Time filter

Source Type

Farajzadeh M.,Tarbiat Modares University | Rahimi M.,Tarbiat Modares University | Kamali G.A.,Atmospheric science and Meteorological Research Center | Mavrommatis T.,Aristotle University of Thessaloniki
Meteorological Applications | Year: 2010

The bud burst phase of orchard trees is the most critical phase in relation to low temperature and frost since the most parts of the bud, especially the ovary, are very sensitive to low temperatures. Therefore, predicting the time of bud burst is important. If a model can predict the time of budding, it would be possible to protect buds from late spring frosts. In this study, the budding time of apple trees at two agrometeorological stations in northeast and northwest Iran was predicted by using a chilling and forcing model. Data for years 2002-2006 were used to calibrate the bud burst prediction model and respective information for the years 2007 and 2008 was used to validate it. For this purpose, five threshold temperatures (TC) and 11 chill requirements (CR) were used. Among 55 combinations of TC and CR, the combination with minimum Root Mean Square Error (RMSE) was selected for predicting bud burst of apple for each region. Meanwhile, the probability of last date of frost in spring was estimated by statistical distribution. By comparing the probability of frost occurrence with the date of predicted bud burst, the risk of frost damage on apple budding was estimated. © 2009 Royal Meteorological Society. Source

Rahimzadeh F.,Atmospheric science and Meteorological Research Center | Nassaji Zavareh M.,Institute of Technical and Vocational Higher Education of Jihad e Agriculture
International Journal of Climatology | Year: 2014

In-situ observations of surface air temperature at 55 weather stations in Iran are analysed for homogeneity and trends over the period 1960-2010. Among them 32 stations have data available for the whole period. The other 23 stations with shorter records are used only to confirm variability during overlapping periods. Discontinuities in the temperature series relate mostly to relocation and changes of environmental conditions at individual stations. These changes alter the statistical characteristics of temperature, including the mean, variance, and frequency distribution and introduce uncertainties in spatially averaged trends. This article determines new estimates of temperature trends over Iran after the detection of artificial change points and application of homogenization. The regional trend of temperature is estimated using seasonal and annual minimum and maximum temperature from stations that have identical variability across the country. The country may be segmented to 10 such regions in terms of trends and variability of temperature. There is little doubt that temperatures have increased in all regions at nearly equal rates of 0.4-0.5 and 0.2-0.3 (°C/decade) for minimum and maximum temperature, respectively in Iran. The finding in earlier work of a few individual stations with negative trends is found to be due to artificial effects such as relocation. © 2013 Royal Meteorological Society. Source

Sharifi M.A.,University of Tehran | Azadi M.,Atmospheric science and Meteorological Research Center | Khaniani A.S.,University of Tehran
Annals of Geophysics | Year: 2016

In this work, the effect of assimilation of synoptic, radiosonde and ground-based GPS precipitable water vapor (PWV) data has been investigated on the short-term prediction of precipitation, vertical relative humidity and PWV fields over north of Iran. We selected two rainfall events (i.e. February 1, 2014, and September 17, 2014) caused by synoptic systems affecting the southern coasts of the Caspian Sea. These systems are often associated with a shallow and cold high pressure located over Russia that extends towards the southern Caspian Sea. The three dimensional variational (3DVAR) data assimilation system of the weather research and forecasting (WRF) model is used in two rainfall cases. In each case, three numerical experiments, namely CTRL, CONVDA and GPSCONVDA, are performed. The CTRL experiment uses the global analysis as the initial and boundary conditions of the model. In the second experiment, surface and radiosonde observations are inserted into the model. Finally, the GPSCONVDA experiment uses the GPS PWV data in the assimilation process in addition to the conventional observations. It is found that in CONVDA experiment, the mean absolute error (MAE) of the accumulated precipitation is reduced about 5 and 13 percent in 24h model simulation of February and September cases, respectively, when compared to CTRL. Also, the results in both cases suggest that the assimilation of GPS data has the greatest impact on model PWV simulations, with maximum root mean squares error (RMSE) reduction of 0.7 mm. In the GPSCONVDA experiment, comparison of the vertical profiles of 12h simulated relative humidity with the corresponding radiosonde observations shows a slight improvement in the lower levels. © 2016 by the Istituto Nazionale di Geofisica e Vulcanologia. All rights reserved. Source

Zarrin A.,Tarbiat Modares University | Azadi M.,Atmospheric science and Meteorological Research Center | Farajzadeh M.,Tarbiat Modares University
International Journal of Climatology | Year: 2010

The National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEPNCAR) monthly mean reanalysis dataset has been used to analyze spatial variations of summertime subtropical anticyclones over the Asia-Africa region. The geopotential height and zonal wind components of 1000, 500, 200, and 100 hPa in a 30-year period (1971-2000) have been used to determine the spatial and temporal variations of the anticyclone centres, their monthly frequency and latitudinal axis variations during April-October. The results revealed that there is a clear difference in the location of the summer anticyclone centres in lower, middle and upper levels of the troposphere. In the lower levels, the Azores subtropical anticyclone is located at the east of North Atlantic. In the middle levels, the frequencies of anticyclone centre are concentrated over the northwest of Africa, Arabian Peninsula and Iranian Plateau. In the upper troposphere, the geographical location of the anticyclone centres and their frequencies in the summer season exhibit a scattered pattern from south of China up to western Iran at 200 hPa, and a bimodal pattern over the Tibetan and the Iranian Plateaus at 100 hPa. In fact, in the entire study domain, the Iranian Plateau is a preferable location of the middle and upper troposphere anticyclones. The highest observed latitude of the subtropical anticyclone at 100, 200 and 500 hPa levels have been seen over north of Tibetan plateau, a large area from east to west of Asia and Iran during August, July-August and July, respectively. The maximum monthly variation in the latitude of the ridgeline is seen at 500, 200, and 100 hPa from June to July which goes even up to 10 degrees at some longitudes. © 2009 Royal Meteorological Society. Source

Tajbakhsh S.,Islamic Republic of Iran Meteorological Organization | Ghafarian P.,Islamic Azad University at Tehran | Sahraian F.,Atmospheric science and Meteorological Research Center
Natural Hazards and Earth System Science | Year: 2012

In this paper, one meteorological case study for two Iranian airports are presented. Attempts have been made to study the predefined threshold amounts of some instability indices such as vertical velocity and relative humidity. Two important output variables from a numerical weather prediction model have been used to survey thunderstorms. The climatological state of thunder days in Iran has been determined to aid in choosing the airports for the case studies. The synoptic pattern, atmospheric thermodynamics and output from a numerical weather prediction model have been studied to evaluate the occurrence of storms and to verify the threshold instability indices that are based on Gordon and Albert (2000) and Miller (1972). Using data from the Statistics and Data Center of the Iran Meteorological Organization, 195 synoptic stations were used to study the climatological pattern of thunderstorm days in Iran during a 15-yr period (1991-2005). Synoptic weather maps and thermodynamic diagrams have been drawn using data from synoptic stations and radiosonde data. A 15-km resolution version of the WRF numerical model has been implemented for the Middle East region with the assistance of global data from University Corporation for Atmospheric Research (UCAR). The Tabriz airport weather station has been selected for further study due to its high frequency of thunderstorms (more than 35 thunderstorm days per year) and the existence of an upper air station. Despite the fact that storms occur less often at the Tehran weather station, the station has been chosen as the second case study site due to its large amount of air traffic. Using these two case studies (Tehran at 00:00 UTC, 31 April 2009 and Tabriz at 12:00 UTC, 31 April 2009), the results of this research show that the threshold amounts of 30 °C for KI, -2 °C for LI and -3 °C for SI suggests the occurrence and non-occurrence of thunderstorms at the Tehran and Tabriz stations, respectively. The WRF model output of vertical velocity and relative humidity are the two most important indices for examining storm occurrence, and they have a numerical threshold of 1 ms -1 and 80%, respectively. These results are comparable to other studies that have examined thunderstorm occurrence. © 2012 Author(s). CC Attribution 3.0 License. Source

Discover hidden collaborations