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Ann R. Webb

Abstract

Spectral measurements of the ultraviolet region of the solar spectrum have been made at Reading, southeast England (51.5°N) since July 1989. The data presented here show the daily and annual variability of and within the ultraviolet-B wave band, and its relation to total solar radiation.

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Andrew R. D. Smedley, Richard C. Kift, and Ann R. Webb

Abstract

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 (<±0.01°C) and stray light removal. Focusing on the TCO retrieval from direct spectra, results are compared to a collocated Brewer spectrophotometer during the study period of May 2013–January 2014. Agreement for individual measurements made within 20 min of a reference Brewer direct sun observation on relatively clear example days is <1.5%. For all valid individual measurements, the study found an overall bias of 1.1 Dobson units (DU; 0.4%) and scatter of ±6.7 DU (2.2%) for retrievals obtained at airmass values < 4. A dependence on air mass of 6.3 DU (2.0%) per airmass unit is observed and a correlation of R 2 = 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.

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Wilawan Kumharn, John S. Rimmer, Andrew R. D. Smedley, Toh Ying Ying, and Ann R. Webb

Abstract

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.

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Evelyn Jäkel, Manfred Wendisch, Mario Blumthaler, Rainer Schmitt, and Ann R. Webb

Abstract

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 (<300 ms time resolution for each spectrum) with improved sensitivity in the UV spectral range. The instrumental setup and its characterization in terms of stray light, dark current, noise, and detection limits are described and compared to a spectroradiometer with a photodiode array (PDA) detector. The new CCD spectroradiometer has a one order of magnitude greater sensitivity than the PDA-based spectroradiometer. However, the stray light of the CCD instrument is wavelength dependent, which requires a more complicated data evaluation procedure than the PDA instrument. Comparison with other UV spectroradiometers (a PDA spectroradiometer and two ground-based double monochromators) shows the advantages of the CCD system for UV-B measurements of actinic flux densities and photolysis frequencies of ozone and nitrogen dioxide, and the improved performance compared to PDA spectroradiometers.

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Piers M. De F. Forster, Keith P. Shine, and Ann R. Webb

Abstract

High-resolution measurements in the spectral region of 280–400 nm using a double monochromator are compared with detailed radiative transfer calculations at Reading, United Kingdom (52°N, 0°), for clear and totally overcast days, using aerosol and cloud information deduced from empirical methods. For clear skies, instrument and model agree well in the UVA (320–400 nm), but agreement is worse in the UVB (280–320 nm). A number of possible reasons for the discrepancies are explored. Volcanic aerosols in the stratosphere of the model are found to improve agreement between the model and the instrument for high solar zenith angles by increasing the model UVB irradiances by as much as 6%. Convolving the model surface irradiances with the bandpass of the instrument leads to smaller differences between instrument and model at short wavelengths and also reduces the noisiness of the difference. When the model included stratospheric aerosol and the instrument's bandpass function, UVB irradiances within 10% of the measured irradiances could be produced by the model for clear skies. For cloudy conditions, differences between instrument and model are larger, reaching 20%, integrated over the UVB.

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Keith A. Browning, Alan M. Blyth, Peter A. Clark, Ulrich Corsmeier, Cyril J. Morcrette, Judith L. Agnew, Sue P. Ballard, Dave Bamber, Christian Barthlott, Lindsay J. Bennett, Karl M. Beswick, Mark Bitter, Karen E. Bozier, Barbara J. Brooks, Chris G. Collier, Fay Davies, Bernhard Deny, Mark A. Dixon, Thomas Feuerle, Richard M. Forbes, Catherine Gaffard, Malcolm D. Gray, Rolf Hankers, Tim J. Hewison, Norbert Kalthoff, Samiro Khodayar, Martin Kohler, Christoph Kottmeier, Stephan Kraut, Michael Kunz, Darcy N. Ladd, Humphrey W. Lean, Jürgen Lenfant, Zhihong Li, John Marsham, James McGregor, Stephan D. Mobbs, John Nicol, Emily Norton, Douglas J. Parker, Felicity Perry, Markus Ramatschi, Hugo M. A. Ricketts, Nigel M. Roberts, Andrew Russell, Helmut Schulz, Elizabeth C. Slack, Geraint Vaughan, Joe Waight, David P. Wareing, Robert J. Watson, Ann R. Webb, and Andreas Wieser

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 rawinsondes 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.

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