Trends in Extreme Precipitation Indices in Iran: 1951–2007

Robert C. Balling,Shouraseni Sen Roy,Mohammad Sadegh Keikhosravi Kiany,Javad Khoshhal

doi:10.1155/2016/2456809

Abstract

We investigate trends in extreme precipitation in Iran for 1951–2007 using the recently released APHRODITE daily rainfall time series. We find that seven different indices of extreme precipitation all show an upward trend through the study period. The seven different precipitation indices include annual precipitation total, number of days above a certain threshold, maximum precipitation received over a certain period of time, maximum one-day precipitation, and number of days with precipitation above the 90th percentile. A principal components analysis reveals one eigenvector explaining much of the variance in the seven indices and reveals that this component exhibits a strong upward trend for the whole of Iran. On a regional level, we find that the upward trend in extreme precipitation has a strong southwest-to-northeast gradient across the country for all the indices. We repeated all the analyses for 42 stations across the country to compare with the results from the gridded data; trends in extreme rainfall generated from the station data compare favorably with the results from the APHRODITE daily rainfall time series thereby reinforcing the robustness of our conclusions.

Highlights

An extreme event is generally defined as the occurrence of a weather or climate event above or below a threshold value near the upper or lower ends of the range of observed values for a specific variable
Some of the main findings from the latest report on climate change and a special report on extreme events from the Intergovernmental Panel on Climate Change indicate a greater consensus within scientific literature about a likely increase in the frequency and intensity of heavy precipitation events over land areas since 1950, with a likely increase in the frequency of heavy precipitation or proportion of heavy precipitation in the 21st century
In a comprehensive study examining the spatial patterns of precipitation extremes, Alexander et al [3] reported a significant increase in precipitation extremes, with less spatially coherent patterns compared with trends in extreme temperatures

Summary

Introduction

An extreme event is generally defined as the occurrence of a weather or climate event above or below a threshold value near the upper or lower ends of the range of observed values for a specific variable. The majority of studies based on observational data indicate a general increase in extreme heavy precipitation events, which is attributed to anthropogenic forcing caused by increased levels of moisture in the atmosphere and warmer temperatures overall [4]. Some of the specific regional level studies for extreme precipitation events show likely increases in North America [13,14,15] and South America [14, 16] for the entire 20th century. Rahimzadeh et al [27] examined extreme temperatures and precipitation across 27 synoptic stations across Iran The results of their analysis indicated marked negative trends in cool days and cool nights, diurnal temperature range (DTR), and positive trends in warm days and tropical nights. In the present study we have analyzed trends in extreme precipitation for Iran which represents a location with precipitation coming largely from cool-season cyclonic events and limited warm-season convective precipitation

Study Area

Station Precipitation Data

Analyses and Results