INPUT: aerosol size distribution, aerosol refractive index, spectral surface ... Quantify the non-sphericity of Saharan dust particles and approximate them by.
Radiative transfer simulations within Saharan dust atmospheres compared to measurements from the SAMUM experiment Sebastian Otto1, Eike Bierwirth2, Thomas Trautmann1, Manfred Wendisch2 (1) Remote Sensing Technology Institute, DLR Oberpfaffenhofen, 82234 Wessling (2) Johannes Gutenberg University, 55099 Mainz
Radiative transfer model
Dust size distributions and refractive index
PlanePlane-parallel radiative transfer in a 120 km high model atmosphere within the maximum spectral range of 0.20.2-100 μm Big uncertainties in literature data1 (dotted)
Spectral resolution: resolution: the lineline-byby-line, line, finefine-band and broadbroad-band modes
„dust-like“ from Volz (1972)2 (dashed black)
Large particle fraction
Solution of the radiative transfer for both the solar extraextra-terrestrial light and Planckian emission source term TwoTwo-, fourfour- and multimulti-stream radiative transfer codes Gas absorption using HITRAN database (2004) with Voigt line shape INPUT: aerosol size distribution, distribution, aerosol refractive index, index, spectral surface albedo etc.
SAMUM: size distributions measured over the Sahara on forenoon of 19.5.06 averaged over 3 dust layers
OUTPUT: optical properties, properties, radiance, radiance, irradiance, irradiance, actinic flux density, density, heating rate, atmospheric radiative effect (ARE)
(1: 1.11.1-3.4 km, 2: 3.43.4-5.0 km, 3: 5.05.0-9.3 km)
Moving average of the complex refractive index of mineral dust (thick solid lines) lines) used for both the ACEACE-2 as well as the SAMUM simulations
Results • Application of the model package to ACEACE-2 measured dust size distribution data (8 levels) levels) in coorporation with S. Borrmann and M. de Reus (University of Mainz)3: - Big influence of large dust particles with diameters larger than 4 μm on the optical properties of the dust (e.g. e.g. ωo(0.55 μm) ~ 0.8 within the dust plume at 4 km altitude) altitude) - ARE at TOA of -53 W/m2 over OCEAN (cooling (cooling)) and +107 W/m2 over DESERT (warming (warming)) as the result of the significantly higher surface albedo (see below) below) over desert • Application of the model package to preliminary SAMUM data (dust particle size distributions and surface albedo) albedo) measured on 19.05.06 at 11.10 UTC : - SizeSize- and band averaged single scattering albedo/ albedo/asymmetry parameter (Mie calculations) calculations) at level 1 are significantly influenced by large particles: particles: they decrease/ decrease/increase them in the shortwave (two left figures) ACE-2 results figures) where e.g. e.g. ωo(0.55 μm) ~ 0.78 being comparable to the ACE- First calculations of the dust optical depth show good closure with measured LIDAR optical depth (third figure from the left) left) - Simulation of the downward/ downward/upward shortwave irradiances in comparison to airborne measurements (right figure): figure): Model nicely follows the measured fluxes
shortwave bands (0.2-3.5 μm) ______________________
longwave bands (3.5-100.0 μm) Big contribution of the large particles
Spectral surface albedos
Outlook • Minimize the uncertainties in the complex refractive index of the Saharan dust particles by considering the chemical particle analyses ... Mixed refractive index of the dust as function of chemical composition and wavelength • Quantify the nonnon-sphericity of Saharan dust particles and approximate them by homogeneous spheroids; spheroids; Calculate their optical properties as function of complex refractive index and size parameter to use these data for comprehensive radiative transfer computations • Explore the dust‘ dust‘s radiative effects on the shortwave and longwave radiation transport compared to the SAMUM radiation measurements (closure) closure) • Investigate the influence of the particle nonnon-sphericity on the radiative transport
Surface albedo of Ocean (multiplied (multiplied by 5) and Desert (sand) sand) from literature used for the ACEACE-2 simulations
1
Measured surface albedo on 19.5.06 at 11.10 UTC over the Sahara used for the SAMUM simulations
• Calculate dust optical properties as function of its nonnon-sphericity for mesoscale and global transport and climate modeling • Simulate TOA radiance spectral signatures and compare them with satellite observations (UV/VIS/IR/TIR)
Carlson, T.N. and S.G. Benjamin: Radiative heating rates for Saharan dust, Am. Meteor. Soc., 37, 193-213, 1980. E.M., D.A. Gilette and B.H. Stockton: Complex index of refraction between 300 and 700 nm for Saharan aerosol, J. Geophys. Res., 82, 3153-3160, 1977. I., A. Andronova and T.C. Johnson: Complex refractive index of atmospheric dust aerosols, Atmos. Environ., 27 A, 2495-2502, 1993. Sokolik, I., O.B. Toon and R.W. Bergstrom: Modeling the radiative characteristics of airborne mineral aerosols at infrared wavelengths, J. Geophys. Res., 103 D8, 8813-8826, 1998. 2 Refractive index from Volz (1972) used in Shettle, E.P. and R.W. Fenn: Models of the aerosols of the lower atmosphere and the effects of humidity variations on their optical properties, Project 7670, Air Force Geoph. Lab., Massechusetts, 1979. Volz, F. E.: Infrared absorption by atmospheric aerosol substances, J. Geophys. Res., 77, 1017-1031, 1972. 3 Results submitted to Atmos. Chem. Phys. Discuss., 7, 7767-7817, 2007. 1 Patterson, 1 Sokolik, 1
