D. Mateos

Quantifying the respective roles of aerosols and clouds in the strong brightening since the early 2000s over the Iberian Peninsula

The contribution of clouds and aerosols to the decadal variations of downward surface shortwave radiation (SSR) is a current controversial topic. This study proposes a method, which is based on surface-based SSR measurements, aerosol observations, and radiative transfer simulations (in cloud-free and cloud- and aerosol-free scenarios), to evaluate cloud-aerosol (CARE), cloud (CRE), and aerosol (ARE) radiative effects. This method is applied to quantify the role played by, separately, clouds and aerosols on the intense brightening of the SSR observed in the Iberian Peninsula. Clouds and Earths Radiation Energy Budget System (CERES) and surface-based data exhibit an increase in SSR between 2003 and 2012, exceeding +10 W m−2 over this period for some areas of the peninsula. The calculations are performed for three surface-based sites: Barcelona and Valladolid (Spain), and Evora (Portugal). Ranges in monthly values of CARE, CRE, and ARE are (−80, −20), (−60, −20), and (−30, 0), respectively (in W m−2). The average trends for the analyzed period of CARE, CRE, and ARE are +7, +5, and +2 W m−2 per decade, respectively. Overall, three fourths of the SSR trend is explained by clouds, while the other one fourth is related to aerosol changes. The SSR trends explained by the clouds and aerosol radiative effects are in line with the observed reductions in total cloud cover and aerosol load (both at the surface and in the whole atmospheric column). Furthermore, the CRE values are compared against CERES data showing good agreement between both data series, although some discrepancies are observed in their trends.

Short-wave radiative forcing at the surface for cloudy systems at a midlatitude site

Six-year data (2006–2011) of short-wave (SW) radiation, aerosol optical depth and cloud fraction were used in the evaluation of cloud radiative forcing (1-min data) at the surface in the SW spectral range at Granada station (South-Western Europe). Three different systems were analysed in this study: cloud radiative forcing (CRFSW), cloud–aerosol radiative forcing (CARFSW) and aerosol radiative forcing under cloudy conditions (ARFSW). Average values of these variables presented a clear dependence on solar zenith angle (SZA), for example, at SZA=30°, the results were CRFSW=−78 W m−2, CARFSW=−100 W m−2 and ARFSW=−22 W m−2, and the values decreased to CRFSW=−50 W m−2, CARFSW=−69 W m−2 and ARFSW=−19 W m−2 at SZA=60°. These three variables exhibited a similar pattern: they increased in the absolute magnitude up to moderate SZAs and strongly decreased towards zero for high SZAs. The hemispherical fractional sky cover (SCV) and the fractional sky cover inside the octant where the Sun is placed (SCV-Sun) also played a key role in the determination of cloud forcing. As expected, the strongest cloud effect appeared when clouds covered the Sun. However, when SCV-Sun was low or moderate and total SCV was over 0.5, there was a high likelihood of enhancement occurrence (i.e., positive cloud forcing values). Finally, the evolution of CRFSW values during a case study (ranging from −600 to +200 W m−2) with a wide variety of cloud conditions could be explained by the temporal evolution of SCV and SCV-Sun.

Observed influence of liquid cloud microphysical properties on ultraviolet surface radiation

Measurements of different UV quantities (UV index, ozone photolysis rates, global and diffuse irradiances, and actinic flux spectra) and cloud properties were collected during a field campaign carried out in Southern Italy in May–June 2010. Independent measurements of cloud liquid water path and optical depth allowed retrieving the cloud effective radius. The cloud modification factor (CMF) is used to investigate the influence of liquid cloud properties on the UV radiation under overcast conditions. CMF was also simulated using a 1-D radiative transfer model. Experimental and simulated CMF values for UV index (under overcast conditions) show a normalized root-mean-square error around 11%. Clouds with small effective radius determine a higher UV radiation attenuation than clouds formed by large particles. The CMFs for the UV index and the global spectral irradiance show a very weak dependence on the solar zenith angle (SZA), while the CMFs for actinic flux (both integrated and spectral) and diffuse spectral irradiance show a variation with SZA. The irradiance is more effectively attenuated at low SZA, while the actinic flux at high SZA. These effects are due to the different weight given to the direct and the diffuse components.

Columnar and surface aerosol load over the Iberian Peninsula establishing annual cycles, trends, and relationships in five geographical sectors

The study of atmospheric aerosol load over the Iberian Peninsula (IP) under a climatological perspective is accomplished by means of PM10 and AOD440 nm measurements from EMEP and AERONET networks, respectively, in the period 2000-2013. The PM10 annual cycles in five Iberian sectors show a main maximum in summer and a secondary maximum in spring, which is only observed in the southern area for the AOD climatology. The characteristics of PM10-AOD annual cycles of each geographical sector are explained by the different climatology of the air mass origins and their apportioning. The two magnitudes are correlated with a factor ranging between 20 and 90 depending on the sector. The temporal evolution of the aerosol load has shown a notable decrease in the IP since the 1980s. Statistically significant trends are obtained in the Northeastern sector with a reduction of 26% (period 1985-2000) for the total suspended particles, which continues for the PM10 data with a value of 35% per decade (2001-2013), and also in the whole column, 61% per decade in the AOD440 nm (2004-2013).

Aerosol properties of mineral dust and its mixtures in a regional background of north-central Iberian Peninsula

To broaden the knowledge about desert dust (DD) aerosols in western Mediterranean Basin, their fingerprints on optical and microphysical properties are analyzed during DD episodes in the north-central plateau of the Iberian Peninsula between 2003 and 2014. Aerosol columnar properties obtained from the AErosol RObotic NETwork (AERONET), such as aerosol optical depth (AOD), Ångström exponent (AE), volume particle size distribution, volume concentration (VC), sphericity, single scattering albedo, among others, are analyzed in order to provide a general characterization, being some of them compared to particle mass surface concentrations PM10, PM2.5, and their ratio, data obtained from EMEP network. The mean intensity of DD episodes exhibits: AOD440nm=0.27±0.12, PM10=24±18μg/m3, AE=0.94±0.40 and PM2.5/PM10=0.54±0.16. The AOD and PM10 annual cycles show maximum intensity in March and summer and minima in winter. A customized threshold of AE=1 distinguishes two types of dusty days, those with a prevailing desert character and those of mixed type, which is corroborated by sphericity values. Three well established intervals are obtained with the fine mode volume fraction (VCF/VCT). Coarse-mode-dominated cases (VCF/VCT≤0.2) present a mineral dust character: e.g., particle maximum concentration about 2μm, non-sphericity, stronger absorption power at shorter wavelengths, among others. The relevance of the fine mode is noticeable in mixtures with a predominance of particles about 0.2-0.3μm radii. Conditions characterized by 0.2<VCF/VCT<0.45 and VCF/VCT≥0.45 present a larger variability in all investigated aerosol properties. Relationships between AOD and columnar particle volume concentration give volume extinction efficiencies between 1.7 and 3.7μm2/μm3 depending on VCF/VCT. Aerosol scale height is obtained from relationships between surface and columnar concentrations displaying very large values up to 10km. The uncertainty associated with the transformation between AOD and PM10 can be partially reduced when the aerosol microphysical properties are known.

A method to determine the ozone radiative forcing in the ultraviolet range from experimental data

This paper proposes a method to calculate the ozone radiative forcing (RF) at surface in the ultraviolet (UV) spectral range for all-sky conditions based on the estimation of the ozone efficiency (OE) from experimental data that are subsequently applied to changes on the total ozone column (TOC) since late 1970s. The OE is defined as the rate at which the solar UV irradiance is “forced” per TOC unit, being estimated for all-sky conditions from UV-B (280–320 nm) and TOC data recorded with a Bentham spectroradiometer at Granada (Spain). The results showed a clear seasonal pattern in the OE values with largest monthly averages (in absolute terms) in July (−4.2 ± 0.3 mW/m2 per Dobson Unit) and the smallest in January (−0.7 ± 0.3 mW/m2 per Dobson Unit). The continuous and consistent TOC data set (1979–2008) provided by the Multisensor Reanalysis over the study site showed that spring months present the largest annual TOC changes relative to 1979 while summer months exhibit small variations. Thus, spring has the largest contribution (~53%) to annual ozone RF followed by summer (~17%), winter (~16%), and autumn (~4%). The evolution of the ozone RF relative to 1979 in the UV-B range at Granada showed positive values for most of years (between 5 and 40 mW/m2). Finally, the long-term evolution of the ozone RF exhibited a positive trend until the mid-1990s and, subsequently, a weak negative trend until the end of the analyzed period.

Sensitivity of UV Erythemal Radiation to Total Ozone Changes under Different Sky Conditions: Results for Granada, Spain

This study focuses on the analysis of the sensitivity of UV erythemal radiation (UVER) to variations in the total ozone column (TOC) under different sky conditions at Granada (southeastern Spain). The sensitivity is studied both in relative terms by means of the Radiation Amplification Factor (RAF) and in absolute terms using the Ozone Efficiency (OE). These two variables are determined for diverse sky conditions characterized by the cloud cover information given by a sky camera (in oktas) and the cloud optical depth (COD) estimated from global solar radiation measurements. As expected, in absolute terms, the TOC variations cause substantially smaller UVER changes during completely overcast situations than during cloud‐free cases. For instance, the OE (SZA = 30°, TOC = 290 DU) decreases from 0.68 mW m−2 per unit of TOC (0 oktas) to 0.50 mW m−2 per unit of TOC (8 oktas). However, the opposite is observed when the analysis is performed in relative terms. Thus, the RAF (determined for SZA cases below 80°) increases from 1.1 for cloud‐free cases (0 oktas) to 1.4 for completely overcast situations (8 oktas). This opposite behavior is also found when both RAF and OE are analyzed as functions of COD. Thus, while the OE strongly decreases with increasing COD, the RAF increases as COD increases.

Comparison of integrated water vapor from GNSS and radiosounding at four GRUAN stations

Integrated water vapor (IWV) data from Global Navigation Satellite Systems (GNSS) and radiosounding (RS) are compared over four sites (Lindenberg, Ny-Ålesund, Lauder and Sodankylä), which are part of the Global Climate Observing System (GCOS) Reference Upper Air Network (GRUAN). Both datasets show an excellent agreement, with a high degree of correlation (R2 over 0.98). Dependences of GNSS-RS differences on several variables are studied in detail. Mean bias error (MBE) and standard deviation (SD) increase with IWV, but in relative term, these variables decrease as IWV increases. The dependence on solar zenith angle (SZA) is partially related to the distribution of IWV with SZA, but the increase of SD for low SZA could be associated with errors in the humidity sensor. Large surface pressures worsen performance, which could be due to the fact that low IWV is typically present in high pressure situations. Cloud cover shows a weak influence on the mentioned MBE and SD. The horizontal displacement of radiosondes generally causes SD to increase and MBE to decrease (increase without sign), as it could be expected. The results point out that GNSS measurements are useful to analyze performance to other instruments measuring IWV.