Dimming Period Research Articles

Trends from 30-Year Observations of Downward Solar Irradiance in Thessaloniki, Greece

The shortwave downward solar irradiance (SDR) is an important factor that drives climate processes and energy production and can affect all living organisms. Observations of SDR at different locations around the world with different environmental characteristics have been used to investigate its long-term variability and trends at different time scales. Periods of positive trends are referred to as brightening periods and of negative trends as dimming periods. In this study we have used 30 years of pyranometer data in Thessaloniki, Greece, to investigate the variability of SDR under three types of sky conditions (clear-, cloudy- and all-sky). The clear-sky data were identified by applying a cloud screening algorithm. We have found a positive trend of 0.38%/year for all-sky, ∼0.1%/year for clear-sky, and 0.41%/year for cloudy conditions. The consistency of these trends, their seasonal variability, and the effect of the solar zenith angle have also been investigated. Under all three sky categories, the SDR trend is stronger in winter, with 0.7, 0.4, and 0.76%/year, respectively, for all-, clear-, and cloudy-sky conditions. The next larger seasonal trends are in autumn—0.42 and 0.19%/year, for all and cloudy skies, respectively. The rest of the seasonal trends are significant smaller, close to zero, with a negative values in summer, for clear and cloudy skies. The SDR trend is increasing with increasing solar zenith angle, except under cloudy skies, where the trend is highly variable and close to zero. Finally, we discuss shorter-term variations in SDR anomalies by examining the patterns of the cumulative sums of monthly anomalies from the climatological mean, both before and after removing the long-term trend.

Analysis of sunshine duration and cloud cover trends in Lisbon for the period 1890–2018

Sunshine duration (SD) represents a valuable parameter for early years when few or none measurements of surface solar radiation (SSR) are available. In the present work, daily and monthly SD records registered in Lisbon (Portugal) for the period 1890–1940 have been digitized to expand the data series available in electronic format, which starts in 1941. The resulting series for the period 1890–2018 can be considered as the earliest one in Portugal and the second one in the Iberian Peninsula. Cloud cover (CC) data for the same period have also been digitized. The SD series exhibits a weak negative trend (without statistical significance) from the 1890s to the 1910s, which is in line with the early dimming period in SSR reported in some regions. Subsequently, no trends are obtained for the period 1910s–1950s, which indicates that the early brightening is not observed in Lisbon unlike other locations in the Iberian Peninsula. After that, two strong statistically significant trends are found for the periods 1950s–1980s and 1980s–2010s in line with the well-known global dimming and brightening periods in SSR, respectively. On the other hand, the CC series presents an increase from 1890 to the 1980s, followed by a decrease up to 2018 (both being statistically significant), which may partially explain the reported SD trends. An analysis of SD under cloudless conditions proved the utility of this quantity to track long-term changes in atmospheric aerosol load. In addition, this analysis and a seasonal one pointed out that aerosols seem to play a relevant role in SD long-term variability.

Northern dimming and southern brightening in eastern China during the first decade of the 21st century

Global solar radiation (GSR) reaching the Earth’s surface can directly alter the energy balance of the climate system. The potential influence of inhomogeneity issues on Chinese GSR records on dimming and brightening in China has been argued for years. In this study, we explored the temporal evolution and spatial distribution of the long-term trend of GSR in eastern China (EC) during 1961–2018 using a homogenized GSR dataset, which removed the artificial signals. Results demonstrate that EC experienced a significant dimming period from 1961 to 1999, a brightening period from 2010 until 2018 and a leveling-off period between them (2000–2010), which resulted from opposite trends of GSR in the northern (dimming) and southern (brightening) regions of EC. The reduction of cloud coverage after the dimming period (2000) induced an early coming brightening in the southern region of China between 2000 and 2010, while the increase in aerosols and the rapidly extending cloud fraction resulted in the continuing dimming in northern China.

Evaluation of surface solar radiation trends over China since the 1960s in the CMIP6 models and potential impact of aerosol emissions

Accurate representation of surface solar radiation (SSR) trends is an important indicator for global climate models (GCMs) to correctly reproducing the historical climate evolution. This study examines the annual mean SSR trends in China under all-sky and clear-sky conditions for the period 1961–2014 in 34 Coupled Model Intercomparison Project Phase 6 (CMIP6) models using the latest homogenized in-situ SSR dataset. The site-observed annual mean SSR over China shows a significant decadal decline during 1961–2005 but an uptrend during 2006–2014, with the trends being −6.4 (−8.6) W m−2 and + 2.5 (+5.9) W m−2 per decade under all-sky (clear-sky) condition, respectively. All CMIP6 models simulate the sustained decline in SSR over China for the period 1961–2005 but significantly underestimate the dimming. The model results show trends of −1.9 ± 0.5 W m−2 and -2.5 ± 0.7 W m−2 per decade during 1961–2005 under all-sky and clear-sky conditions, respectively, which are around one third of the observed results. Furthermore, the models fail to capture the reversal of SSR trends in China during 2006–2014, with the trends being −1.1 ± 1.7 W m−2 and -2.2 ± 0.9 W m−2 per decade under all-sky and clear-sky conditions, respectively. We infer that the underestimation of anthropogenic aerosol emissions, especially absorbing black carbon emissions cause the underestimated simulation of SSR in dimming period over China. After 2005, the unseasonal increase in carbonaceous aerosol emissions and the weaker decline of sulfur dioxide emissions in China in the models result in an opposite SSR trends relative to the trends based on the site-observations. Our results suggest that improving the anthropogenic aerosol emissions inventory will be useful for generating a more accurate reproduction of the regional SSR evolution over China in GCMs.

Variability of Surface Radiation Budget Components Over the U.S. From 1996 to 2019—Has Brightening Ceased?

AbstractThe record of downwelling solar irradiance and other surface radiation budget components for the U.S. has been extended through 2019 using SURFRAD Network data. Brightening of surface solar irradiance of +7.36 Wm−2/decade occurred from 1996 through 2012. In 2013, surface solar radiation sharply decreased to the long‐term mean (representing 1996–2019) and remained near that level through 2017. Successive decreases in 2018 and 2019 yielded a dimming trend of −3.90 Wm−2/decade after 2012, but with a high uncertainty owing to the observed variability and brief period covered. Individually, all stations but Penn State showed brightening trends consistent with the network average, and surface solar irradiance decreased at all stations after 2012. Total surface net radiation showed similar tendencies but the reversal from increasing to decreasing was more gradual because of the response of surface net longwave to the changing solar input. Aerosol optical depth decreased continuously throughout the tenure of the network but accounted for only 3% of the variability of surface solar irradiance, while cloud fraction explained 62%. The mean cloud fraction was 2.4% greater during the dimming period than the brightening period but showed no trends due to high interannual variability. However, annual anomalies of direct‐normal solar radiation, which relate to sun duration and clouds, generally increased to 2012 and then decreased thereafter. Collectively, these results indicate that changing cloud cover was the primary source of brightening and dimming over the U.S. from 1996 to 2019.

Cloud ‘shrinking’ and ‘optical thinning’ in the ‘dimming’ period and a subsequent recovery in the ‘brightening’ period over China

There was a dramatic increase in aerosol loading in China after the 1960s due to rapid industrialization, followed by a gradual reduction due to improvements in air quality since the early 2000s. They are deemed to be the main cause of ‘dimming’ and ‘brightening’ in China, respectively. China, therefore, provides an ideal testbed to investigate the multi-decadal evolution of clouds under a background of large variations in the amount of atmospheric aerosols. We used a unique combination of long-term in-situ observational records combined with a radiative transfer model to investigate the changes in clouds and aerosols over the last 60 years (1958–2018) over China. We found during the dimming period, the clouds over China shrunk in area steadily, gradually thinned in terms of optical depth, and thereby blocked less and less solar radiation. This situation reversed during the brightening period. The clouds over China showed a quick expansion in extent and thickening in terms of optical depth, and accordingly the amounts of solar radiation blocked by clouds recovered rapidly. It is observationally demonstrated that high levels of pollution and the associated amount of aerosols cause a suppression of cloud growth and a reduction of optical depth.

Impact of global dimming/brightening on estimating Angström-Prescott parameters based on geographically weighted regression in China

Variations of solar radiation is related to energy planning and architectural designing. It is important to estimate solar radiation from other climate factors because of the limited measurement solar radiation data. In this study, temporospatial changes of solar radiation and sunshine hour are analysed for global dimming/brightening periods based on meteorological data in China. Results show that the trends of solar radiation and sunshine hour are both decreasing dramatically for 1961–1990 (the global dimming period) but levelling off for 1991–2010 (the global brightening period), which indicates that sunshine hour can be used the reliable indicator of solar radiation. Spatially, the change trends of solar radiation are consistent with sunshine hour in 36 stations during the period of 1961–1990, but they are consistent in just 27 stations during the period of 1991–2010. To assess the impact of changing solar radiation on Angstrom-Prescott parameters (a and b) in the equation of solar radiation estimation from sunshine hour, geographically weighted regression is developed to calculate a and b. There is not dramatic difference in the spatial patterns of a and b for the period of 1961–2010 and 1961–1990. However, there is significantly inconsistent changes in a and b during the global brightening period compared with the global dimming period, which suggests that global dimming/brightening has important impact on estimating solar radiation from sunshine hour, with dramatic variability among different regions.

Comparison of an intermittent, short-dawn/dusk photoperiod with an increasing, long-dawn/dusk photoperiod on broiler growth, stress, and welfare

Photoperiod has been shown to significantly affect broiler performance. However, the effects of the traditional 1-min dimming period on broiler growth and welfare are unclear. In this study, 4 consecutive trials were conducted to compare the effects of an intermittent, short-dawn/dusk photoperiod (ISD) to an increasing, long-dawn/dusk photoperiod (ILD) on broiler growth, stress, and welfare. Straight run day-of-hatch Ross 708 × Ross 708 broilers were placed in 1 of 4 commercial broiler barns (2 b of 26,200 birds each per treatment) and grown to 45 D of age. The photoperiod in the ISD treatment consisted of 24L:0D day 0 to 6, 16L:8D day 7 to 13, 12L:4D:2L:6D day 14 to 20, 12L:4D:3L:5D day 21 to 27, 12L:4D:4L:4D day 28 to 41, and 13L:3D:5L:3D day 42 to 45, with a 1-min transition between light and dark periods. The photoperiod in the ILD treatment consisted of 23L:1D day 0 to 7, 16L:8D day 8 to 21, 18L:6D day 22 to 32, and 20L:4D day 33 to 45, with a 1-min light/dark transition period day 0 to 7 and a 30-min transition period day 8 to 45. Treatments were rotated among the barns between trials. On day 45, blood samples were collected from 20 birds/barn (n = 40/treatment) to assess plasma corticosterone (CORT) and heterophil/lymphocyte (H/L) ratio. One hundred birds per barn (n = 200/treatment) were weighed individually and assigned scores for hock burn, foot pad dermatitis, and feather condition on day 45. Trial differences were observed for all measures (P ≤ 0.001). Birds in the ISD treatment were heavier on day 45 (P < 0.001) and had lower hock burn (P = 0.044) and foot pad dermatitis (P < 0.001) scores. Birds in the ILD treatment had lower plasma CORT (P < 0.001) and H/L ratio (P < 0.001). No treatment differences were observed for feather condition (P > 0.05). Overall, birds reared under the intermittent, short-dawn/dusk photoperiod had higher day 45 live body weights and lower hock burn and foot pad dermatitis scores, whereas those reared under the increasing, long-dawn/dusk photoperiod had reduced measures of short-term and long-term stress.

Causes of Dimming and Brightening in China Inferred from Homogenized Daily Clear-Sky and All-Sky in situ Surface Solar Radiation Records (1958–2016)

AbstractThis paper presents a study on long-term surface solar radiation (SSR) changes over China under clear- and all-sky conditions and analyzes the causes of the “dimming” and “brightening.” To eliminate the nonclimatic signals in the historical records, the daily SSR dataset was first homogenized using quantile-matching (QM) adjustment. The results reveal rapid dimming before 2000 not only under all-sky conditions, but also under clear-sky conditions, at a decline rate of −9.7 ± 0.4 W m−2 decade−1 (1958–99). This is slightly stronger than that under all-sky conditions at −7.4 ± 0.4 W m−2 decade−1, since the clear-sky dimming stopped 15 years later. A rapid “wettening” of about 40-Pa surface water vapor pressure (SWVP) from 1985 to 2000 was found over China. It contributed 2.2% to the SSR decline under clear-sky conditions during the whole dimming period (1958–99). Therefore, water vapor cannot be the main cause of the long-term dimming in China. After a stable decade (1999–2008), an intensive brightening appeared under the clear-sky conditions at a rate of 10.6 ± 2.0 W m−2 decade−1, whereas a much weaker brightening (−0.8 ± 3.1 W m−2 decade−1) has been observed under all-sky conditions between 2008 and 2016. The remarkable divergence between clear- and all-sky trends in recent decades indicates that the clouds played two opposite roles in the SSR changes during the past 30 years, by compensating for the declining SSR under the cloud-free conditions in 1985–99 and by counteracting the increasing SSR under cloud-free conditions in 2008–16. Aerosols remain as the main cause of dimming and brightening over China in the last 60 years, although the clouds counteract the effects of aerosols after 2000.

Estimated spatiotemporal variability of total, direct and diffuse solar radiation across China during 1958–2016

The long‐term variability of total, direct and diffuse solar radiation across China during 1958–2016 is investigated based on a ground‐measured daily radiation dataset. Missing data are estimated using a 3‐day average moving window and a backpropagation artificial neural network (BP network). The BP network achieves better estimates of direct (R2 = 0.32–0.96) than diffuse radiation (R2 = 0.00–0.81). A dimming period during 1958–1990 and a “From Dimming to Brightening” transition between 1990 and 1993 have been detected across China. The declining ratio of direct to diffuse radiation suggests a degrading air quality caused by increasing aerosols in eastern China. To study the aerosol effect on radiation, two empirical models are developed from 2000–2016 using the ground‐measured total radiation, sunshine duration and satellite‐retrieved total aerosol concentration. Both models perform well in the estimate of direct (R2 = 0.71–0.89) and diffuse radiation (R2 = 0.63–0.95). The increasing total radiation in eastern China since 2000 is mainly contributed by diffuse radiation. Besides, small anthropogenic aerosols can increase diffuse fraction, the proportion of diffuse in total radiation, whereas large natural aerosols may reduce it. The BP network and empirical models exhibit a better agreement in the estimate of direct than diffuse radiation in eastern China, which highlights the impact of aerosols on diffuse radiation in the recent decade.

Maximum and Minimum Soil Surface Temperature Trends Over China, 1965–2014

AbstractMinimum and maximum soil surface temperatures have shown significant warming trends since the midtwentieth century in China, with minimum temperatures rising at a faster pace than maximums, resulting in a decrease in the soil surface diurnal temperature range. Similar to air temperature, these changes demonstrate spatial coherence, with regions at higher latitudes experiencing relatively more change. While increases in air temperatures in China slowed after 1998, paralleling the global “warming hiatus,” soil temperatures at the surface and in shallow layers have continued to rise across China. The increases in soil surface temperatures are fastest at night since 1998, reducing the soil surface diurnal temperature range. Thus, we find that while the decrease in the diurnal air temperature range mainly happened during the earlier “dimming” period ending around 1988, the decrease in the diurnal soil surface temperature range is detectable only during the renewed dimming period after 1998. This has caused soil temperatures at the surface and in shallow layers to diverge from air temperatures since 1998. Although changes in soil temperatures were associated with changes in air temperature, the patterns of change are different. Calculations using only nonurban stations show similar warming rates, indicating that the effects of urbanization are small.

Long-term series and trends in surface solar radiation in Athens, Greece

Abstract. We present a long-term series of surface solar radiation (SSR) from the city of Athens, Greece. SSR measurements were performed from 1954 to 2012, and before that (1900–1953) sunshine duration (SD) records were used in order to reconstruct monthly SSR. Analysis of the whole data set (1900–2012) mainly showed very small (0.02 %) changes in SSR from 1900 to 1953, including a maximum decrease of −2.9 % decade−1 in SSR during the 1910 to 1940 period, assuming a linear change. For the dimming period 1955–1980, −2 % decade−1 was observed that matches various European long-term SSR-measurement-related studies. This percentage in Athens is in the lower limit, compared to other studies in the Mediterranean area. For the brightening period 1980–2012 we calculated +1.5 % decade−1, which is also in the lower limit of the reported positive changes in SSR around Europe. Comparing the 30-year periods 1954–1983 and 1983–2012, we found a difference of 4.5 %. However, measurements of the first 30-year period are associated with higher uncertainties than those of the second period, especially when looking at year-to-year changes. The difference between the two periods was observed for all seasons except winter. Analyzing SSR calculations of all-sky and clear-sky (cloudless) conditions/days, we report that most of the observed changes in SSR after 1954 can be attributed partly to cloudiness and mostly to aerosol load changes.

Sunshine duration and global radiation trends in Italy (1959–2013): To what extent do they agree?

AbstractTwo Italian homogenized data sets of sunshine duration (SD) and global radiation (Eg↓) relative anomalies are used to investigate to what extent these two variables agree with respect to their temporal evolution. They are compared for northern and southern Italy over the period 1959–2013. Both under all‐sky and clear‐sky conditions, the SD records tend to show a shorter and less intense decrease until the 1980s (“global dimming”) with respect to the Eg↓ ones, while there is a better agreement in the subsequent period when both variables increase (“brightening period”). To investigate whether such behavior can be explained by a different sensitivity of SD and Eg↓ to atmospheric turbidity variations, the observed clear‐sky trends are compared to those estimated by a model based both on Lambert‐Beer's law and on a simple estimation of diffuse radiation. Results show that most of the differences observed in the trends of the clear‐sky SD and Eg↓ records can be explained considering a realistic pattern of atmospheric turbidity in the 1959–2013 period. The only exception concerns winter and autumn in northern Italy where clear‐sky SD does not decrease in the dimming period as much as it would be expected on the basis of the corresponding increase in atmospheric turbidity. One reason for this discrepancy could be the influence of other variables like relative humidity. This case study highlights that changes in atmospheric turbidity have to be kept in mind when SD is used to investigate the multidecadal evolution of Eg↓.

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.

Lighting schedule and dimming period in early life: consequences for broiler chicken leg bone development

Prolonged (>20 h) light periods during grow-out of broiler chickens have been shown to increase the occurrence of skeletal abnormalities, but the effects of early life light-dark schedules are not well known. The present experiment investigated the effect of lighting schedule and light-dark transition during the first days of a broiler chicken's life on leg bone development. In 2 experiments, Ross-308 broiler chicks (n = 2,500 per experiment) were subjected to 1 of 5 treatments for 4 d: 24L; 2L:1D lighting schedule with either an abrupt or gradual light-dark transition (“dimming”); and a 2L:6D lighting schedule with an abrupt transition or dimming. At d 4, tibia and femur weight, length, and diameter, yolk free body mass, organ weights, realized weight gain, feed intake, feed conversion ratio, and mortality were determined. In Experiment 2, chick length and relative asymmetry of the femur and tibia were determined additionally. Data were analyzed using orthogonal contrasts. 24L resulted in higher femur diameter (P < 0.028; both experiments), tibia diameter (P < 0.001; Experiment 1), relative asymmetry of tibia length (P = 0.002; Experiment 2), and relative asymmetry of femur length (P = 0.003) than applying a light-dark schedule. A 2L:1D lighting schedule resulted in higher femur length (P = 0.039; Experiment 1) and relative asymmetry of tibia length (P = 0.032; Experiment 2) and lower relative asymmetry of tibia diameter (P = 0.016) than a 2L:6D lighting schedule. An abrupt light-dark transition resulted in higher relative asymmetry of tibia length (P = 0.004; Experiment 2) and relative asymmetry of tibia diameter (P = 0.018) than dimming. To conclude, leg bone development in the first 4 d of a broiler chicken's life was higher for 24L than when a lighting schedule was applied, but relative asymmetry was higher as well, suggesting developmental instability. The effect of dimming on leg bone development was less pronounced, but the decreased relative asymmetry levels in the dimming treatment suggested lower environmental stress than for the abrupt light-dark transition.

Dimmable and Cost-Effective DC Driving Technique for Flicker Mitigation in LED Lighting

In LED lighting systems, consideration about dimming and flicker is necessary. It is found that conventional DC-mode technique produces flicker-free light and high luminous efficacy but poor dimming function, whereas pulse-width-modulation (PWM) technique offers better dimming flexibility but inherent flicker in comparison to DC technique. Although increasing the PWM dimming frequency could make the flicker invisible to the human eyes, the flicker still exists, and the potential health risks should not be ignored. In this paper, a cost-effective DC driving technique is proposed to improve dimming flexibility of existing DC technique while retaining flicker-free and high luminous efficacy. A current-controlled buck converter is operated in discontinuous conduction mode (DCM). LED current is regulated by the number of inductor current pulses during the dimming period. A method to distribute PWM pulses over the dimming period is utilized to ensure a small output voltage ripple without a large output capacitor for the buck converter. A RGB LED lamp was used in the experimental verifications. The results show that linear constant DC through LEDs could be obtained and flicker could not be found. The main tradeoff is the reduction of control precision, which is acceptable for LED outdoor lighting systems.

Visible light communications employing PPM and PWM formats for simultaneous data transmission and dimming

A visible light communication system with simultaneous brightness control and data transmission is experimentally demonstrated in this paper. A pulse-width modulation (PWM) scheme is adopted for brightness control of the light-emitting diode and pulse-position modulation (PPM) superimposed on top of PWM is used for data communications. We show that independent control of light dimming is possible while transmitting data within a typical room environment, provided the PWM dimming period is an integer multiple of the PPM slot duration. To validate this, we show the measured eye diagrams for a range of dimming levels. In addition, we qualitatively illustrate examples of different dimming levels by changing the dimming level while maintaining the PPM data stream.

Influence of changes in solar radiation on changes of surface temperature in China

The long-term trends of total surface solar radiation (SSR), surface diffuse radiation, and surface air temperature were analyzed in this study based on updated 48-yr data from 55 observational stations in China, and then the correlation between SSR and the diurnal temperature range (DTR) was studied. The effect of total solar radiation on surface air temperature in China was investigated on the basis of the above analyses. A strong correlation between SSR and DTR was found for the period 1961–2008 in China. The highest correlation and steepest regression line slope occurred in winter, indicating that the solar radiation effect on DTR was the largest in this season. Clouds and water vapor have strong influences on both SSR and DTR, and hence on their relationship. The largest correlations between SSR and DTR occurred in wintertime in northern China, regardless of all-day (including clear days and cloudy days) or clear-day cases. Our results also showed that radiation arriving at the surface in China decreased significantly during 1961–1989 (dimming period), but began to increase during 1990–2008 (brightening period), in agreement with previous global studies. The reduction of total SSR offset partially the greenhouse warming during 1961–1989. However, with the increase of SSR after 1990, this offsetting effect vanished; on the contrary, it even made a contribution to the accelerated warming. Nonetheless, the greenhouse warming still played a controlling role because of the increasing of minimum and mean surface temperatures in the whole study period of 1961–2008. We estimated that the greenhouse gases alone may have caused surface temperatures to rise by 0.31–0.46°C (10 yr)−1 during 1961–2008, which is higher than previously estimated. Analysis of the corresponding changes in total solar radiation, diffuse radiation, and total cloud cover indicated that the dimming and brightening phenomena in China were likely attributable to increases in absorptive and scattering aerosols in the atmosphere, respectively.

Urbanization Effect on the Diurnal Temperature Range: Different Roles under Solar Dimming and Brightening*

Abstract Based on the 1960–2009 meteorological data from 559 stations across China, the urbanization effect on the diurnal temperature range (DTR) was evaluated in this study. Different roles of urbanization were specially detected under solar dimming and solar brightening. During the solar dimming time, both urban and rural stations showed decreasing trends in maximum temperature (Tmax) because of decreased radiation, suggesting that the dimming effects are not only evident in urban areas but also in rural areas. However, minimum temperature (Tmin) increased more substantially in urban areas than in rural areas during the dimming period, resulting in a greater decrease in the DTR in the urban areas. When the radiation reversed from dimming to brightening, the change in the DTR became different. The Tmax increased faster in rural areas, suggesting that the brightening could be much stronger in rural areas than in urban areas. Similar trends of Tmin between urban and rural areas appeared during the brightening period. The urban DTR continued to show a decreasing trend because of the urbanization effect, while the rural DTR presented an increasing trend. The remarkable DTR difference in the urban and rural areas showed a significant urbanization effect in the solar brightening time.

Impact of global dimming and brightening on reference evapotranspiration in Greece

[1] In this study, the consistency of trends in radiation and temperature records and their implications for the hydrological cycle and especially the trends in reference evapotranspiration are examined during the period 1950–2001. The new reference evapotranspiration model for complex terrains (REMCT), with monthly time step, is used for estimating trends of reference evapotranspiration in Greece. REMCT is applied after developing a methodology for calibrating its parameter values with Penman-Monteith estimates. The calibrated REMCT estimates are independently validated against available pan evaporation measurements. The evolution of available sunshine duration anomalies measured in Athens during the period 1951–2001 are used for highlighting global dimming or brightening periods in Greece. The sign of trends in the modeled reference evapotranspiration and precipitation are examined according to the dimming or brightening periods 1950–1983 and 1958–1983 (for 16 and 22 stations, respectively) or 1983–2001 (for 16, 22, and 29 stations). The trends of REMCT estimates, precipitation, number of rainy days, and mean, maximum, and minimum air temperature for all sets of stations considered “as a whole” are evaluated during the same periods. The results show that the annual calibrated reference evapotranspiration trend shows a decline from 1950 until the early 1980s, followed by an upward trend until 2001, while the annual precipitation and rainy days indicate a downward trend during the whole period 1950–2001. The trends of mean, maximum, and minimum air temperature are found almost negligible during the dimming period and rather increased during the brightening period.