Radiative Flux Variability in the Recent Decade

To examine the solar radiation trend in the recent decade, Soni et al. (2016) further analyzed the data from 1971 to 2010 over 12 different stations in India (Fig. 4). The all-sky global irradiance showed a decreasing trend of 0.6 W m 2 year 1 during 1971-2000 and 0.2 W m 2 year 1 during 2001-2010. A reversal in all-sky global irradiance after 2001 at some stations also was noticed. Figure 4a shows that after 2001 there seems to be stagnation and slight upward trend. This indicates solar brightening over India, which may be ascertained further with long-term data sets.

Fig. 4 Linear, third-order polynomial and 5-year moving average fits to annual time series of a all-sky global irradiance averaged over 12 solar radiation stations and b clear-sky global irradiance averaged over eight stations (Soni et al. 2016)

The clear-sky global irradiance showed a linear decreasing trend of 0.4 W m- year-1 for the period 1971-2000 over India. While, interestingly, a positive and significant trend of magnitude 1.3 W m-2 year-1 is observed during the latter period 2001-2010. The reversal in clear-sky solar trend is observed after 2001. However, station-wise analysis indicates reversal in trend at different years for different stations. It is noted that at Ahmedabad and Jodhpur the decreasing trend reversed from 2001, at Nagpur from 2003, at Pune from 2005, at Visakhapatnam and Shillong from 2004. During 2001-2010, the increasing linear trend in clear-sky irradiance was evident at stations Ahmedabad, Delhi, Jodhpur, Nagpur, and Pune. Thus, station-wise trends highlight the fact that dimming/brightening is region dependent due to regional sources and meteorology. The main factors that are responsible are changes in the atmospheric loading due to natural as well as various man-made activities, cloud cover and other properties of clouds. The data analysis of total cloud cover, based upon 172 surface meteorological stations (under the network of IMD) from 1961 to 2007, indicated a general decrease over most parts of India (Jaswal et al. 2010). The extension of their study to 195 stations from 1951 to 2010 showed that 74 stations reported decreasing trends, while 28 stations reported increasing trends; both the trends are statistically significant at the 95 % confidence level. The remaining stations showed either increasing or decreasing nonsignificant trends. For the country as a whole, annual mean total cloud cover showed statistically significant decreasing trend (0.44 % per decade), the trend being more after 2000. This seems to be consistent with the increasing trend in all-sky global irra- diance in the late 2000.

Diffuse irradiance has shown a more complicated spatial and temporal variability. All-sky diffuse radiation, averaged over 12 stations, showed increasing trend from 1971 to 2010. The trend was 0.1 W m 2 year 1 during the period 1971-2000 and 0.4 W m 2 year 1 during 2001-2010 (Fig. 5). The diffuse irra- diance showed high values in 1982 and 1983 followed by El-Chichon volcanic eruption in Chiapas (Mexico) in March 1982, and also during 1991-1992 peaking in 1992 followed by Mt. Pinatubo volcanic eruption in Philippines in June 1991. Seasonal variability of all-sky diffuse irradiance showed increasing trend for the period 1971-2000 and 2001-2010 except during the monsoon season. High trend values are observed during winter and post-monsoon seasons. These two seasons play an important role in the temporal variability of annual diffuse radiation. Under clear-sky conditions, the increasing trend in diffuse irradiance was much higher during the period 2001-2010 as compared to the period 1971-2000 over India. For changes in solar radiation under clear-sky conditions, aerosols are considered to be the main source of extinction in the atmosphere. Other possible causes could be the changes in frequency of occurrence of cloud-free days. However, further analysis is required to understand the changes in clear-sky global irradiance during 2001-2010.

Fig. 5 Linear, third-order polynomial and 5-year moving average fits to annual time series of a all-sky diffuse irradiance averaged over 12 solar radiation stations and b clear-sky diffuse irradiance averaged over eight stations (Soni et al. 2016)

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