The present study aims to evaluate the effect of heat waves on the early melting of snow cover in the Karkheh catchment of Iran.After collecting daily data on the maximum temperature of meteorological stations in the catchment during the statistical period (2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015)(2016)(2017)(2018)(2019), three indices of WSDI, Baldi, and ocular method were used to determine the threshold of days with heat wave. By extracting the hot waves and applying programming, the snow cover maps were drawn in the Google Earth Engine system and the area of snow cover zones was calculated in the Model Builder environment. Finally, the atmospheric data were received from the NCEP/NCAR site and their generating patterns were examined after drawing in Grads software environment after determining the atmospheric synoptic patterns. The results indicated that the slope trend of hot waves is increasing at the catchment level.The average annual frequency of heat waves extracted for the snow cover growth period (November to May) was equal to 24 days of hot waves during the year. Regarding the ground pressure conditions and synoptic conditions of high atmospheric levels, two dominant synoptic patterns of heat waves were identi ed in the region as follows: 1) The Siberian high-pressure synoptic pattern at the ground level and the Saudi-African high-pressure ridge at high atmospheric levels. This synoptic pattern reduces snow cover area by an average of 40% in the Karkheh catchment and the highest frequency of the occurrence of this synoptic pattern is in February and March. 2) Pakistan-Africa low-pressure synoptic pattern at ground level and Saudi-African highpressure ridge at high atmospheric levels, the frequency of the occurrence of which is higher in April and May months and reduces the snow cover area in the Karkheh catchment by an average of more than 55%.

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The present study aims to evaluate the effect of heat waves on the early melting of snow cover in the Karkheh catchment of Iran. After collecting daily data on the maximum temperature of meteorological stations in the catchment during the statistical period (2000-2019), three indices of WSDI, Baldi, and ocular method were used to determine the threshold of days with heat wave. By extracting the hot waves and applying programming, the snow cover maps were drawn in the Google Earth Engine system and the area of ​​snow cover zones was calculated in the Model Builder environment. Finally, the atmospheric data were received from the NCEP/NCAR site and their generating patterns were examined after drawing in Grads software environment after determining the atmospheric synoptic patterns. The results indicated that the slope trend of hot waves is increasing at the catchment level. The average annual frequency of heat waves extracted for the snow cover growth period (November to May) was equal to 24 days of hot waves during the year. Regarding the ground pressure conditions and synoptic conditions of high atmospheric levels, two dominant synoptic patterns of heat waves were identified in the region as follows: 1) The Siberian high-pressure synoptic pattern at the ground level and the Saudi-African high-pressure ridge at high atmospheric levels. This synoptic pattern reduces snow cover area by an average of 40% in the Karkheh catchment and the highest frequency of the occurrence of this synoptic pattern is in February and March. 2) Pakistan-Africa low-pressure synoptic pattern at ground level and Saudi-African high-pressure ridge at high atmospheric levels, the frequency of the occurrence of which is higher in April and May months and reduces the snow cover area in the Karkheh catchment by an average of more than 55%.

The present study was conducted in order to identify the predominant synoptic patterns of fire occurrence in the western Alborz Mountains. For this purpose, first the days of fire were obtained from the Department of Natural Resources of Gilan Province and product (MCD14) Modis sensor (2007–2020). The longitude and latitude of the affected regions was added to GIS and the spatial database of the fire was prepared. In order to determine atmospheric synoptic patterns, atmospheric maps were obtained from NCEP / NCAR site on all fire days with a spatial resolution of 2.5 * 2.5 degree. Totally, 240 fires occurred in the study area during this 14-year period. Among the months of the year, July, August and December have the highest incidence of fires with the occurrence frequency of 38, 37 and 24 days, respectively. Based on the fires that occurred, the three predominant synoptic patterns have the highest frequency of fires, which are 1) Saudi-African high-pressure dominant in the hot and cold seasons of the year, with three forms of omega blocking (16.5%), ridge (24.8%) and intrusion (22.2%), in total with a frequency of 63.5%, 2) European Migrant high-pressure system intrusion dominant in the hot period of the year, with a frequency of 21.7% and 3) Sub polar low-pressure which is generally dominant during the cold period of the year and responsible for about 14.8% of the fires in the region.