Analyses of Changes in Extreme Precipitation in Zanjan

IntroductionThe recent scientific assessment of the United Nations Intergovernmental Panel on Climate Change (IPCC, 2007) acknowledges that there is increasing concerns that extreme precipitation events may be changing infrequency and intensity as a result of human influences on climate. The conceptual basis for changes in precipitation has been detailed among many experts, and numerical climate models consistently predict an increase in extreme rainfall events in many parts of the world given the continued buildup of greenhouse gases (IPCC, 2007). In simplest terms, warming of the planet increases evapo-transpiration rates, a warmer atmosphere potentially holds more water, the higher moisture levels and temperatures tend to destabilize the atmosphere, and changes then occur in the type, amount, frequency, intensity, and duration of precipitation(Sen 2004). While the theoretical basis for expecting an increase in extreme precipitation is well-developed, many climatologists have examined records from throughout the world in an attempt to identify trends in extremerainfall events. Although it is difficult for drawing a consistent picture of changes in all over the world. Regional studies show increasing precipitation during 20th century over several regions of the world while extreme droughts were reported in some regions. In this investigation, Zanjan city is studied for trends in extreme precipitation. Extreme weather and climate events have received increased attention in the recent years due to their potentials for severe and adverse impact on human life, civil infrastructure, and natural ecosystems. Accordingly extreme climatic events have drawn more and more concerns from public, government and academic circles because of the tremendous impacts on environments. Data and Study areaThe study area is Zanjan city that located in Northwest of Iran. The main factors governing Zanjan climate are the high latitude and mountains of this region. The elevation is about 1600 m at the synoptic station.Historical daily precipitation data are available from 1961 to 2006 for this weather station. The data were obtained from the Islamic Republic of Iran Meteorological Organization (IRIMO). Prior to creating the daily databases, the station data time series were evaluated for potential irregularities through time. Then, from the daily data it has been created the annualized data throughout the 1961-2006 study periods. In the end, it has been annualized the precipitation for each year.Methods and IndicesThe objective of this investigation is to analyze trends in various extreme rainfall indices computed from Zanjan synoptic station in Northwest of Iran. Any review of the recent literature will reveal a large number of indices used to represent temporal and/or spatial variations in extreme precipitation events (IPCC, 2007). Many scientists have used measures of extreme frequency that count the number of events each year above fixed threshold values (e.g., events > 100 mm per day) or above thresholds determined by the precipitation climatology at individual stations (e.g., events > 95th percentile). Others have focused on extreme intensity levels with choices ranging from the largest one-day event to the mean of the events in the 95th percentile to the annual total divided by the number of rain days. Other popular choices involve quantifying the proportion of annual precipitation coming from extreme events (e.g., total from events in the 95th percentile/annual total). In some cases, it can be surprisingly difficult to determine exactly how the researcher provides various definitions of extreme events. Furthermore, it is not clear how these various indices of extreme rainfall events are related to one another, and how the selection of the indices influences the resulting temporal or spatial variations. It has carefully examined the methods used to quantify extreme rainfall events in many recent articles and found dozens of often redundant indices that generally fall into three broad categorie