Effect of aerosols on the downward shortwave irradiances at the surface: Measurements versus calculations with MODTRAN4.1

Abstract

[1] A detailed analysis of measurements and model calculations of clear-sky shortwave irradiances at the surface is presented for a set of 18 cases collected during 3 cloudless days in the Netherlands in 2000. The analysis is focused on the influence of the optical and physical properties of aerosols on simulations of direct and diffuse downward solar irradiance at the surface. The properties of aerosols in the boundary layer are derived from surface measurements, under the assumption that all aerosol is confined to a well-mixed atmospheric boundary layer. The simulations of the irradiances are performed with the radiative transfer model MODTRAN 4, version 1.1. The analysis reveals no discernable differences between model and measurement for the direct irradiance, but several significant differences for the diffuse irradiance. The model always overestimates the diffuse irradiance measurements by 7 to 44 Wm À2 (average: 25 Wm À2). On the basis of an estimated uncertainty in the differences of 18 Wm À2 , it appears that for 13 out of 18 cases the model significantly overestimates the measurements. This number decreases if instrumental errors (e.g., pyranometer zero-offset) and assumptions on the model input (e.g., wavelength-independent surface albedo) are considered. Nevertheless, the analysis presented here points to a persistent and significant positive model-measurement difference for the diffuse irradiance, which typically amounts to 1-4% of the top-ofatmosphere irradiance, and does not depend on the solar zenith angle. The reason for the discrepancy may be found in the presence of ultrafine absorbing aerosol particles that were not detected by the surface instrument for measuring aerosol absorption. It is also possible that these particles are not present near the surface, due to dry deposition, but do contribute to the total extinction if they are situated higher up in the boundary layer.

Surface shortwave aerosol radiative forcing during the Atmospheric Radiation Measurement Mobile Facility deployment in Niamey, Niger

“…These flux differences are consistent with values seen in previous aerosol radiative flux closure studies [ Halthore and Schwartz , 2000; Henzing et al , 2004], despite the considerably larger aerosol optical depths seen at Niamey. A more recent study, performed during a field experiment at the ARM Southern Great Plains (SGP) site in Oklahoma found smaller flux differences, with instantaneous biases in direct flux less than 1% and diffuse flux less than 2% [ Michalsky et al , 2006].…”

Complex experiment on studying the microphysical, chemical, and optical properties of aerosol particles and estimating the contribution of atmospheric aerosol-to-earth radiation budget

“…The analysis also shows that the average differences between empirical data and model calculations are close to results obtained by other authors in other regions (Kato et al, 1997;Henzing et al, 2004;Michalsky et al, 2006;Nowak et al, 2008;Wang et al, 2009;Halthore et al, 1998Halthore et al, , 2004Plakhina et al, 1998;Chubarova et al, 1999). It is important that our results were obtained under the conditions of high and moderate atmospheric transmittance, where the optical depth, scattering phase function, and aerosol single scattering albedo errors increase significantly.…”

On the application of the MODTRAN4 atmospheric radiative transfer code to optical remote sensing

“…Examples of applications using MODTRAN4 are mostly in the field of atmospheric correction (Staenz and Williams 1997;AdlerGolden et al 1999;Richter and Schlaepfer 2002;Miller 2002;Miesch et al 2005;Guanter et al 2007), but there are also codes for sensor simulation (Bö rner et al 2001), generation of realistic scenarios Bach 2003, 2007), or assessment of instrument's calibration (Green 1998;Green et al 2003;Guanter et al 2006) using MODTRAN4 as well. The topic of the simulation of atmospheric radiative transfer is also a key tool in the field of climate research (Henzing et al 2004;Ogunjobi and Kim 2007;Kondratyev and Varotsos 1995).…”