Climatology
Atefeh Hoseini Sadr; bromand salahi; Gholam Hasan Mohammadi
Abstract
The aim of this study is to investigate the long-term fluctuations and trend in horizontal visibility in the northwest of Iran. For this purpose, hourly horizontal visibility data from 7 synoptic stations were used for the period of 1951-2020. The Koschmieder approach was used to calculate the extinction ...
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The aim of this study is to investigate the long-term fluctuations and trend in horizontal visibility in the northwest of Iran. For this purpose, hourly horizontal visibility data from 7 synoptic stations were used for the period of 1951-2020. The Koschmieder approach was used to calculate the extinction coefficient. Moreover, the Mann-Kendall and Rdit tests were applied to examine the trend of horizontal visibility. Also, the percentages of very good visibility (>19 km) compared with bad (<10 km) visibilities. Based on the results the annual average of horizontal visibility in northwest of Iran is ~13 km. This study showed three different fluctuation periods in the regional average of horizontal visibility: the first period (1951-1985) showed a sharp decrease in the visibility, the second period (1987-2005) was characterized by a low and stable visibility, and the third period showed a recent relative improvement. The regional average of horizontal visibility (extinction coefficient) exhibited a significant decreasing (increasing) trend of -0.167(0.0017) km per year at a confidence level of 0.01. The significant decreasing trend was confirmed in all stations except for the Ardabil station. The most severe decreasing trend was detected in Sanandaj and Zanjan stations with rates of 0.183 and 0.179 km year-1, respectively. The region-average of Rdit statistic in northwest Iran in the early 1950s was ~0.85, but it decreased to around 0.3 in the 1990s. Despite the recent improvement in horizontal visibility, reaching the reference distribution (i.e. Rdit=~0.5), the decreasing trend of horizontal visibility was still confirmed. The percentage of trend analysis of very good and bad visibility showed an increase in bad visibilities (from 5% to 25%) and a decrease in very good visibilities (from 80% to 5%), which confirms the decreasing trend in horizontal visibility. Hazy condition with 38.7% was the most influential weather phenomenon in visibility degradation.
Climatology
shahnaz Rashedi; Saeed jahanbakhsh; Ali Khorshiddoust; Gholam Hasan Mohammadi
Abstract
For this purpose, data on the type, amount, and height of different cloud layers and daily precipitation of 36 synoptic stations located on the southern coast of the Caspian Sea were received from the Meteorological Organization. MODIS images were used to investigate the relationship between precipitation ...
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For this purpose, data on the type, amount, and height of different cloud layers and daily precipitation of 36 synoptic stations located on the southern coast of the Caspian Sea were received from the Meteorological Organization. MODIS images were used to investigate the relationship between precipitation and cloud microphysical parameters (CTT, CTH, COT, CER, CWP). ERA5 and NCEP/NCAR data were also used to identify synoptic patterns leading to cloud formation. Finally, HYSPLIT model and regression method were used to identify the path of moisture flow. The results of observational data showed that Caspian clouds were observed in the form of low Stratus clouds and middle clouds of Altocumulus type in the region. So that among the low clouds, the heights of 750 and 900 meters and among the middle clouds, the heights of 2700 meters had the highest frequency. The results of Caspian clouds rainfall showed that in most areas, 1 to 5 mm of precipitation has occurred. Correlation results showed that precipitation was positively correlated with CTH,COT, CER and CWP, and negatively correlated with CTT. Multivariate regression predicted 17% of precipitation by cloud parameters. The results of the study of synoptic maps showed that with the establishment of a 1012 hPa high pressure core in the north of the Caspian Sea, the north-south wind flow along with the transfer of sea moisture to the south shore of the Caspian Sea, ascending the air mass and the formation of clouds and limited rainfall in the region. Vertical profiles showed maximum specific humidity in the lower levels of the atmosphere (1000 to 900 hPa). The results of HYSPLIT model moisture flow path showed that the main source of regional moisture was the Caspian Sea.
Climatology
Atefeh Hoseini Sadr; Gholam Hasan Mohammadi; Firooz Abdoul Alizade; Vaheid Khjaste Golamei
Volume 23, Issue 70 , March 2020, , Pages 79-100
Abstract
Heavy rainfall occurrence on April 14th, 2017 which followed by flood in Azarshahr and Ajabshir regions caused 48 deaths at East Azerbaijan province. Detecting synoptic mechanisms for such event was our main motivation for this study. In this regard, observational, upper atmospheric and SkewT data were ...
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Heavy rainfall occurrence on April 14th, 2017 which followed by flood in Azarshahr and Ajabshir regions caused 48 deaths at East Azerbaijan province. Detecting synoptic mechanisms for such event was our main motivation for this study. In this regard, observational, upper atmospheric and SkewT data were used. First, spatial distribution of precipitation in the study area was plotted. Then, Geo-potential height, vorticity and omega, specific humidity, moisture transition, jet stream for upper level and Huff-Muller chart were analyzed for different atmosphere layers. Finally, some atmospheric stability indexes were surveyed by using SkewT charts. Results showed that, highest rainfall was occurred as a rain corridor between southwest of West Azerbaijan province and middle regions of Urmia Lake to the central regions of East Azerbaijan Province with 30 to 57 mm range. Synoptic maps showed that in this day simultaneous expansion of the ascending area of Mediterranean trough in mid atmospheric layers and the core of polar Jet stream in upper levels provided favorable conditions for unstable atmosphere on the northwest of Iran, so that the amount of omega was less than -0.3 Pascal per second. Specific humidity maps indicate that, at the same time a significant moisture flux has flown from the Red Sea to the northwest of Iran. Therefore, the rapid conversion of atmospheric moisture into the rain in the ascending area of Mediterranean trough was the main cause of the heavy rainfall occurring on this day. This issue was proved by convergence moisture flux map and the Hoff-Müller graph. SkewT charts and instability indexes of Tabriz station also confirmed high amount of moisture and favourable ascending conditions in the atmosphere on April 14th
Climatology
Saeid Jahanbaksh; Gholam Hasan Mohammadi; shahnaz rashedi; Atefeh Hoseini Sadr
Abstract
In this study, non-parametric Mann-Kendall test was used to analyze the trend of monthly mean minimum temperature in the Northwest of Iran. For this purpose, the data from 35 synoptic stations in the cold part of year including November, December and January-to-April have been used in the period of 24 ...
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In this study, non-parametric Mann-Kendall test was used to analyze the trend of monthly mean minimum temperature in the Northwest of Iran. For this purpose, the data from 35 synoptic stations in the cold part of year including November, December and January-to-April have been used in the period of 24 years (1987-2010). Kriging interpolation method was used to obtain spatial distribution of mean minimum temperature over study area. The results showed that the minimum temperature in most parts of study area is rising so that in the months of February, March and December respectively in 22, 19 and 17 stations of the 35 stations were determined an increasing trend. While only in April, at 6 weather stations decreased and this parameter is relatively stable in November and January. Spatial distribution of monthly mean minimum temperature indicated that this element is lower in the mountainous parts of study area (including Northwest and Southeast) and whiles in lowlands (including Northeast and Southwest parts), is higher than its regional average. It was also found that regional minimum temperature and its trend in the Northwest of Iran Does not have the same characteristics in different months, so that this parameter in January and in April increased and decreased in November and December. Finally, in November and December minimum temperature has not substantially changed.
