Weather Types and their Influence on the Precipitation of Iran

Weather typing of surface pressure patterns over Iran and the surrounding region (20°N to 47.5°N and 35°E to 67.5°E) has been accomplished through the use of daily 12 GMT grid - point pressure values of the NCEP reanalysis data for the months November to March during the 1966-1995 period. The NCEP data were checked against reality by comparison with observed station data. The results showed excellent agreement with correlations >0.85 on the daily time scale.
The complete data set was divided into two halves and 5 monthly sub-periods. Principal component analysis with S-mode and Varimax rotation of the correlation matrix was applied to the standardised series. Because of the high spatial correlation of all the sub-period series, only the full 30-year series was selected for subsequent studies. Using the scree test, 6 orthogonal factors were retained. Factor scores of these components were then used to classify the daily pressure patterns of the study period by assigning each day to the factor with the highest absolute score. Days with each factor were then divided into three clusters based on factor scores. The resulting 18 cluster maps were then compared by their spatial correlation. They highly correlated maps were combined and 9 final weather types were produced. Each of the nine resulting weather types were named according to the location of the dominant system relative to Iran. The monthly and annual frequencies of the types were computed and their related 500 hpa composite flow patterns were produced. The significance and importance of the surface weather types to the climate of Iran were checked by analyzing their contribution to the frequency and daily intensity of precipitation.
Among the weather types, the Zonal (Z) type was the most frequent and persistent representing the mean distribution of pressure over the area. Then Northeastern high (NEH) and Western low (WL) types were the next most frequent and persistent types. In terms of contribution to precipitation, the cyclonic type of WL showed the highest contribution all over Iran except the Caspian area where the Northwestern high (NWH) was the most important type. Each type showed a monthly preference and each month had one dominantly frequent weather type.