Synoptic Anomalies Resulting in Pervasive Frosts in Iran

Although temperatures in 0 and below 0 C° in the cold season of the year and mid latitudes are not necessarily considered an extreme phenomenon، but their long-term durability in a widespread area or their sudden occurrence in the beginning and end of the cold season can make this faetor an extreme extent phenomenon. The extreme phenomena are resulted from interaction between large scale atmospheric circulations and the local landscaps of the study area (terrain، altitude، etc.). Considering the geographical situation of Iran and relatively high altitude of most of its areas as well as entering different air masses to this land، Freezing occurrence involves most parts of the country each year، so in some years، even the southern parts of the country is affected، and makes a lot of damages. Thus، identifying the circulation patterns causing such an extreme phenomenon can be helpful in more accurate forecasting as well as management of the freezing hazard. Therefore، a goal to be sought by this study is to achieve circulation patterns identifying، which create pervasive frosts in Iran. Study area: Iran with an area over 1''648''195 square km is located between 25N to 40N latitudes and 44E to 64E longitudes. Iran''s landscape is dominated by rugged mountain ranges that separate various basins or plateaus from one another. The populous western part is the most mountainous، with ranges such as the Zagros and Alborz Mountains، the latter of which also contains Iran''s highest point، the Damavand at 5،671 m (18،606 ft). The eastern half consists mostly of uninhabited desert basins with the occasional salt lake. The only large plains are found along the coast of the Caspian Sea and at the northern end of the Persian Gulf، where Iran borders on the mouth of the Arvand River (Shatt al-Arab). Smaller، discontinuous plains are found along the remaining coast of the Persian Gulf، the Strait of Hormuz and the Sea of Oman. The climate of Iran is mostly arid or semiarid، though subtropical along the Caspian beaches. This study is focused on synoptic anomalies resulting in pervasive frosts in Iran with the aim of improving the predictive capabilities of this phenomenon in Iran. So، to achieve this aim، Iran''s frosts were classified based on a location principle into three categories، including pervasive frosts (occurring simultaneously in more than 65 percent of Iran''s area)، semi-pervasive frosts (occurring simultaneously in 25 to 65 percent of Iran''s area) and partial frosts (occurring simultaneously in less than 25 percent of Iran''s area). It should be noted that this analysis only covers the years with maximum and minimum frequencies the pervasive freezing days with pervasive frosts that were a standard deviation above and below the average (respectively، and) for a 43-year Records from 1962 to 2004. Them after، the frosts were assessed in three scales of annual، seasonally (from October to November، from December to February and from March to April) and monthly. The variables used to describe the extracted anomalies، include: Sea level pressure، upper and lower level geopotential height، surface temperature، humidity at the level of freezing 850 hPa، wind lines at 850 hPa and 250 hPa، associated with zonal and meridional wind at 250 hPa. As noted، the anomalies were extracted with respect to the study period (1962-2004) for all three sub-periods، including annual، seasonal and monthly periods. Data used in this section are the re-analyzed data obtained from the National Center Environmental Prediction (NCEP) with the spatial resolution of 2. 5 * 2. 5 degrees. Composites prepared for the maximum and minimum frequency of pervasive frost days show different synoptic anomalies for study variables so that the composites for the maximum and the minimum frequency of indicate negative temperature and humidity anomalies for the entire studied periods. Also، at low levels of atmosphere for the maximum frequency of the pervasive freezing contempory with days with pervasive frosts، a bipolar pressure anomaly with a positive value in the north east and a weak negative value in the north west of Iran above the Turkey. In other words، with a strengthened Siberian high-pressure system and extending its western flow on the Mediterranean sea، the conditions for establishment of a high- pressure system on this sea is provided that the result of such an establishment would be pushing back the low-pressure system flow of Sudan monsoon to the lower latitudes as well as provided synoptic conditions for moving the cold air from the higher latitudes to the western part of Iran. The wind field in the high levels also shows a one-polar anomaly of the stream function generally with positive sign (except for November) over Iran that such a one-polar anomaly causes an anticyclone circulation (with the exception of November) over Iran that such an anomaly shows the weakening of subtropical jet stream over Iran. It is also seen for the minimum frequency of days with pervasive frosts that the entire studied area is dominated by a negative anomaly، which its central focus of severity is located over the Turkey. Thus، the spatial patterns configuration of this group indicates the weakening of the Siberian high pressure or anticyclone and subsequent extension of its western stream to the Mediterranean sea that as a result، the conditions for establishment of a low pressure system or depression with a cyclonic circulation over the Mediterranean sea will be provided; this pattern is well coordinated with the wind anomaly at 850 hPa level، therefore، under these circumstances and with the expansion of Sudan monsoon low pressure stream and humidity injection from anticyclone circulation of the Indian Ocean (Arabian Sea)، Iran''s climate experiences a more humid and warmer conditions، which reduces the frequency of days with inclusive frosts over Iran. Based on a location criterion، the frosts in Iran were divided into three groups: Pervasive frosts، semi-pervasive frosts and partial frosts، and then، the anomalies of general circulation average of the atmosphere were investigated for years، seasons and months that had maximum and minimum frequencies of days with pervasive frosts. The atmosphere for the maximum frequency of the days with pervasive frosts، a bipolar pressure anomaly a positive value in the North East and a weak negative value in the North West of Iran over Turkey can be seen at low levels of atmosphere. Based on this anomaly، the configuration of spatial patterns of this group changes in a way that conditions for entering the anticyclones، both from the North East and the North West of Iran، will be provided. In other words، by strengthened Siberian high-pressure and its extended western stream over the Mediterranean Sea، conditions for establishment of a high pressure system over the sea is provided that results in pushing back the Sudan monsoon low pressure system to the lower latitudes as well as provided synoptic conditions for entering the cold air from the upper latitudes towards the western half of Iran. It is also observed for the minimum frequency of pervasive freezing days that the entire studied area (20 to 60 North degrees and 20 to 75 east degrees) is dominated by a negative anomaly that its central intensity focus is located over Turkey. Thus، the configuration of spatial patterns of this group indicates the weakening of the Siberian high-pressure and subsequent non-development of its Western flowing air up to the Mediterranean Sea; therefore، the conditions are provided for establishment of a low-pressure cyclonic circulation over the Mediterranean Sea that such a pattern is also well coordinated to the wind anomaly at 850 hPa level. Thus، under these conditions and with expansion of Sudan monsoon low pressure system and humidity injection from the Indian Ocean anti-cyclone (Arabian Sea)، under the abow Conditions، the Climate charechtrestics become warmer and wetter to cause the reduction of inclusive freezing days over the hole country.