K. Tourpali

Spectral measurements of solar UVB radiation and its relations to total ozone, SO2, and clouds

Spectral solar UV radiation measurements performed at Thessaloniki, Greece (40°N), are presented, and the influence of various atmospheric constituents such as total ozone, cloud cover, and columnar SO2 on these measurements is examined. By comparing UV radiation measurements at days with different total ozone amounts the magnification factor was calculated. Its values range from 1 to 20, depending on the wavelength and the total ozone. A relationship between the UV radiation and the cloud cover was established, being representative only for measurements at 50° solar zenith angle. In addition, the influence of columnar SO2 variations on UV irradiances was also studied. Finally, an attempt was made to compare the relative influence of these parameters on UV radiation, which proved that total ozone is the major factor controlling the solar UVB radiation received at the ground.

Recent variability of the solar spectral irradiance and its impact on climate modelling

The lack of long and reliable time series of solar spectral irradiance (SSI) measurements makes an accurate quantification of solar contributions to recent climate change difficult. Whereas earlier SSI observations and models provided a qualitatively consistent picture of the SSI variability, recent measurements by the SORCE (SOlar Radiation and Climate Experiment) satellite suggest a significantly stronger variability in the ultraviolet (UV) spectral range and changes in the visible and near-infrared (NIR) bands in anti-phase with the solar cycle. A number of recent chemistry-climate model (CCM) simulations have shown that this might have significant implications on the Earths atmosphere. Motivated by these results, we summarize here our current knowledge of SSI variability and its impact on Earths climate. We present a detailed overview of existing SSI measurements and provide thorough comparison of models available to date. SSI changes influence the Earths atmosphere, both directly, through changes in shortwave (SW) heating and therefore, temperature and ozone distributions in the stratosphere, and indirectly, through dynamical feedbacks. We investigate these direct and indirect effects using several state-of-the art CCM simulations forced with measured and modelled SSI changes. A unique asset of this study is the use of a common comprehensive approach for an issue that is usually addressed separately by different communities. We show that the SORCE measurements are difficult to reconcile with earlier observations and with SSI models. Of the five SSI models discussed here, specifically NRLSSI (Naval Research Laboratory Solar Spectral Irradiance), SATIRE-S (Spectral And Total Irradiance REconstructions for the Satellite era), COSI (COde for Solar Irradiance), SRPM (Solar Radiation Physical Modelling), and OAR (Osservatorio Astronomico di Roma), only one shows a behaviour of the UV and visible irradiance qualitatively resembling that of the recent SORCE measurements. However, the integral of the SSI computed with this model over the entire spectral range does not reproduce the measured cyclical changes of the total solar irradiance, which is an essential requisite for realistic evaluations of solar effects on the Earths climate in CCMs. We show that within the range provided by the recent SSI observations and semi-empirical models discussed here, the NRLSSI model and SORCE observations represent the lower and upper limits in the magnitude of the SSI solar cycle variation. The results of the CCM simulations, forced with the SSI solar cycle variations estimated from the NRLSSI model and from SORCE measurements, show that the direct solar response in the stratosphere is larger for the SORCE than for the NRLSSI data. Correspondingly, larger UV forcing also leads to a larger surface response. Finally, we discuss the reliability of the available data and we propose additional coordinated work, first to build composite SSI data sets out of scattered observations and to refine current SSI models, and second, to run coordinated CCM experiments.

Coupled chemistry climate model simulations of the solar cycle in ozone and temperature

The 11-year solar cycles in ozone and temperature are examined using newsimulations of coupled chemistry climate models. The results show a secondary maximumin stratospheric tropical ozone, in agreement with satellite observations and in contrastwith most previously published simulations. The mean model response varies by upto about 2.5% in ozone and 0.8 K in temperature during a typical solar cycle, at the lowerend of the observed ranges of peak responses. Neither the upper atmospheric effectsof energetic particles nor the presence of the quasi biennial oscillation is necessaryto simulate the lower stratospheric response in the observed low latitude ozoneconcentration. Comparisons are also made between model simulations and observed totalcolumn ozone. As in previous studies, the model simulations agree well with observations.For those models which cover the full temporal range 1960–2005, the ozone solarsignal below 50 hPa changes substantially from the first two solar cycles to the last twosolar cycles. Further investigation suggests that this difference is due to an aliasingbetween the sea surface temperatures and the solar cycle during the first part of the period.The relationship between these results and the overall structure in the tropical solarozone response is discussed. Further understanding of solar processes requiresimprovement in the observations of the vertically varying and column integrated ozone.

Solar activity-total column ozone relationships : Observations and model studies with heterogeneous chemistry

In the present paper we examine the effect of long-term solar variability on total column ozone using the longest available ground-based (1964-1994) and satellite (1979-1992) zonally averaged total column ozone records. Numerical simulations with a two-dimensional (2-D) model, which incorporates heterogeneous chemistry, transport, and the representation of the long-term solar variability, are compared to the observations. Our analysis of the total column ozone records shows that the solar activity signal in total column ozone is better seen in the tropics and during periods with no volcanoes and no synergistic effects from El Nino-Southern Oscillation (ENSO) and quasibiennial oscillation (QBO). On decadal timescales the solar activity component in total column ozone is confirmed to have a relatively small amplitude (1-2% of the 1964-1994 mean). The slowly varying decadal solar activity component in total column ozone has an amplitude 3 to 5 times larger than that of the 27-day solar rotation oscillation as seen in total ozone. The observations confirm model calculations of a larger amplitude in total column ozone at solar maximum as compared to solar activity minimum conditions for the 27-day period. The heterogeneous 2-D chemical transport model results are in good agreement with the long-term ground-based observations. It is noted that in the tropics both the interannual variability of total column ozone and the long-term impact from chlorofluorocarbons (CFCs) through the action of polar stratospheric clouds (PSCs) are small and therefore the 1-2% solar activity signal in the data can be more clearly seen above the noise level at these lower latitudes.

Further studies on possible volcanic signal to the ozone layer

In this paper the authors report the results of a study of the global ozone profiles, following the eruptions of El Chichon and Mt Pinatubo, after factoring out of the results the effect of known variations. This includes removing effects of El Nino and Quasi Bienniel Oscillations from the measurements. The remaining residual effects show a variation due to the volcanic eruptions in the range of 2-4% in equatorial regions, and only slightly larger toward the poles.

Quasi‐biennial and longer‐term changes in clear sky UV‐B solar irradiance

Using the longest available clear sky time series of spectral UV-B data (from November 1990 to present) at Thessaloniki, Greece (40°N), estimates are provided on the variability of UV-B on annual and longer time scales. Solar irradiance at two distinct wavelengths are used: the strongly ozone dependent 305nm and the weakly dependent on ozone 325nm. The results are based on Brewer measurements of spectral and erythemally weighted irradiances at 63° and 50° solar zenith angles. The long-term change in UV-B solar irradiance, which can be attributed solely to the observed ozone change of 4.5% per decade, is the order of 10% per decade at 305 nm, 63° SZA‥ These long-term change is checked against model calculations and changes imposed by the variability in air quality at Thessaloniki and is statistically significant. The peak-to-peak amplitude of the QBO in total ozone is 8% and its found to be associated with a 21% peak-to-peak amplitude relative to the mean for the solar irradiance at 305nm. Similar results are obtained with the erythemally weighted irradiance and are in agreement with results obtained from model calculations. The observed QBO in solar UV-B has many implications not only because of the QBO biorhythm imposed to many species but also because of its importance in atmospheric chemistry.

Changes in surface solar UV irradiances and total ozone during the solar eclipse of August 11, 1999

During the solar eclipse of August 11, 1999, intensive measurements of UV solar irradiance and total ozone were performed at a number of observatories located near the path of the Moons shadow. At the Laboratory of Atmospheric Physics (LAP) of the Aristotle University of Thessaloniki, Greece, global and direct spectra of UV solar irradiances (285–365 nm) were recorded with a double monochromator, and erythemal irradiances were measured with broadband pyranometers. In addition, higher-frequency measurements of global and direct irradiances at six UV wavelengths were performed with a single Brewer spectrophotometer. Total ozone measurements were also performed with Dobson and Brewer spectrophotometers at Hradec Kralove (Czech Republic), Ispra (Italy), Sestola (Italy), Hohenpeissenberg (Germany), Bucharest (Romania), Arosa (Switzerland), and Thessaloniki (Greece). From the spectral UV measurements the limb darkening effect of the solar disk was tentatively quantified from differences of measured solar spectral irradiances at the peak of the eclipse (near to limb conditions) and before the eclipse. Two blackbody curves were fit to the preeclipse and peak eclipse spectra, which have shown a difference in effective temperatures of about 165°K between the limb and the whole of the solar disk. The limb darkening effect is larger at the shorter UV wavelengths. The ratio of the diffuse to direct solar irradiances during the eclipse shows that the diffuse component is reduced much less compared to the decline of the direct solar irradiance at the shorter wavelengths. Moreover, a 20-min oscillation of erythemal UV-B solar irradiance was observed before and after the time of the eclipse maximum under clear skies, indicating a possible 20-min fluctuation in total ozone, presumably caused by the eclipse-induced gravity waves. This work also shows that routine total ozone measurements with a Brewer or a Dobson spectrophotometer should be used with caution during a solar eclipse. This is because the diffuse light increases by more than 30% with respect to the direct solar radiation, increasing more at the shorter wavelength side of the UV spectrum. This plausible mechanism introduces an artificial decrease in total ozone during solar eclipse of more than 30 Dobson units (DU), which is confirmed by all Brewer and Dobson measurements. Changes in total ozone cited earlier in the refereed literature have not been confirmed in the present study.

Skin Cancer Risks Avoided by the Montreal Protocol—Worldwide Modeling Integrating Coupled Climate-Chemistry Models with a Risk Model for UV

The assessment model for ultraviolet radiation and risk “AMOUR” is applied to output from two chemistry‐climate models (CCMs). Results from the UK Chemistry and Aerosols CCM are used to quantify the worldwide skin cancer risk avoided by the Montreal Protocol and its amendments: by the year 2030, two million cases of skin cancer have been prevented yearly, which is 14% fewer skin cancer cases per year. In the “World Avoided,” excess skin cancer incidence will continue to grow dramatically after 2030. Results from the CCM E39C‐A are used to estimate skin cancer risk that had already been inevitably committed once ozone depletion was recognized: excess incidence will peak mid 21st century and then recover or even super‐recover at the end of the century. When compared with a “No Depletion” scenario, with ozone undepleted and cloud characteristics as in the 1960s throughout, excess incidence (extra yearly cases skin cancer per million people) of the “Full Compliance with Montreal Protocol” scenario is in the ranges: New Zealand: 100–150, Congo: −10–0, Patagonia: 20–50, Western Europe: 30–40, China: 90–120, South‐West USA: 80–110, Mediterranean: 90–100 and North‐East Australia: 170–200. This is up to 4% of total local incidence in the Full Compliance scenario in the peak year.

Effect of tropopause height changes on the calculation of ozone trends and their radiative forcing

Three separate estimates of ozone trends since 1970 are obtained from ozonesonde analyses in the Northern Hemisphere, at up to 11 point locations. Our analysis shows that there has been an increase in the annually averaged tropopause height of between 330-520 m at all stations, accompanied by an overall upward movement of the ozone profile. This movement of the ozone profile has led to an extra 2-15% decade -1 ozone depletion between roughly 8 and 22 km. We argue that as the part of the ozone trend associated with dynamical movement of the ozone profile is not a direct result of anthropogenic chemical ozone depletion, it ought not to be included in anthropogenic radiative forcing calculations. This leads to a stratospheric ozone radiative forcing which is roughly 50% of the original estimate. Combining stratospheric and tropospheric ozone changes to calculate the total ozone radiative forcing in the Northern Hemisphere for the 1970-1997 period gives a radiative forcing of -0.09 W m -2 , excluding the effects of dynamical movement of the ozone profile, which were calculated to have led to a further ozone feedback of -0.1 W m -2 , These results suggest estimating ozone radiative forcing from observational ozone trend data, without accounting for the movement of the ozone profile could be fundamentally flawed, due to the unquantified feedbacks in the climate system, making it difficult to separate out the part of the ozone trend associated directly with the anthropogenic forcing.

Long term solar induced variations in total ozone, stratospheric temperatures and the tropopause

Abstract The variations of total ozone, stratospheric temperature and tropopause temperature are presented for the past 3 solar cycles for the summer months of the northern hemisphere. Ground-based, 30-year total column ozone series, filtered from its seasonal, QBO, El Nino/Southern Oscillation (ENSO) and trend components are found to be correlated to the 11-year solar cycle. Model calculations with a 2D chemical transport model are consistent with the observations. Mean stratospheric temperature variations, between levels 100 and 10 hPa, show also the same variation, correlated with the observed 11-year solar cycle, and the tropopause temperature increases in the same manner, in response to a warmer stratosphere during solar maxima.