely driven by a 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 the agriculturalists in Iran and elsewhere.