Clear-sky Surface Solar Radiation Research Articles

Variability of surface solar radiation under clear skies over Qinghai-Tibet Plateau: Role of aerosols and water vapor

Surface solar radiation (SSR) plays a vital role in the global energy cycle and the interaction between the earth and atmosphere. It has a significant impact on climate change. Aerosols and water vapor are important factors affecting the variations in clear-sky SSR. This study analyzed the effects of aerosols and water vapor on clear-sky SSR in the Qinghai-Tibet Plateau (QTP), one of the most sensitive areas to global climate change, over the past 20 years. The results show that the 87.0% of the monthly clear-sky SSR over QTP can be explained by the monthly aerosols and water vapor. The aerosol direct radiative effect (ADRE) value in most QTP areas (93.35%) is lower than −10 W m−2. The effect of water vapor on the monthly value of clear-sky SSR is greater than that of aerosols. The reduced clear-sky SSR (about 61.36% of QTP areas) in the past 20 years is mainly concentrated at the south–north boundary. The significant increases in sulfate and carbonaceous aerosols from India have contributed to the decrease in clear-sky SSR in the South. Moreover, the increasing water vapor influx from the southern boundary is also an important reason for the reduction in clear-sky SSR in the South and North. The results also indicate that the decrease in dust aerosols, originating from the Taklamakan and Qaidam Deserts, results in the increase in clear-sky SSR in the Northeast.

Long-term evolution of clear sky surface solar radiation and its driving factors over East Asia

This study evaluated the performance of clear sky surface solar radiation (SSR) and its long-term evolution mechanism in the second Modern-Era Retrospective Analysis for Research and Applications (MERRA-2) in East Asia. The results revealed that although MERRA-2 overestimated the clearness index and clear sky SSR, the reanalysis performed well in northwestern, northern, northeastern, and central East Asia. From 1980 to 2019, clear sky SSRs decreased significantly in most areas. Aerosols play a dominant role in the annual variation of clear sky SSR rather than water vapor. In northwestern East Asia, owing to the influence of the increase in aerosol optical depth (AOD), the clear sky SSR showed a strong downward trend during 1993–2013. After 2007, the increase in water vapor played an important role in the weakening of the clear sky SSR. In northern, northeastern, and central East Asia, clear sky SSRs ushered in a brightening stage at the beginning of the 21st century because of the reduction in AOD. The magnitude of the change in Central East Asia is even more pronounced. In the Tarim Basin and northwestern India, dust plays an important role in the variation of clear sky SSR, whereas anthropogenic aerosols drive clear sky SSR in highly industrialized regions.

Long-term variations of surface solar radiation in China from routine meteorological observations

Accurate observations of long-term surface solar radiation (SSR) are crucial for hydrological processes, agricultural production, and photovoltaic power generation. The main purpose of this study is to investigate the spatial-temporal variations and driving factors of SSR over mainland China. Firstly, ten selected models for estimating SSR were evaluated at 61 China Meteorological Administration (CMA) stations with SSR measurements. Then, the Yang's hybrid model (YHM) with the best model accuracy was used to construct a daily mean SSR dataset at 2474 CMA meteorological stations in China during 1961–2014. The results indicated that the annual mean SSR showed a fluctuating downward trend (−0.16 × 10−1 MJ m−2 year−1) during 1961–2014 under all sky condition. The annual mean SSR showed dimming trend over mainland China during 1961–1989 and 1990–2005, while the annual mean SSR experienced an increasing trend (0.02 × 10−1 MJ m−2 year−1) during 2005–2014 under all sky condition. Meanwhile, the annual mean SSR showed downward trend (−0.02 × 10−1 MJ m−2 year−1) during 1980–2014 under clear sky condition, while an increasing trend (0.06 × 10−1 MJ m−2 year−1) appeared under clear sky conditions from 2005 to 2014. The Qinghai-Tibet Plateau was always the area with the relatively higher SSR values, while the Sichuan Basin was generally the area with the relatively lower SSR values. Finally, the potential relationships between SSR and climate factors were analyzed. The result showed that the variation of annual mean SSR negatively correlates with the optical thickness of all cloud (TC) and the aerosol optical depth (AOD). The SSR had a significant positive correlation with sunshine duration (SD) under all sky conditions. The aerosol loading and SD were the primary factor of variation in clear sky SSR. The research will contribute to the studies related to climate change and land surface process.

Effects of aerosols and water vapour on spatial-temporal variations of the clear-sky surface solar radiation in China

Clear-sky solar radiation is an important indicator describing the potential of solar energy. The aerosols and water vapour, apart from geometrical/astronomical ones, are the main factors resulting in the losses of the surface solar radiation (SSR) under clear sky conditions. This study uses satellite aerosol data, reanalysis water vapour data, and the Mesoscale Atmospheric Irradiance Code to create the clear-sky SSR dataset for China in 2001–2015. The aerosol direct radiative effect (ADRE) and water vapour radiative effect (WVRE) are calculated to study the clear-sky SSR losses due to aerosols and water vapour. The coefficient of determination (R2) of the observations and simulations is 0.944. The mean absolute error (MAE) and root mean square error (RMSE) are 6.67% and 9.00%, respectively. The strongest attenuation in clear-sky SSR due to aerosols occurs in April in most areas of China, while in June in North China and the Yangtze River Delta, water vapour most strongly attenuates the clear-sky SSR in July, and the monthly solar radiation losses due to water vapour are higher than those due to aerosols. The high aerosol and water vapour contents are the reasons why the clear-sky SSR in North China and the Yangtze River Delta in June are lower than those in May. From 2001 to 2015, the clear-sky trend with the greatest increase of up to 0.389 W m−2 per year (2.48% in 15 years) occurs over central Inner Mongolia, which is related to the reduction in dust. The increasing concentration of particulate matter in the atmosphere results in the most significant decreasing clear-sky trend in North China (−0.510 W m−2 year−1, 3.35% in 15 years). Generally, the ADRE variations are the main factor controlling the annual variations in clear-sky SSR.

Clear-Sky Surface Solar Radiation and the Radiative Effect of Aerosol and Water Vapor Based on Simulations and Satellite Observations over Northern China

The distribution and trend of clear-sky surface solar radiation (SSR) and the quantitative effects of aerosol and water vapor are investigated in northern China during 2001–2015 using radiation simulations and satellite observations. Clear-sky SSR in northern China is high in summer and low in winter, which is dominated by astronomical factors and strongly modulated by the seasonal variations of radiative effects of aerosol (ARE) and water vapor (WVRE). The larger variation of WVRE than ARE indicates that water vapor plays a more important role in moderating the seasonal variation of clear-sky SSR. Clear-sky SSR shows an overall decreasing trend of –0.12 W/m2 per year, with decrease more strongly than –0.60 W/m2 per year in west-central Shandong and increase (about 0.40 W/m2) in south-central Inner Mongolia. The consistency of spatial distribution and high correlation between clear-sky SSR and ARE trend indicate that the clear-sky SSR trend is mainly determined by aerosol variation. Dust mass concentration decreases about 16% in south-central Inner Mongolia from 2001 to 2015, resulting in the increase in clear-sky SSR. In contrast, sulfate aerosol increases about 92% in west-central Shandong, leading to the decreasing trend of clear-sky SSR.

Trends in downward surface solar radiation from satellites and ground observations over Europe during 1983–2010

Trends of all-sky downward surface solar radiation (SSR) from satellite-derived data over Europe (1983–2010) are first presented. The results show a widespread (i.e., non-local dimension) increase in the major part of Europe, especially since the mid-1990s in the central and northern areas and in springtime. There is a mean increase of SSR of at least 2Wm−2 per decade from 1983 to 2010 over the whole Europe, which, taking into account that the satellite-derived product lacks of aerosol variations, can be mostly related to a decrease in the cloud radiative effects over Europe. Secondly, residual series have been derived as the result of the difference between ground-based and satellite-derived all-sky SSR data. The residual mean series points to a significant increase during the period 1983–2010, with higher rates of around 2Wm−2 per decade over central and eastern Europe. The spatial variation of these residual time series, which are in line with clear-sky SSR trends over Europe reported in the literature, seem to suggest that these differences in the residual series are not just explained by calibration issues in the satellite-derived product.

Detection of dimming/brightening in Italy from homogenized all-sky and clear-sky surface solar radiation records and underlying causes (1959–2013)

Abstract. A dataset of 54 daily Italian downward surface solar radiation (SSR) records has been set up collecting data for the 1959–2013 period. Special emphasis is given to the quality control and the homogenization of the records in order to ensure the reliability of the resulting trends. This step has been shown as necessary due to the large differences obtained between the raw and homogenized dataset, especially during the first decades of the study period. In addition, SSR series under clear-sky conditions were obtained considering only the cloudless days from corresponding ground-based cloudiness observations. Subsequently, records were interpolated onto a regular grid and clustered into two regions, northern and southern Italy, which were averaged in order to get all-sky and clear-sky regional SSR records. Their temporal evolution is presented, and possible reasons for differences between all-sky and clear-sky conditions and between the two regions are discussed in order to determine to what extent SSR variability depends on aerosols or clouds. Specifically, the all-sky SSR records show a decrease until the mid-1980s (dimming period), and a following increase until the end of the series (brightening period) even though strength and persistence of tendencies are not the same in all seasons. Clear-sky records present stronger tendencies than all-sky records during the dimming period in all seasons and during the brightening period in winter and autumn. This suggests that, under all-sky conditions, the variations caused by the increase/decrease in the aerosol content have been partially masked by cloud cover variations, especially during the dimming period. Under clear sky the observed dimming is stronger in the south than in the north. This peculiarity could be a consequence of a significant contribution of mineral dust variations to the SSR variability.

Dimming over the oceans: Transient anthropogenic aerosol plumes in the twentieth century

AbstractAnthropogenic aerosols reduce incoming surface solar radiation (SSR), but the magnitude of this effect for reducing sea surface temperatures (SST) is still debated. Using simulations from the global climate model ECHAM5 with the Hamburg Aerosol Module and prescribed SSTs, we quantify anthropogenic aerosol dimming over sea surfaces by comparing ensembles, which only differ in anthropogenic aerosol emissions. We isolate the anthropogenic aerosol effect on SSR with sufficiently large ensemble sizes to provide statistically significant results. The following simulation results are obtained: Dimming plumes extend from their source regions with clear seasonality. The latter is predominantly shaped by atmospheric circulation, while interdecadal changes follow the gradual increase in anthropogenic aerosol emissions. Comparing the 1990s with the 1870s, on average, 9.4% (clear‐sky SSR) or 15.4% (all‐sky SSR) of the entire ocean surface was affected by anthropogenic aerosol dimming larger than −4 Wm−2 (decadal mean). Comparing the same time periods, global average anthropogenic dimming over oceans is −2.3 Wm−2 and −3.4 Wm−2 for clear‐sky and all‐sky SSR, respectively. Surface dimming is hemispherically asymmetrical with stronger Northern Hemispheric dimming by 2.3 Wm−2 and 4.5 Wm−2 for clear‐sky and all‐sky SSR, respectively. Zonal average clear‐sky dimming reaches its maximum (−5.5 Wm−2) near the equator. All‐sky dimming peaks at 40°N (−8 Wm−2) and is regionally larger than clear‐sky dimming. Regionally, surface dimming can reach values up to 9.5 Wm−2 (clear‐sky) and 25 Wm−2 (all‐sky). Results are a contribution toward better quantifying spatially heterogeneous and time‐dependent anthropogenic dimming effects on SSTs.

Decadal variations in estimated surface solar radiation over Switzerland since the late 19th century

Abstract. Our knowledge on trends in surface solar radiation (SSR) involves uncertainties due to the scarcity of long-term time series of SSR, especially with records before the second half of the 20th century. Here we study the trends of all-sky SSR from 1885 to 2010 in Switzerland, which have been estimated using a homogenous dataset of sunshine duration series. This variable is shown to be a useful proxy data of all-sky SSR, which can help to solve some of the current open issues in the dimming/brightening phenomenon. All-sky SSR has been fairly stable with little variations in the first half of the 20th century, unlike the second half of the 20th century that is characterized also in Switzerland by a dimming from the 1950s to the 1980s and a subsequent brightening. Cloud cover changes seem to explain the major part of the decadal variability observed in all-sky SSR, at least from 1885 to the 1970s; at this point, a discrepancy in the sign of the trend is visible in the all-sky SSR and cloud cover series from the 1970s to the present. Finally, an attempt to estimate SSR series for clear-sky conditions, based also on sunshine duration records since the 1930s, has been made for the first time. The mean clear-sky SSR series shows no relevant changes between the 1930s to the 1950s, then a decrease, smaller than the observed in the all-sky SSR, from the 1960s to 1970s, and ends with a strong increase from the 1980s up to the present. During the three decades from 1981 to 2010 the estimated clear-sky SSR trends reported in this study are in line with previous findings over Switzerland based on direct radiative flux measurements. Moreover, the signal of the El Chichón and Pinatubo volcanic eruption visible in the estimated clear-sky SSR records further demonstrates the potential to infer aerosol-induced radiation changes from sunshine duration observations.

Erratum to: Decadal variation of surface solar radiation in the Tibetan Plateau from observations, reanalysis and model simulations

In this study, the annual and seasonal variations of all-sky and clear-sky surface solar radiation (SSR) in the eastern and central Tibetan Plateau (TP) during the period 1960-2009 are investigated, based on surface observational data, reanalyses and ensemble simulations with the global climate model ECHAM5-HAM. The mean annual all-sky SSR series shows a decreasing trend with a rate of -1.00 Wm -2 decade -1 , which is mainly seen in autumn and secondly in summer and winter. A stronger decrease of -2.80 Wm -2 decade -1 is found in the mean annual clear- sky SSR series, especially during winter and autumn. Overall, these results confirm a tendency towards a decrease of SSR in the TP during the last five decades. The comparisons with reanalysis show that both NCEP/NCAR and ERA-40 reanalyses do not capture the decadal varia- tions of the all-sky and clear-sky SSR. This is probably due to a missing consideration of aerosols in the reanalysis assimilation model. The SSR simulated with the ECHAM5-HAM global climate model under both all-sky and clear-sky conditions reproduce the decrease seen in the surface observations, especially after 1980. The steadily increasing aerosol optical depth (AOD) at 550 nm over the TP in the ECHAM5-HAM results suggests transient aero- sol emissions as a plausible cause.

Clear-Sky Surface Solar Radiation during the South China Sea Monsoon Experiment

Downward total solar fluxes measured at Dungsha coral island (20°42'N, 116°43'E) during the South China Sea Monsoon Experiment (May-June 1998) have been calibrated and compared with radiative transfer calculations for three clear-sky days. Model calculations use water vapor and temperature profiles from radiosonde measurements, and the aerosol optical thickness is derived from sunphotometric radiance measurements. Results show that the difference between observed and model-calculated downward surface fluxes is ＜ 3 % of the daily mean. Averaged over the three clear days, the difference reduces to 1 %. The averaged downward surface fluxes from observations and calculations for the three selected dates are 314 and 317 W/m2, respectively. This result is consistent with a previous study using TOGA CO ARE measurements, which found good agreement between observations and model calculations. This study provides an extra piece of useful information for the modeling of radiative transfer, which helps to fill in the puzzle of the absorption of solar radiation in the atmosphere.