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Soltani M.,Karlsruhe Institute of Technology | Laux P.,Karlsruhe Institute of Technology | Kunstmann H.,Karlsruhe Institute of Technology | Kunstmann H.,University of Augsburg | And 14 more authors.
Theoretical and Applied Climatology | Year: 2015

In this study, changes in the spatial and temporal patterns of climate extreme indices were analyzed. Daily maximum and minimum air temperature, precipitation, and their association with climate change were used as the basis for tracking changes at 50 meteorological stations in Iran over the period 1975–2010. Sixteen indices of extreme temperature and 11 indices of extreme precipitation, which have been quality controlled and tested for homogeneity and missing data, are examined. Temperature extremes show a warming trend, with a large proportion of stations having statistically significant trends for all temperature indices. Over the last 15 years (1995–2010), the annual frequency of warm days and nights has increased by 12 and 14 days/decade, respectively. The number of cold days and nights has decreased by 4 and 3 days/decade, respectively. The annual mean maximum and minimum temperatures averaged across Iran both increased by 0.031 and 0.059 °C/decade. The probability of cold nights has gradually decreased from more than 20 % in 1975–1986 to less than 15 % in 1999–2010, whereas the mean frequency of warm days has increased abruptly between the first 12-year period (1975–1986) and the recent 12-year period (1999–2010) from 18 to 40 %, respectively. There are no systematic regional trends over the study period in total precipitation or in the frequency and duration of extreme precipitation events. Statistically significant trends in extreme precipitation events are observed at less than 15 % of all weather stations, with no spatially coherent pattern of change, whereas statistically significant changes in extreme temperature events have occurred at more than 85 % of all weather stations, forming strongly coherent spatial patterns. © 2015 The Author(s)

Mofidi A.,Ferdowsi University of Mashhad | Soltanzadeh I.,Meteorological Service of New Zealand MetService | Yousefi Y.,University of Mazandaran | Zarrin A.,Ferdowsi University of Mashhad | And 4 more authors.
Natural Hazards | Year: 2014

An exceptional southerly Foehn in the Alborz Mountains in northern Iran is investigated by using a combination of observations, reanalysis, and simulation data. A synoptic analysis is used as well as a high-resolution numerical modeling to clarify the Foehn event at different scales. The event resulted in an extensive and high-intensity fire in the Gilan and Mazandaran forests in northern Iran. The results indicate that a mechanically driven Foehn occurred in the Alborz Mountains during December 16–18, 2005. On the synoptic scale, the Foehn event occurred due to the presence of high pressure over the interior regions of Iran and lee cyclone over the southern Caspian Sea, with a strong south–north pressure gradient across the Alborz Mountains. In the mesoscale, the results suggest that mountain waves generated over the northern slopes of the Alborz Mountains are the primary source of the localized southerly wind maximum around the lee side of Alborz. A numerical simulation reveals that strong meridional surface pressure differences along with southerly flow, which is blocked upstream of the Alborz Mountains, result in higher nonlinearity and create large-amplitude vertically propagating mountain waves over the Alborz Mountains. The study also indicates that the wave-breaking region on the lee side with a critical level ranging from 600 to 400 hPa is responsible for reflecting the mountain wave energy back to the ground and for creating severe downslope wind (Garmij) in leeward side of Alborz Mountains. The Foehn event first appeared early on December 16, due to a wave-breaking at 550 hPa in western part of the Alborz Mountains. © 2014, Springer Science+Business Media Dordrecht.

Crouch J.F.,Meteorological Service of New Zealand MetService | Pardo N.,Massey University | Miller C.A.,Institute of Geological & Nuclear Sciences
Journal of Volcanology and Geothermal Research | Year: 2014

The 6 August 2012 eruption of Mt. Tongariro from Upper Te Maari Crater in the central North Island of New Zealand was the first volcanic eruption observed by an operational weather radar in New Zealand, and is believed to be one of only a small number of eruptions observed by a dual-polarisation radar worldwide. The eruption was also observed by a GeoNet webcam, and detailed ash deposit studies have permitted analysis of the plume characteristics. A combination of radar and webcam imagery show 5 pulses within the first 13. min of the eruption, and also the subsequent ash transport downwind. Comparison with ash samples show the radar was likely detecting ash particles down to about 0.5. mm diameter. The maximum plume height estimated by the radar is 7.8 ± 1.0. km above mean sea level (amsl), although it is possible this may be a slight under estimation if very small ash particles not detected by the radar rose higher and comprised the very top of the plume. The correlation coefficient and differential reflectivity fields that are additionally measured by the dual polarisation radar provide extra information about the structure and composition of the eruption column and ash cloud. The correlation coefficient easily discriminates between the eruption column and the ash plume, and provides some information about the diversity of ash particle size within both the ash plume and the subsequent detached ash cloud drifting downwind. The differential reflectivity shows that the larger ash particles are falling with a horizontal orientation, and indicates that ice nucleation and aggregation of fine ash particles was probably occurring at high altitudes within 20-25. min of the eruption. © 2014 Elsevier B.V.

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