Analysis of Atmospheric Circulation Patterns Triggering of Flooding Precipitations in Babol-Rud Catchment

Introduction
The variations of amount, intensity and type of precipitation variable are transformed through spatial and temporal dimensions. Flooding precipitation as a dangerous natural hazard is caused of such variations. The annual heavy precipitations are triggered many hazardous floods over the most of catchments in northern Iran. Hence, investigation of the generation and evolution mechanism of synoptic circulation patterns triggering of flooding and heavy precipitations could play a key role during risk management and mitigation measures of such natural hazards.
In this study, we aimed to investigate and analysis of synoptic condition of perceptible systems in Babol-Rud catchment. Two main questions were as below: how do the annual heavy precipitations follow the occurrences cycles? and how do the synoptic factors impact on annual heavy precipitations in Babol-Rud catchment?
Data
In this paper, both type of ground level and remotely sensed data were used. Diurnal data were gathered from eleven rain gauge stations in addition of a hydrometric station named as Quran-Talar. Number of days with heavy precipitation were considered by confidence level of P=95%. Atmospheric variations were collected as sea level pressure and geo-potential heights in 500 hPa with spatial resolution of 2.5*2.5 arch degrees. Water discharge of Quran-Talar hydrometric station was selected as monitoring indicator of the study area. 
Methods
Spectral analysis is a proper procedure to extract of cycles in time series. Its merit is related to low-volume statistical operations based on Fourier transform. In order to extract the heavy precipitation and number of days with heavy precipitation, the percentile indices and the thresholds of US climate variability and predictability program (CLIVAR) were used. In this regard, equal to 435 days from 7305 total days in time series were selected as days with heavy precipitation in the study area. Based on clustering analysis, all temporal sequences were classified as three precipitation clusters. 
Results
In this study, variations of annual heavy precipitation and water discharge data were investigated using synoptic and statistic techniques. Trends and occurrences cycles were investigated during temporal time series. Results revealed the significant annual and biennial curvilinear cycles in confidence level of P=95%.
Northern wards of Iran have been recorded as the prone of heavy precipitation and flooding events. The main aim of the present study was to investigate and analysis of synoptic condition of perceptible systems in Babol-Rud catchment. For this purpose, the heavy precipitation and number of days with heavy precipitation were extracted within time series. Thereafter, the synoptic conditions of perceptible systems were analyzed through sea level and height of 500 hPa. By assigning three clusters for heavy precipitation patterns in the study area, results revealed that the generation of short waves inside the main Rossby troughs is an important factor of heavy precipitation in Caspian coastal regions including the study area.