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The response of the ozone column across Europe to the extreme 2020 Arctic ozone depletion was examined by analyzing ground-based observations at 38 European stations. The ozone decrease at the northernmost site, Ny-Ålesund (79°N) was about 43% with respect to a climatology of more than 30 years. The magnitude of the decrease declined by about 0.7% deg−1 moving south to reach nearly 15% at 40°N. In addition, it was found that the variations of the ozone column at each of the selected stations in March-May were similar to those observed at Ny-Ålesund but with a delay increasing to about 20 days at mid-latitudes with a gradient of approximately 0.5 days deg−1. The distributions of reconstructed ozone column anomalies over a sector covering a large European area show decreasing ozone that started from the north at the beginning of April 2020 and spread south. Such behavior was shown to be similar to that observed after the Arctic ozone depletion in 2011. Stratospheric dynamical patterns in March–May 2011 and during 2020 suggested that the migration of ozone-poor air masses from polar areas to the south after the vortex breakup caused the observed ozone responses. A brief survey of the ozone mass mixing ratios at three stratospheric levels showed the exceptional strength of the 2020 episode. Despite the stronger and longer-lasting Arctic ozone loss in 2020, the analysis in this work indicates a similar ozone response at latitudes below 50°N to both 2011 and 2020 phenomena. An Unprecedented Arctic Ozone Depletion Event During Spring 2020 and Its Impacts Across Europe

Long-term trends of total column ozone (TCO), assessments of stratospheric ozone recovery, and satellite validation are underpinned by a reliance on daily best representative values from Brewer spectrophotometers and other ground-based ozone instruments. In turn reporting of these daily total column ozone values to the World Ozone and Ultraviolet Radiation Data Centre (WOUDC) has traditionally been predicated upon a simple choice between direct sun (DS) and zenith sky (ZS) observations. For mid- and high-latitude monitoring sites impacted by cloud cover we discuss the potential deficiencies of this approach in terms of its rejection of otherwise valid observations and capability to evenly sample throughout the day. A new methodology is proposed that makes full use of all valid direct sun and zenith sky observations, accounting for unevenly spaced observations and their relative uncertainty, to calculate an improved estimate of the daily mean total column ozone. It is demonstrated that this method can increase the number of contributing observations by a factor of 2.5, increases the sampled time span, and reduces the spread of the representative time by half. The largest improvements in the daily mean estimate are seen on days with the smallest number of contributing direct sun observations. No effect on longer-term trends is detected, though for the sample data analysed we observe a mean increase of 2.8 DU (0.82 %) with respect to the traditional direct sun vs. zenith sky average choice. To complement the new calculation of a best representative value of total column ozone and separate its uncertainty from the spread of observations, we also propose reporting its standard error rather than the standard deviation, together with measures of the full range of values observed.

This study describes a dual-channel array spectrometer system designed to make high-frequency simultaneous spectral global irradiance and direct solar irradiance measurements covering the visible and ultraviolet wavelength ranges. The dual-channel nature of the instrument allows spectrally integrated quantities (e.g., erythema or vitamin D) to be calculated at a rate similar to broadband instruments while retrieving total column ozone (TCO) from the direct solar channel. The characterization and calibration of the instrument is discussed, with emphasis on temperature stabilization (R2 = 0.954 is found for all individual measurements, although this is reduced to 0.908 for daily means. TCO retrievals are limited to airmass values < 4 primarily because of residual structure in the transmission spectrum that cannot be attributed to other trace gases. These results are encouraging and suggest that similar instrument designs could make a significant and relatively low-cost contribution to surface measurements of atmospheric radiation.

Two radiative transfer models are presented that simplify calculations of street canyon spectral irradiances with minimum data input requirements, allowing better assessment of urban exposures than can be provided by standard unobstructed radiation measurements alone. Fast calculations improve the computational performance of radiation models, when numerous repetitions are required in time and location. The core of the models is the calculation of the spectral diffuse-to-global ratios (DGR) from an unobstructed global spectral measurement. The models are based on, and have been tested against, outcomes of the SMARTS2 algorithm (i.e. Simple Model of the Atmospheric Radiative Transfer of Sunshine). The modelled DGRs can then be used to partition global spectral irradiance values into their direct and diffuse components for different solar zenith angles. Finally, the effects of canyon obstructions can be evaluated independently on the direct and diffuse components, which are then recombined to give the total canyon irradiance. The first model allows ozone and aerosol inputs, while the second provides a further simplification, restricted to average ozone and aerosol contents but specifically designed for faster calculations. To assess the effect of obstructions and validate the calculations, a set of experiments with simulated obstructions (simulated canyons) were performed. The greatest source of uncertainty in the simplified calculations is in the treatment of diffuse radiation. The measurement-model agreement is therefore dependent on the region of the sky obscured and ranges from

Aerosols play an important role in attenuating solar radiation reaching the earth's surface and are thus important inputs to climate models. Aerosol optical depth is routinely measured in the visible range but little data in the ultraviolet (UV) are available. In the UV range it can be determined from Langley plots of direct-sun measurements from the Brewer spectrophotometer (where conditions allow) and can also be determined as the residual once the ozone and sulfur dioxide have been accounted for in the extinction observed during a normal Brewer direct-sun measurement. By comparing aerosol optical depth derived from Brewer direct-sun data in both the United Kingdom and Malaysia, two very different locations, it is determined that while most of the existing global Brewer network could contribute to aerosol optical depth data, further analysis, such as calculation of the Ångström parameter, would be dependent on latitude and sky conditions.

In this study we present the UK's total ozone and UV data records, their measurement and preliminary processing, and discuss the observed trends. Notably, we combine the records from the Camborne and Reading sites to obtain a single ozone data series for southern England—a total duration of 30 years. Two-section linear trends and cross-over years are determined for Lerwick and southern England, both annually and seasonally. The observed minimum using this technique in the UK ozone record is found to occur during the year 1993. Significant trends of − 4.8% per decade for southern England and − 5.8% per decade for Lerwick are seen prior to the mid-1990s; the rate of decrease in the UK being at the upper limit of other European stations. No significant ozone trend is seen for the latter period in either location in contrast with Europe as a whole. We, additionally, compare the satellite-derived total ozone trends for both locations. The UV record at Reading showed a significant trend from 1993 to 2008 in the daily maximum UV index record (6.6% per decade), despite no significant recovery in the total ozone data series, suggesting a reduction in cloud cover during the midday period. Filtering data to remove other factors shows the erythemal UV to have an ozone-dependent trend of − 0.11% per year, but this is not statistically significant. We find no correlation between anomalies in the surface UV and total ozone records suggesting that the majority of the inter-annual variability is due to changes in cloud cover and other effects. Copyright © 2011 Royal Meteorological Society

Several UV data products derived from satellite measurements, 1-D and 3-D radiative transfer modeling are compared with high-quality ground-based measurements. Data products include the UV index, erythemally weighted daily dose and spectrally resolved UV irradiances at 305, 310, 324 and 380 nm. The study focuses on the UV radiation climate in mountainous terrain under cloud-free conditions. The results show, that overall the 3-D- and the 1-D-model agree best with the measurements (average ratio 1.10 and 1.13, range 0.88-1.6). It is also found that snow and local topography have a rather minor impact on ground UV-irradiance, while altitude plays a significant role >5 %). Satellite-retrieved values significantly underestimate irradiance for most of our stations due to erroneous cloud correction (average ratio 0.89, range 0.6-1.35). However, if one compares the uncorrected (cloud-free) satellite-retrieved values to the measurements, the ratios are only slightly larger (average ratio 1.14, range 0.8 - 1.6) than for the 1-D- and 3-D-model. The main deficiencies arise in determining the correct surface height and albedo within the satellite-retrieval algorithm.

This paper describes the PROMOTE UV Record, which aims to provide a global long-term record of the surface UV radiation. The algorithm developed takes as input cloud information from the International Satellite Cloud Climatology Project (ISCCP) and a recently developed multisensor assimilated record of the total ozone column. Aerosols and surface albedo are based on climatologies. Here, first validation results of the PROMOTE UV Record are presented through comparison against ground-based measurements of daily erythemal UV doses at eight European stations. The validation shows that the method is working reasonably, although there is a clear tendency toward overestimation. Typically, the median bias as compared to measurements is 3%-10% and 56%-68% of the daily doses are within plusmn20% from the ground-based reference. The prototype version of the PROMOTE UV Record included in this paper covers the period from July 2002 to June 2005. The time series will later be extended to start in 1983.

The Convective Storm Initiation Project (CSIP) took place during the summers of 2004 and 2005, centred on the research radar at Chilbolton, UK. Precursors to convective precipitation were studied, using a comprehensive and broad-based range of fieldwork and modelling. The principal aim of CSIP was the detection of the primary and secondary initiation of convective cells. The Universities Facility for Atmospheric Measurements (UFAM) Cessna 182 was used to map temperature and humidity fields over a broad area within and beyond the Chilbolton radar beam. Additionally, air motion was measured using a new turbulence probe, the AIMMS20AQ. The performance of the probe is critically appraised, based on calibrations, test flights and data flights flown during CSIP intensive operating periods. In general, the probe performed well, although some aspects require more careful data interpretation which we describe in detail.

Solar irradiance spectral measurements were performed during a total solar eclipse. The spectral effect of the limb darkening to the global, direct irradiance and actinic flux measurements was investigated. This effect leads to wavelength dependent changes in the measured solar spectra showing a much more pronounced decrease in the radiation at the lower wavelengths. Radiative transfer model results were used for the computation of a correction for the total ozone measurements due to the limb darkening. This correction was found too small to explain the large decrease in total ozone column derived from the standard Brewer measurements, which is an artifact in the measured irradiance due to the increasing contribution of diffuse radiation against the decreasing direct irradiance caused by the eclipse. Calculations of the Extraterrestrial spectrum and the effective sun's temperatures, as measured from ground based direct irradiance measurements, showed an artificial change in the calculations of both quantities due to the fact that radiation coming from the visible part of the sun during the eclipse phases differs from the black body radiation described by the Planck's law.

A new spectroradiometer for spectral measurements of ultraviolet (UV) atmospheric radiation (290–400 nm) using a charge coupled device (CCD) as a detector is introduced. The instrument development is motivated by the need for measurements with (a) high accuracy in the UV-B spectral range (290–315 nm) for photochemistry applications and (b) high temporal resolution in quickly changing atmospheric conditions such as partial cloud cover. The new CCD instrument is mainly intended for airborne use. It allows fast data collection (

Measuring ultraviolet (UV) radiation is an intrinsic part of undertaking an UV effect study, but the science of UV measurement is not usually the main interest of the plant scientist. Nonetheless, if the UV measurement is not carefully calibrated and characterized then the trial results may be misleading, or unrealistic in terms of what might be expected from ozone depletion. This paper discusses the various methods of measuring UV radiation, and their applicability for field and forest research.

Oversampling of optical array probes (OAPs) is described as a novel technique to increase the quality of small-particle data available in cloud microphysics. The slice rate of a 10-μm-resolution grayscale OAP is increased by a factor f with respect to that which would produce images with an aspect ratio of 1:1 for the given air velocity and probe resolution. The Fresnel diffraction pattern is simulated and the probe response calculated for normal (f = 1) and oversampled (f = 10) cases. The increased number of pixels imaged by the OAP permits the fraction of 25%–50% shadowed pixels to be used to obtain a much improved estimate of the actual droplet diameter for droplets between 7 and 50 μm in diameter. The process of oversampling is also found to significantly increase the sample area for the smallest particles. Example laboratory particle size distributions are provided. The oversampling factor for an individual probe is limited by the maximum sampling frequency, the airspeed, and the probe resol...

Routinely monitored radiation parameters, including those at UV wavelengths, most commonly refer to irradiance, that is the radiation incident on a flat horizontal plate. However, in the study of atmospheric chemistry, where UV radiation is a prime photochemical driver, the target molecules are approximately spherical and the actinic flux (radiation on the surface of a sphere) is a better measure of the effective radiation. Unfortunately actinic flux measurements (also known as scalar irradiance) are uncommon research measurements. The ability to convert irradiance data to actinic flux data within a reasonable degree of uncertainty would provide an actinic flux database mapped from existing irradiance databases, thus vastly increasing knowledge of actinic flux variability and climatology. Synchronised spectral UV irradiance and actinic flux measurements have been made during the ADMIRA project, and used to develop an empirical method for converting irradiance to actinic flux. With some prior knowledge of the sky conditions during the irradiance measurements the actinic flux can be estimated to within a few percent. If no knowledge of the sky conditions is available then the empirical method still returns actinic fluxes within 10% of those measured at the site for which the conversion was developed.

Results are presented from the Actinic Flux Determination from Measurements of Irradiance (ADMIRA) campaign to measure spectral global UV irradiance and actinic flux at the ground, beneath an atmosphere well defined by supporting measurements. Actinic flux is required to calculate photolysis rates for atmospheric chemistry, yet most spectral UV measurements are of irradiance. This work represents the first part of a project to provide algorithms for converting irradiances to actinic fluxes with specified uncertainties. The campaign took place in northern Greece in August 2000 and provided an intercomparison of UV spectroradiometers measuring different radiation parameters, as well as a comprehensive radiation and atmospheric dataset. The independently calibrated spectroradiometers measuring irradiance and actinic flux agreed to within 5%, while measurements of spectral direct irradiance differed by 9%. Relative agreement for all parameters proved to be very stable during the campaign. A polarizat...

Sky radiance distributions pertaining to surface UV have been investigated under clear-sky and broken cloud conditions. Comparison between the measured clear-sky distribution and either a radiative transfer model or an empirical function showed reasonable agreement in each case. The radiance distribution measurements performed during broken cloud conditions were then investigated and compared to the clear-sky radiance. Radiances from cloudy sky locations were either higher (by up to 2.5 times) or lower (with minimum values that were 8% of the clear-sky radiances) than the clear-sky case. It has been shown that clouds have an enhancing effect on the ground UV radiance at locations with scattering angles between 30° and 60° and a sight zenith angle less than 30°, or with scattering angles greater than 60° irrespective of the sight zenith angle. These results represent an advance toward the accurate description and modeling of ground UV under broken cloud conditions.

Spectral, ultraviolet irradiance measurements have been made since 1990 at Reading (England). Ancillary measurements using a photodiode show that UV measurements are frequently affected by clouds. Consequently, it is not practical to simply use irradiance measurements to study possible ozone-related changes in UV-B over the timescale of the measurements. However, taking ratios of irradiances in bands covering UV-B and UV-A wavelengths can be used to reduce the cloud influence on measurements. Using these ratios and ozone vertical column measurements, the anticorrelation between ozone and UV-B irradiances measured at the ground is demonstrated. These data also show that between 1993 and 1997, UV-B at Reading has increased by 4.3%, compared to a decrease in ozone of 5.9%. The UV-B increase is not a statistically significant trend, however, and serves only as an indication of the changes observed.

Spectral reflectivities at ultraviolet and visible wavelengths (290-500 nm) have been measured from an airborne platform (Cessna light aircraft) at several heights in the boundary layer, and at ground level, using an Optronic Laboratories 742 spectroradiometer with specially adapted input optics. Ground level measurements of vegetation and man-made surfaces show a general increase in reflectivity with wavelength. Airborne measurements over vegetation and ocean (from heights of 700-5500 ft) show an increase in effective reflectivity with the height at which measurements were made and also a reversal in spectral character, so reflectivity decreases with increasing wavelength at height. This suggests significant backscattering from the boundary layer air which rapidly dominates the surface influence for low-albedo surfaces.

We present a unique case study of the solar global irradiance in a highly heterogeneous albedo environment at the arctic coast. Diodearray spectroradiometers were deployed at three sites around Ny Ålesund, Svalbard, and spectral irradiances were simultaneously measured under clear sky conditions during a 24 h period. The 3-D radiative transfer model MYSTIC is applied to simulate the measurements in various model scenarios. First, we model the effective albedos of ocean and snow and consequently around each measurement site. The effective albedos at 340 nm increase from 0.57 to 0.75, from the coastal site in the west towards the site 20 km east, away from the coast. The observed ratios of the global irradiance indicate a 15% higher average irradiance at east relative to west at 340 nm due to the higher albedo. The comparison to our model scenarios suggest a snow albedo of >0.9 and confirm the observation that drift ice has moved into the Fjord during the day. The local time shift between the locations causes a hysteresis-like behavior of these east-west ratios with solar zenith angle (SZA). The observed hysteresis, however, is larger and, at 340 nm, can be explained by the drift ice. At 500 nm, a plausible explanation is a detector tilt of about 1°. The ratios between afternoon and morning irradiances at the same SZA are investigated, which confirm the above conclusions. At the coastal site, the measured irradiance is significantly higher in the afternoon than in the morning. Besides the effect of changing drift ice and detector tilt, the small variations of the aerosol optical depth have to be considered also at the other stations to reduce the discrepancies between model and observations. Remaining discrepancies are possibly due to high thin clouds.

International audience; The strong ozone depletion event that occurred in Arctic during spring 2011 was found to cause appreciable reduction in the ozone column (OC) in Europe, even at lower latitudes. The features of this episode have been analysed using the data recorded at 34 ground-based stations located in the European area and compared with the similar events in 2000 and 2005. The results provided evidence that OC as far south as 40°N latitude was considerably influenced by the Arctic ozone loss in spring 2011. The reduction of OC at the northernmost sites was about 40% with respect to the mean value calculated over the previous six-year period, while a similar decrease at the southern extreme ranged between 15 and 18%, and were delayed by nearly two weeks compared to the Arctic region. The ozone distributions reconstructed for the West Europe sector show that the decline of OC lasted from late March to late April 2011. The echo of the Arctic ozone depletion on mid-latitude UV irradiance has been analysed trough model computations that show an increase of the midday erythemal dose by 3-4 SED (1 SED = 100 J m-2) that was slightly higher than at polar regions. On the other hand it was assessed that the biosystems in the northernmost regions were a subject of about 4 times higher UV stress than those at mid-latitudes. Despite indications of an OC recovery, the event examined here shows that the issue of ozone depletion episodes cannot be belittled.

Even in idealized, clear sky conditions, spectral UV models need well-specified input parameters such as solar zenith angle, pressure, ozone and aerosol optical depth to produce realistic results. UV models based on exact analytical methods are widely considered to be absolutely accurate with errors in results widely attributed to inaccurate input parameters. The uncertainties in the standard determinations of each input parameter were determined for the following methods of specifying each parameter: (1) ozone from satellites or ground-based Dobson spectrometer measurements; (2) aerosol optical depth from sunphotometer measurements; (3) single-scattering albedo; three different procedures using ground-based spectrometer measurements and radiative transfer calculations were studied; (4) ground albedo; the determination of the ground albedo is achieved using satellite measurements, which have a resolution of some 100 km; ground-based measurements may give some information about the albedo of the ground, but the albedo needed for radiative transfer calculations is a weighted average of the surrounding albedos. The resulting inaccuracies in UV calculations from errors in input parameters were simulated with the discrete ordinate method model by Stamnes et al. [1988] using the uncertainties specified above for the different input parameters. The simulations of the errors of the spectral UV calculations were carried out for each input parameter at 0°, 50°, and 80° zenith angle and analyzed at 305 and 380 nm. The overall maximum error may be determined by combining the different errors.

In this work, spectral UV calculations based on the discrete ordinate model by Stamnes et al. [1988] are compared with measurements from two different campaigns in Panorama (Greece) and on Jungfraujoch (Switzerland). Ozone input data were taken from Brewer or spectrophotometer measurements; spectral aerosol optical depths were measured by sunphotomelers and by direct spectral irradiance measurements. The single-scattering albedo(SSA) and the reflectance were inverted using the procedure by King and Herman [1979]. The cosine error for global irradiance is taken into account and a correction is applied to the data if needed. We analyze the uncertainty interval of calculations as well as the uncertainty interval of measurements. We find deviations of the UV calculations around ±10% or slightly more in Panorama and ±5% to 10% at Jungfraujoch. Taking into consideration the uncertainty interval of the measurements, there is a large probability of reaching agreement between calculations and measurements. Comparisons of the UV calculations performed in this study with calculations performed with SSA and asymmetry model input parameters taken from aerosol climatology by d'Almeida and from IAMAP show that the calculated UV irradiances differ by up to 14% between model runs.

The public health implications of a warming urban environment mean that appropriate action by planners, designers and health workers will be necessary to minimise risk under future climate scenarios. Data at an appropriate spatial scale are required by user groups in order to identify key areas of vulnerability. Thermal mapping of a UK urban conurbation was carried out during the summers of 2007 and 2008 with the aim of providing high spatial resolution temperature data. The air temperature results showed an average daytime (night time) urban–rural thermal contrast of 3°C (5°C) on summer days (nights) with ideal urban heat island (UHI) conditions. The intensity of the daytime surface temperature heat island was found to exceed 10°C. The measured data were used to derive an empirical model of spatial temperature patterns based upon characteristics of land use, distance from urban centre and building geometry. This model can be used to provide sub-kilometre resolution temperature data which are required by decision makers and can provide a mechanism for downscaling climate model output.

During three field campaigns spectral actinic flux was measured from 290–500 nm under clear sky conditions in Alpine terrain and the associated O3- and NO2-photolysis frequencies were calculated and the measurement products were then compared with 1-D- and 3-D-model calculations. To do this 3-D-radiative transfer model was adapted for actinic flux calculations in mountainous terrain and the maps of the actinic flux field at the surface, calculated with the 3-D-radiative transfer model, are given. The differences between the 3-D- and 1-D-model results for selected days during the campaigns are shown, together with the ratios of the modeled actinic flux values to the measurements. In many cases the 1-D-model overestimates actinic flux by more than the measurement uncertainty of 10%. The results of using a 3-D-model generally show significantly lower values, and can underestimate the actinic flux by up to 30%. This case study attempts to quantify the impact of snow cover in combination with topography on spectral actinic flux. The impact of snow cover on the actinic flux was ~ 25% in narrow snow covered valleys, but for snow free areas there were no significant changes due snow cover in the surrounding area and it is found that the effect snow-cover at distances over 5 km from the point of interest was below 5%. Overall the 3-D-model can calculate actinic flux to the same accuracy as the 1-D-model for single points, but gives a much more realistic view of the surface actinic flux field in mountains as topography and obstruction of the horizon are taken into account., Atmospheric Research, 102 (3), ISSN:0169-8095, ISSN:1873-2895

[1] The Ozone Monitoring Instrument (OMI), on board the NASA EOS Aura spacecraft since July 2004, provides a global view of surface spectral ultraviolet (UV) irradiance at 305, 310, 324, and 380 nm; erythemal dose rate both at overpass time and local noontime; and erythemal daily dose. Previous studies have shown comparisons of the OMI erythemal UV irradiances and ground-based UV measurements in areas of midlatitude and high latitudes, predominantly in the Northern Hemisphere. In this study the noontime erythemal UV dose rates retrieved from OMI and measured from broadband instruments at four sites in Thailand were compared. The comparisons show a positive bias for the OMI data with respect to the ground-based measurements. The differences between the two data sets were 30%–60% for all data and were 10%–40% for cloudless data. The differences for the cleanest site showed better agreement than those for the more urban sites. Using the Libradtran radiative transfer model, we show that aerosol is responsible for much of the positive bias in polluted areas. Since absorbing aerosol is not taken into account in the OMI surface UV algorithm, aerosol absorption correction factors have been introduced as a function of aerosol absorption optical thickness provided by OMI to improve the OMI UV data for urban and maritime sites. The differences between the corrected erythemal UV data and the ground-based data were reduced to less than 20%.

In the last few years diode array spectroradiometers have become useful complements to traditional scanning instruments when measuring visible and ultraviolet solar radiation incident on the ground. This study describes the application of such an instrument to the problem of measuring the radiation scattered by different cloud-types in a laboratory environment. Details of how the instrument is incorporated into the experimental set-up are given together with the development of the system as a whole. The capability to measure a full spectrum for each scattering angle is an undoubted advantage, although the limited sensitivity impacts on the usefulness for optically thin clouds. Nevertheless example results are presented: (1) scattering phase functions at a range of wavelengths recorded simultaneously for water clouds, showing spectral deviation at the rainbow angle and verification of Mie theory; (2) likewise for mixed phase clouds, with evidence of both halo and rainbow features in a single scattering function; and, (3) detail of the forward scattering region in a glaciated cloud showing a barely perceptible halo feature, with implications for the small-scale structure of the ice crystals produced.

Ultraviolet radiation is the key factor driving tropospheric photochemistry. It is strongly modulated by clouds and aerosols. A quantitative understanding of the radiation field and its effect on photochemistry is thus only possible with a detailed knowledge of the interaction between clouds and radiation. The overall objective of the project INSPECTRO was the characterization of the three-dimensional actinic radiation field under cloudy conditions. This was achieved during two measurement campaigns in Norfolk (East Anglia, UK) and Lower Bavaria (Germany) combining space-based, aircraft and ground-based measurements as well as simulations with the one-dimensional radiation transfer model UVSPEC and the three-dimensional radiation transfer model MYSTIC. During both campaigns the spectral actinic flux density was measured at several locations at ground level and in the air by up to four different aircraft. This allows the comparison of measured and simulated actinic radiation profiles. In addition satellite data were used to complete the information of the three dimensional input data set for the simulation. A three-dimensional simulation of actinic flux density data under cloudy sky conditions requires a realistic simulation of the cloud field to be used as an input for the 3-D radiation transfer model calculations. Two different approaches were applied, to derive high- and low-resolution data sets, with a grid resolution of about 100 m and 1 km, respectively. The results of the measured and simulated radiation profiles as well as the results of the ground based measurements are presented in terms of photolysis rate profiles for ozone and nitrogen dioxide. During both campaigns all spectroradiometer systems agreed within ±10% if mandatory corrections e.g. stray light correction were applied. Stability changes of the systems were below 5% over the 4 week campaign periods and negligible over a few days. The J(O1D) data of the single monochromator systems can be evaluated for zenith angles less than 70°, which was satisfied by nearly all airborne measurements during both campaigns. The comparison of the airborne measurements with corresponding simulations is presented for the total, downward and upward flux during selected clear sky periods of both campaigns. The compliance between the measured (from three aircraft) and simulated downward and total flux profiles lies in the range of ±15%. © Author(s) 2008. This work is distributed under the Creative Commons Attribution 3.0 License.

Copyright @ 2007 AMS The Convective Storm Initiation Project (CSIP) is an international project to understand precisely where, when, and how convective clouds form and develop into showers in the mainly maritime environment of southern England. A major aim of CSIP is to compare the results of the very high resolution Met Office weather forecasting model with detailed observations of the early stages of convective clouds and to use the newly gained understanding to improve the predictions of the model. A large array of ground-based instruments plus two instrumented aircraft, from the U.K. National Centre for Atmospheric Science (NCAS) and the German Institute for Meteorology and Climate Research (IMK), Karlsruhe, were deployed in southern England, over an area centered on the meteorological radars at Chilbolton, during the summers of 2004 and 2005. In addition to a variety of ground-based remote-sensing instruments, numerous rawin-sondes were released at one- to two-hourly intervals from six closely spaced sites. The Met Office weather radar network and Meteosat satellite imagery were used to provide context for the observations made by the instruments deployed during CSIP. This article presents an overview of the CSIP field campaign and examples from CSIP of the types of convective initiation phenomena that are typical in the United Kingdom. It shows the way in which certain kinds of observational data are able to reveal these phenomena and gives an explanation of how the analyses of data from the field campaign will be used in the development of an improved very high resolution NWP model for operational use. This work is funded by the National Environment Research Council following an initial award from the HEFCE Joint Infrastructure Fund.

[1] Solar UV radiation plays a very significant role in terms of atmospheric chemistry. The UV radiation that reaches the troposphere drives the photodissociation of tropospheric species whose photolysis energies fall in the range of these wavelengths. The relevant measure of radiation for atmospheric chemistry is actinic flux, but the great majority of UV measurements, not common in themselves, are UV irradiance ( radiation on a flat surface). Spectral UV irradiance was measured in Manchester, UK ( 78 m altitude, 53.28 degreesN, 2.14 degreesW) using a moderate bandwidth filter radiometer (GUV-541) that has five channels in the UV region with center wavelengths at 305, 313, 320, 340, and 380 nm and band-pass functions of approximately 10 nm. This study describes a new method of deriving the photolysis rate of O-3, NO2, and CH2O utilizing the GUV irradiance measurements. The measured irradiance was compared with independently measured photolysis rates determined from the actinic flux measured by a Bentham spectroradiometer on a clear sky day. On the reference day, a linear regression of GUV global irradiance to simultaneous photolysis rate measurements was performed to generate scale factors for each optical channel. The results compare favorably with the general uncertainty in photolysis measurements of about 20%. A further separate comparison is made with comparable filter radiometers.

[1] Spectral albedo measurements have been made by three airborne systems (two aircraft and one balloon) over the flat arable land of East Anglia, UK, and above clouds. The spectral albedos measured at the various flight altitudes include the backscattering of the atmosphere beneath the aircraft/balloon, which has to be removed to get the areal spectral surface albedo. Data derived from the airborne spectroradiometers showed the areal surface albedo to be about 0.02 in the UVB (wavelength range between 280–320 nm), increasing with wavelength to about 0.05 at 500 nm. The cloud albedos were measured above two different stratus layers, one completely overcast and one broken. Above the overcast cloud sheet the albedo was independent of wavelength with values between 0.62 and 0.85 depending on position above cloud and altitude. Above the broken cloud layer the averaged albedo spectra showed clear spectral reflection features from the underlying surface mixed with the cloud albedo signatures.

[1] The actinic flux describes the number of photons incident at a point, while the irradiance describes the radiant energy crossing a surface. An algorithm for conversion of irradiance to downwelling actinic flux has been developed and tested. The algorithm uses a simple method to distinguish between a cloudy and a cloudless sky. It is necessary to separate cloudy and cloudless situations, as the irradiance to actinic flux conversion depends on the radiance field which is rather different for cloudy and cloudless skies. Surface UV irradiance and downwelling actinic flux spectra were measured at five European locations which were representative of different climates. A total of 9015 synchronized actinic flux and irradiance spectra were available to test the proposed algorithm. The measured irradiance spectra were used to estimate downwelling actinic flux spectra. The estimated actinic flux spectra were compared with the measured actinic flux spectra for all cloud and aerosol situations encountered, a wide range of solar zenith angles, and surface conditions. The average ratio of the reproduced to measured downwelling actinic flux is 1.021 ± 0.085 in the UV-B and 1.015 ± 0.105 in the UV-A. In general, the performance of the algorithm is better at smaller solar zenith angles.

In September 2002, the first INSPECTRO campaign to study the influence of clouds on the spectral actinic flux in the lower troposphere was carried out in East Anglia, England. Measurements of the actinic flux, the irradiance and aerosol and cloud properties were made from four ground stations and by aircraft. The radiation measurements were modelled using the uvspec model and ancillary data. For cloudless conditions, the measurements of the actinic flux were reproduced by 1-D radiative transfer modelling within the measurement and model uncertainties of about ±10%. For overcast days, the ground-based and aircraft radiation measurements and the cloud microphysical property measurements are consistent within the framework of 1-D radiative transfer and within experimental uncertainties. Furthermore, the actinic flux is increased by between 60-100% above the cloud when compared to a cloudless sky, with the largest increase for the optically thickest cloud. Correspondingly, the below cloud actinic flux is decreased by about 55-65%. Just below the cloud top, the downwelling actinic flux has a maximum that is seen in both the measurements and the model results. For broken clouds the traditional cloud fraction approximation is not able to simultaneously reproduce the measured above-cloud enhancement and below-cloud reduction in the actinic flux. © Author(s) 2005. This work is licensed under the Creative Commons Attribution-NonCommercial-ShareAlike 2.5 License.