Ianetz, A.

This study targeted the correlations between global dimming and seasonal and diurnal climatic conditions. Global, diffuse, and direct radiation measured during the last 40 years at the Israel Meteorological Service observatory at Bet Dagan, on the eastern coast of the Mediterranean Sea, where large decreases in annual global radiation have been observed and extensive climate data is available, were examined in order to determine the climatic conditions most correlated with these changes. Israel's Mediterranean climate enables clear differentiation between the cool, wet, rainy season, occurring between November and March, and the hot dry summer season between June and September. Examination of the seasonality in trends of radiative forcing shows that the largest mid-day decreases in global radiation occurred at the height of the rainy season in February, while in June, the least cloudy time of the year with zero mean cloudiness, no change has occurred. However, diffuse and direct radiation at noon and early afternoon in June have decreased and increased, respectively, during the past 10 years. Decreases in global radiation are significant at low sun angles in the mornings and evenings during most of the year, reflecting increasing haziness at these times. The significant seasonal-diurnal changes are highly correlated with wind azimuth, with the largest decreases in global radiation occurring when westerly, off-sea winds prevail. Regression of monthly average global radiation on sunshine duration for five year periods shows that there has been a gradual decrease in the extrapolated value of global radiation when sunshine duration is zero. These and other changes will be discussed with relation to their implications for solar dimming climate interactions.