A set of Surface Radiation Budget Network (SURFRAD) measurements across the lower 48 United States has allowed a closer inspection of weather model representations of downward shortwave radiation in the last several years. In this study, it is found that downward shortwave radiation (SW_) is excessive for the NOAA 3-km HRRR model at each of the 14 SURFRAD stations distributed across the lower United States when averaged over 2-month periods. Possible causes for this station-consistent SW_ bias error were hypothesized. Three were eliminated by this study and two were then evaluated in this study. We found that this error was not from clear-sky errors but from insufficient attenuation by clouds. It was also found that this cloud deficiency was partly caused by a dry bias in atmospheric water vapor initial conditions. New experiments using the hourly cycled HRRR model-assimilation system were designed and carried out for three seasons with modified data assimilation addressing the dry bias problem and reduction of effective radius for cloud water droplets for both explicit and subgrid-scale clouds. The assimilation and cloud optical parameter changes contributed similarly toward a combined reduced SW_ radiation bias by 80% in the fall season and 84% in the winter season but by only 35% in the summer season. Even with the improved data assimilation, a dry bias contributing to deficient clouds continues, which is a topic to be explored in a following study.

Several studies proposed relationships linking irradiances in the photosynthetically active radiation (PAR) range and broadband irradiances. A previous study published in 2024 by the same authors proposes a linear model relating clear-sky indices in the PAR and broadband ranges that has been validated in clear and overcast conditions only. The present work extends this study for broken-cloud conditions by using ground-based measurements obtained from the Surface Radiation Budget Network in the U.S.A. mainland. As expected, the clear-sky indices are highly correlated and are linked by affine functions whose parameters depend on the fractional sky cover (FSC), the year, and the site. The previous linear model is also efficient in broken-cloud conditions, with the same level of accuracy as in overcast conditions. When this model is combined with a PAR clear-sky model, the result tends to overestimate the PAR as the FSC decreases, i.e., when fewer and fewer scattered clouds are present. The bias is equal to 1 W m−2 in overcast conditions, up to 18 W m−2 when the FSC is small, and 6 W m−2 when all cloudy conditions are merged. The RMSEs are, respectively, 5, 24, and 15 W m−2. The linear and the clear-sky models can be combined with estimates of the broadband irradiance from satellites to yield estimates of PAR.

Ocean surface radiation measurement best practices have been developed as a first step to support the interoperability of radiation measurements across multiple ocean platforms and between land and ocean networks. This document describes the consensus by a working group of radiation measurement experts from land, ocean, and aircraft communities. The scope was limited to broadband shortwave (solar) and longwave (terrestrial infrared) surface irradiance measurements for quantification of the surface radiation budget. Best practices for spectral measurements for biological purposes like photosynthetically active radiation and ocean color are only mentioned briefly to motivate future interactions between the physical surface flux and biological radiation measurement communities. Topics discussed in these best practices include instrument selection, handling of sensors and installation, data quality monitoring, data processing, and calibration. It is recognized that platform and resource limitations may prohibit incorporating all best practices into all measurements and that spatial coverage is also an important motivator for expanding current networks. Thus, one of the key recommendations is to perform interoperability experiments that can help quantify the uncertainty of different practices and lay the groundwork for a multi-tiered global network with a mix of high-accuracy reference stations and lower-cost platforms and practices that can fill in spatial gaps.

This study provides empirical relationships between Photosynthetically Active Radiation (PAR) and broadband clear-sky indices at ground level for both the PAR global irradiance and its direct component. Once multiplied by the irradiance in clear-sky conditions, the clear-sky index provides the irradiance under cloudy conditions. The relationships are developed by the means of radiative transfer simulations of various realistic atmospheric states including both ice and water cloud phases. For the direct component, the PAR clear-sky index is equal to the broadband clear-sky index. For global irradiance, several linear relationships are proposed depending on the availability of cloud properties namely cloud phase and cloud optical depth. The developed relationships are validated numerically and experimentally by using ground-based measurements from the SURFRAD network in the U.S.A. overall, it has been found, a squared correlation coefficient R2 close to 1.00, relative bias (relative root mean square error) in absolute value less than 3 % (6 %) with respect to the means of the relevant measurements demonstrating a high level of accuracy of the proposed relationships.

Multidecadal dimming and brightening of solar radiation at Earth’s surface has been shown to occur over all continents. Trends have been especially well documented over the Northern Hemisphere (NH) with dimming from the 1950s through the middle 1980s followed by brightening through the first decade of the 2000s in the United States, Europe, and parts of Asia. Trends in Europe and China have been attributed to both aerosols and clouds, but in the U.S. cloud variability has been dominant. A recent analysis shows that U.S. brightening of 7.4 Wm-2/decade peaked in 2012 and then dramatically dropped to near normal values in 2013. Since then, surface solar radiation in the U.S. has remained within 1 Wm-2 of the long-term average. However, in Europe surface solar radiation has remained high, at least through 2017. It has been shown that the direct effect of aerosols cannot account for the magnitude of the latest brightening in the U.S. It has also been shown that the second indirect effect of aerosols may explain brightening into the first decade of the 2000s, but is in opposition to the observed dimming after 2012. High aerosol content does explain perpetual dimming in India and industrial parts of China, but, given that the magnitude and period of dimming and brightening trends from the 1950s through the first decade of the 2000s are similar over North America, Europe, and parts of Asia, I speculate the primary cause is meteorological. A recent study documents a strong association between multidecadal surface solar radiation trends over NH continents and long-term North Pacific and North Atlantic sea surface temperature (SST) patterns. For example, the reversal of the Pacific Decadal Oscillation (PDO) index in the mid-1980s is nearly simultaneous with the change from dimming to brightening over NH continents. A similar association is shown between Atlantic SST patterns and continental surface solar radiation trends but with a decade lag. Using reanalysis and observed SST patterns it is demonstrated that persistent warm SST anomalies support overlying semipermanent geopotential height ridges at tropospheric mid-levels that dynamically induce persistent troughs downstream over adjacent continents, if positioning is favorable. Semipermanent troughs over the continents cause greater than average cloud cover and dimming. Conversely, long-term cool SSTs produce the opposite scenario and yield less clouds and brightening downstream over the continents. Further, marine heat waves on either side of North America are shown to be associated with the recent dimming in the midcontinent from 2013 to the present, and warm SSTs in the Mediterranean and North Seas in the last decade are likely responsible for a persistent midlevel geopotential ridge pattern and continued high surface solar radiation there. Recent studies present evidence that the observed increase in frequency and variability of marine heat waves in the past few decades may be associated with global warming, possibly linking warming to trends in surface solar radiation.

The long‐term variation of North Pacific and North Atlantic sea surface temperatures (SSTs) is shown to be associated with multidecadal trends of surface solar radiation in North America, Europe, and Asia. Long‐term, large‐scale warm SST anomalies lead to a mid‐level planetary wave anomaly pattern of geopotential height ridges over the warm water and dynamically‐induced lower heights on either side, sometimes extending over adjacent continents. Geopotential height troughs over the continents encourage more cloud cover and dimming of surface solar radiation. Conversely, cool SST anomalies correspond to a pattern of lower mid‐level geopotential heights over the cool water and compensating high pressure on either side that encourages decreasing cloud cover and brightening over the continents, if the wave positioning is favorable. Additionally, these effects are observed to be latitude dependent, showing stronger SST‐geopotential height associations in the northern half of the Northern Hemisphere. The change from continental dimming to brightening and the reversal of North Pacific SST trends are nearly simultaneous. A similar SST‐geopotential height association is seen in the North Atlantic and leads to brightening and dimming in Europe and North America, but there is a ∼12‐year lag between the transition of dimming to brightening and SST reversals there, which is yet unexplained. The next step is to support these connections and their latitudinal dependence with carefully designed numerical experiments that consider variable greenhouse gas and aerosol concentrations.

The 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.