K. Eleftheratos

A review of various strategies for contrail avoidance

A review is given of various contrail avoidance strategies that have been developed since the publication of the Intergovernmental Panel on Climate Change (IPCC) special report on Aviation and the Global Atmosphere. The goal of this review is to provide an overview of the various options for contrail mitigation, to describe the state of the art, and to indicate future directions for research.

Solar dimming and brightening over Thessaloniki, Greece, and Beijing, China

This work presents evidence that ultraviolet (UV)-A solar irradiances show increasing trends at Thessaloniki, Greece, where air quality has been improving because of air pollution abatement strategies. In contrast, over Beijing, China, where air quality measures were taken later, solar brightening was delayed. It is shown that until the early 1990s, UV-A irradiances over Thessaloniki show a downward trend of –0.5% yr−1, which reverses sign and becomes positive in the last decade (+0.8% yr−1). This brightening is related to a decreasing trend in local aerosol amounts. Both the negative rate of change (dimming) and the positive rate of change (brightening) are amplified in the UV-A solar irradiances, compared with the total solar irradiance, by a factor of 2.6. Satellite derived short-wave radiation over Beijing showed negative changes of –0.4% (1984–1991) and –0.1% yr−1 during 1994–2006. The negative trend in solar radiation continued even during 2000–2006. Satellite-derived aerosol optical depth (AOD) increased by +1.0% yr−1 during 2000–2006, in agreement with in situ measurements of increasing AOD. Therefore, a statistically significant change from dimming to brightening in Beijing could not be seen in the last decade, but it is expected to occur in the near future.

Evaluation of ozone column measurements over Greece with NILU-UV multi-channel radiometers

Measurements of total ozone over Greece have been performed with NILU-UV multi-channel radiometers since 2005. In this study, the measurements at two stations, Thessaloniki and Athens, of the ultraviolet (UV) monitoring network are compared with ozone values derived from Brewer spectroradiometers at the same sites. The overall percentage difference on daily averages (including measurements for solar zenith angle below 70˚) is below (1 ± 3.5)% at both sites. The cloud modification factor (CMF), specified as the ratio of measured irradiance with a model-derived cloud-free value, is being used to examine the effect of cloudiness on the retrieved ozone abundances. During days with average CMF values above 0.7, the difference in total ozone measurements is within ± 5%. The use of momentary ozone values from the radiometers with this limitation in CMF reduces the difference of the average values to (0.0 ± 2.2)% for Athens and (0.21 ± 2.8)% for Thessaloniki.

Ozone and Spectroradiometric UV Changes in the Past 20 Years over High Latitudes

Abstract This study analyzes changes in solar ultraviolet (UV) irradiances at 305 and 325 nm at selected sites located at high latitudes of both hemispheres. Site selection was restricted to the availability of the most complete UV spectroradiometric datasets of the past twenty years (1990–2011). The results show that over northern high latitudes, between 55° and 70°N, UV irradiances at 305 nm decreased significantly by 3.9% per decade, whereas UV irradiance at 325 nm remained stable with no significant long-term change. Over southern high latitudes (55°–70°S), UV irradiances did not show any significant long-term changes at either 305 or 325 nm. Changes in solar UV irradiances are discussed in the context of long-term ozone and other atmospheric parameters affecting UV variability at ground level.

Total ozone column measurements using an ultraviolet multi-filter radiometer

We have developed and used a method to retrieve total ozone column (TOC), from Ultraviolet Multi-filter Rotating Shadowband Radiometer (UVMFR) measurements in combination with radiative transfer model calculations. Look-up tables of ratios of the direct solar irradiance at (DI) 305 and 325nm in terms of TOC, solar zenith angle, and aerosol optical depth (AOD) have been constructed and compared with TOC retrievals estimated directly from UVMFR irradiance measurements. Sensitivity analysis of the influence of AOD on the calculated TOC has been investigated and found to be 1 Dobson unit per 0.1 change in AOD. We also examined the impact of ozone effective temperature on the TOC retrieval and found that it leads to a 0.9% change in TOC per K. UVMFR direct irradiance measurements in Athens, Greece, during the period July 2009–May 2014 were used to create a time series of high-temporal-frequency measurements (1 min for cloudless conditions) of TOC, which facilitated an analysis of the diurnal variation of TOC. Comparison of the TOC retrievals from the UVMFR with co-located and synchronous daily TOC retrievals from a Brewer MKIV spectrophotometer showed very good agreement (correlation coefficient 0.98). Daily TOC retrievals from the UVMFR were within ±3% compared with the ones measured by the Ozone Monitoring Instrument overpasses on board the Aura satellite.

Manmade Changes in Cirrus Clouds from 1984 to 2007: A Preliminary Study

We analyse cirrus cloud data over congested air traffic corridors during the period 1984–2007 and look into manmade changes in cirrus clouds due to air traffic. The analysis is based on the International Satellite Cloud Climatology Project (ISCCP) D2 data set for the period 1984–2007. Manmade changes in cirrus clouds were determined from the comparison of cirrus clouds over higher traffic regions and over lower traffic regions. These comparisons were done by calculating the differences between adjacent high and low air traffic areas over North America, Europe, North Atlantic and North Pacific. In all cases the differences show a positive trend which is consistent with the increasing trend of global air traffic in the past 20 years. Over North America, Europe, North Atlantic and North Pacific cirrus clouds increased by 0.9 % per decade over high air traffic regions relative to their low traffic counterparts. The result of manmade cirrus increase of 0.9 % over the western air traffic corridors is expected to have a measurable effect in radiative forcing.

Estimation of precipitation change over Greece during the 21st century, using RCM simulations

In the present study, an estimation of the future change in precipitation within the 21st century was carried out for the 12 climatic zones of Greece. The estimation is based on the results of 12 Regional Climate Models (RCM) simulations of the ENSEMBLES Program. The precipitation is projected to decrease mildly in the near future (2021-2050) and moderately in the far future (2071-2100) by 5% and 20% respectively for the area of Greece. The decrease seems to be most prominent during spring and summer, less prominent during winter. The autumn precipitation is projected to increase mildly in the near future and to decrease mildly in the far future. The projected precipitation change in Greece is expected to result in drastic changes on the ground water balance and on several sectors of human activity and environment parameters, as well.