Influence of clouds on the spectral actinic flux density in the lower troposphere (INSPECTRO): overview of the field campaigns

Abstract

Abstract. Ultraviolet radiation is the key factor driving tropospheric photochemistry. It is strongly modulated by clouds and aerosols. A quantitative understanding of the raCorrespondence to: S. Thiel (stephan.thiel@imk.fzk.de) diation 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 threedimensional actinic radiation field under cloudy conditions. This was achieved during two measurement campaigns in Norfolk (East Anglia, UK) and Lower Bavaria (Germany)Published by Copernicus Publications on behalf of the European Geosciences Union. 1790 S. Thiel et al.: Influence of clouds on the spectral actinic flux density 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(O 1 D) 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%.

Characterisation and improvement of <i>j</i>(O<sup>1</sup>D) filter radiometers

“…More recently also singlemonochromator-based spectroradiometers that utilise photodiode arrays (PDAs) or charge coupled devices (CCDs) were employed for the measurement of photolysis frequencies. However, for j (O 1 D) in particular and UV-B measurements in general, these instruments require special care with regard to stray light correction in both laboratory calibrations and field measurements (Edwards and Monks, 2003;Kanaya et al, 2003;Hofzumahaus et al, 2004;Jäkel et al, 2006Jäkel et al, , 2007Thiel et al, 2008).…”

The influence of the spatial resolution of topographic input data on the accuracy of 3-D UV actinic flux and irradiance calculations

“…So far only a limited number of studies report on the direct measurement of actinic flux (e.g., Nader and White, 1969;Van Weele et al, 1995;Kazadzis et al, 2000;McKenzie et al, 2001;Webb et al, 2002;Kylling et al, 2005;Thiel et al, 2008;Wagner et al, 2011). More recently numerical modeling studies, with 1-D and/or 3-D radiative transfer models, of actinic flux have been performed (e.g., Thiel et al, 2008;Wagner et al, 2011), showing reasonable agreement with the measurements, especially under clear-sky conditions (agreement within 10 %). However, modeling of UV spectral actinic flux in mountainous areas, especially in the presence of snow, is complex and still poses a scientific challenge.…”

Influence of spatial heterogeneity of local surface albedo on the area-averaged surface albedo retrieved from airborne irradiance measurements

“…Airborne radiation measurements were collected during the INSPECTRO (INfluence of clouds on SPECtral actinic flux in the lower TROposphere) campaign in East Anglia, UK, in September 2002 Thiel et al, 2008). In particular, data measured in cloudless conditions on 12 September 2002 are used in this paper.…”

“…In particular, data measured in cloudless conditions on 12 September 2002 are used in this paper. Flights were made over sea and land surfaces (predominantly agriculture, villages, and occasional areas of woodland) in a triangular horizontal flight pattern within an area of 20 × 20 km 2 at an altitude of z flight = 500 m. The input parameters for the corresponding extrapolation method are taken from previous IN-SPECTRO publications Thiel et al, 2008): the AOD is approximated by using theÅngstrøm formula AOD = β · λ −α and respectiveÅngstrøm coefficients α = 1.3, and β = 0.044. This gives an AOD = 0.1 at 532 nm wavelength.…”

“…(12) is applied to airborne surface albedo measurements performed during the INSPEC-TRO campaign (Thiel et al, 2008). The example flight track contained an overpass over a mixture of small settlements and agriculture land with variable vegetation and the North Sea.…”