Journal of Sustainable Urban & Regional Development Studies (JSURDS)

Abstract Background and Objective: Late spring frost is one of the major compound climate–phenological hazards affecting apple orchards in the mountainous regions of Iran. Its damage primarily results from the coincidence of temperature decline with sensitive phenological stages, particularly flowering, and is not necessarily directly proportional to the absolute minimum temperature recorded at a weather station. Semirom Township, one of Iran’s major apple-producing regions, is particularly susceptible to this phenomenon due to its elevation, topography, and spring temperature regime. This study aimed to analyze the synoptic atmospheric circulation patterns associated with late spring frost and to examine the events of 2002, 2006, and 2013 within the statistical framework of the 20-year period from 2002 to 2021.
Methodology: Daily minimum temperature data from the Semirom synoptic station were analyzed for the critical phenological window for apple trees, from 4 April to 27 April, during 2002–2021. To characterize the near-surface conditions during the three selected events (2002, 2006, and 2013), daily minimum, maximum, and mean temperatures, 24-hour precipitation, mean relative humidity, and mean wind speed were extracted from the same station for the synoptic window of 1–20 April. Synoptic analysis of the three events was conducted using NCEP/NCAR reanalysis data and GrADS version 2.1.
Results and Findings: Of the 480 days within the study window, 473 had valid observations. During this period, 62 days were classified as weak and 31 days as moderate frost events, while no days met the criteria for severe frost; thus, a total of 93 days experienced frost conditions. The lowest recorded temperature was −1.5 °C, and no day with Tmin ≤ −2 °C was observed. The minimum temperatures during the 2002, 2006, and 2013 events were −1.0, −0.4, and −0.6 °C, respectively; consequently, all three events were classified as moderate frost events. Nevertheless, the greater damage reported in 2013 was associated with the coincidence of cold conditions and flowering, which increased phenological vulnerability. Synoptic analysis of all three events indicated the intrusion of cold air from higher latitudes and the establishment of a mid-tropospheric trough. Moreover, the occurrence of precipitation concurrently with the coldest conditions in 2006 and 2013 was inconsistent with purely radiative cooling. The results indicate that frost damage severity in Semirom is not directly determined by station-based minimum temperature alone, but is influenced by the interaction between atmospheric conditions and crop phenological stage. Therefore, integrating long-term statistical assessment with synoptic analysis of representative frost events can provide a more appropriate basis for characterizing frost risk and developing early-warning systems for apple orchards in Semirom.