Experimental and modelled characterization of diffuse spectral UV irradiance under cloudy conditions: impact of aerosol properties D. Mateos1, A. di Sarra2, J. Bilbao1, M. Cacciani3, G. Casasanta3, D. Meloni2, G. Pace2, and A. de Miguel1 1
Atmosphere and Energy Lab, University of Valladolid, Valladolid, 47005, Spain 2 UTMEA-TER, ENEA, Rome, 00123, Italy 3 Physics Department, University of Rome Sapienza, Rome, 00185, Italy Keywords: radiative properties, cloud microphysics, aerosol optics, cloudiness. Presenting author email:
[email protected]
1 CMF
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LWP = 0.08 kg m-2 reff = 4 μm
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LWP = 0.36 kg m-2 reff = 10 μm
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320 340 360 380 Wavelength (nm) Figure 1. Spectral total and diffuse CMF at SZA=18º for overcast conditions with different LWP and reff.
A specific case was analyzed with the aim of understanding the aerosol impact on total and diffuse CMF comparing measurements and model simulations. The case study case presents an aerosol layer below the cloud during a desert dust event. The COT was 30, and the aerosol optical depth at 550 nm was 0.2. The radiative transfer model libRadtran was initialized with all the available measurements. The model reproduced both the experimental total and diffuse CMF within ±5% at all wavelengths. Varying the aerosol optical properties (single scatting albedo and asymmetry factor) did not significantly affect the total CMF, while did not allow the model to reproduce diffuse CMF within the same uncertainty range. Figure 2 shows the ratio of the spectral modelled and measured diffuse CMF as a function of the aerosol type. This study highlights the need of an adequate characterization of the aerosol optical properties under cloudy conditions, if the effects on the diffuse component have to be analyzed. CMFmodel / CMFexperimental
Measurements of total and diffuse spectral UV irradiance, cloud optical thickness (COT), liquid water path (LWP), cloud effective radius (reff), cloud base and top heights, columnar ozone and spectral aerosol optical depth are used, together with radiative transfer simulations, to evaluate the spectral cloud transmittance or cloud modification factor (CMF) for the total and for the diffuse components under overcast conditions. The observations were carried out during a 2-month measurement campaign held in May and June 2010 at the ENEA-Trisaia Research Centre (40.16ºN, 16.64ºE, 40 m a.s.l.), in Southern Italy. The total and diffuse UV irradiances were measured by an Ultraviolet Multifilter Rotating Shadowband Radiometer (UV-MFRSR) at seven narrowband channels. The HATPRO (Humidity And Temperature PROfiler) microwave radiometer allowed to derive the cloud LWP. With the spectral irradiance measurements by a visible MFRSR, COT and reff are derived, using the cloud LWP (Min and Harrison, 1996). Cloud top and base heights were measured by a LIDAR. The spectral cloud transmittances (for the total and the diffuse components) of two cases with overcast conditions at the same solar zenith angle (SZA) are plotted in Figure 1. The two cases only differ in the cloud properties, i.e., the COT, LWP and reff values. Previous studies (e.g., Kylling et al., 1997; Mateos et al., 2011) showed that the total CMF decreases with wavelength above 315-320 nm and increases at shorter wavelengths (280-315 nm). These results for the total component were confirmed by the measurements of the campaign (see solid lines of figure 1). Conversely, the CMF of the UV diffuse irradiance component increases with wavelength throughout the spectral interval (the CMF depends also on SZA, not shown here), significantly different from the total CMF. 1.2
1.4 1.2
Desert Urban Maritime clean Continental clean
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300 320 340 360 380 Wavelength (nm) Figure 2. Ratio between modelled and experimental spectral diffuse CMF for different aerosol types.
This work was supported by the Spanish Government under the Projects CGL2010-12140E and CGL2011-25363 and the Italian Ministry for Environment through the MINNI Project. Kylling, A., Albold, A., Seckmeyer, G. (1997) Geophys. Res. Lett. 24, 397–400. Mateos, D., di Sarra, A., Meloni, D., di Biagio, C., Sferlazzo, D.M. (2011) J. Atmos. Sol.-Terr. Phy. 73, 1739–1746, doi:10.1016/j.jastp.2011.04.003. Min, Q., Harrison, L.C. (1996) Geophys. Res. Lett. 23, 1641-1644.
