scholarly journals Particle shape and internal inhomogeneity effects on the optical properties of tropospheric aerosols of relevance to climate forcing

1998 ◽  
Vol 103 (D4) ◽  
pp. 3789-3800 ◽  
Author(s):  
Christodoulos Pilinis ◽  
Xu Li
Keyword(s):  
Optical Properties ◽  
Particle Shape ◽  
Climate Forcing ◽  
Tropospheric Aerosols ◽  
Internal Inhomogeneity

scholarly journals The influence of dust optical properties on the colour of simulated MSG-SEVIRI Desert Dust infrared imagery

2018 ◽  
Vol 18 (13) ◽  
pp. 9681-9703 ◽  
Author(s):  
Jamie R. Banks ◽  
Kerstin Schepanski ◽  
Bernd Heinold ◽  
Anja Hünerbein ◽  
Helen E. Brindley
Keyword(s):  
Refractive Index ◽  
Optical Properties ◽  
Particle Shape ◽  
Transport Model ◽  
Mineral Dust ◽  
Spherical Particles ◽  
Upper And Lower Bounds ◽  
Infrared Imagery ◽  
Desert Dust ◽  
Infrared Extinction

Abstract. Satellite imagery of atmospheric mineral dust is sensitive to the optical properties of the dust, governed by the mineral refractive indices, particle size, and particle shape. In infrared channels the imagery is also sensitive to the dust layer height and to the surface and atmospheric environment. Simulations of mineral dust in infrared Desert Dust imagery from the Spinning Enhanced Visible and InfraRed Imager (SEVIRI) have been performed, using the COSMO-MUSCAT (COSMO: COnsortium for Small-scale MOdelling; MUSCAT: MUltiScale Chemistry Aerosol Transport Model) dust transport model and the Radiative Transfer for TOVS (RTTOV) program, in order to investigate the sensitivity of the imagery to assumed dust properties. This paper introduces the technique and performs initial validation and comparisons with SEVIRI measurements over North Africa for daytime hours during 6 months covering June and July of 2011–2013. Using T-matrix scattering theory and assuming the dust particles to be spherical or spheroidal, wavelength- and size-dependent dust extinction values are calculated for a number of different dust refractive index databases, along with several values of the particle aspect ratio, denoting the particle shape. The consequences for the infrared extinction values of both the particle shape and the particle orientation are explored: this analysis shows that as the particle asphericity increases, the extinctions increase if the particles are aligned horizontally, and decrease if they are aligned vertically. Randomly oriented spheroidal particles have very similar infrared extinction properties as spherical particles, whereas the horizontally and vertically aligned particles can be considered to be the upper and lower bounds on the extinction values. Inputting these values into COSMO-MUSCAT-RTTOV, it is found that spherical particles do not appear to be sufficient to describe fully the resultant colour of the dust in the infrared imagery. Comparisons of SEVIRI and simulation colours indicate that of the dust types tested, the dust refractive index dataset produced by Volz (1973) shows the most similarity in the colour response to dust in the SEVIRI imagery, although the simulations have a smaller range of colour than do the observations. It is also found that the thermal imagery is most sensitive to intermediately sized particles (radii between 0.9 and 2.6 µm): larger particles are present in too small a concentration in the simulations, as well as with insufficient contrast in extinction between wavelength channels, to have much ability to perturb the resultant colour in the SEVIRI dust imagery.

scholarly journals On the microwave optical properties of randomly oriented ice hydrometeors

2014 ◽  
Vol 7 (12) ◽  
pp. 12873-12927
Author(s):  
P. Eriksson ◽  
M. Jamali ◽  
J. Mendrok ◽  
S. A. Buehler
Keyword(s):  
Particle Size ◽  
Optical Properties ◽  
Particle Shape ◽  
Reference Data ◽  
Microwave Remote Sensing ◽  
Particle Orientation ◽  
Maximum Dimension ◽  
Single Observation ◽  
Single Temperature ◽  
Selection Of

Abstract. Microwave remote sensing is important for observing the mass of ice hydrometeors. One of the main error sources of microwave ice mass retrievals is that approximations around the shape of the particles are unavoidable. One common approach to represent particles of irregular shape is the soft particle approximation (SPA). We show that it is possible to define a SPA that mimics mean optical particles of available reference data over narrow frequency ranges, considering a single observation technique at the time, but SPA does not work in a broader context. Most critically, the required air fraction varies with frequency and application, as well as with particle size. In addition, the air fraction matching established density parameterisations results in far too soft particles, at least for frequencies above 90 GHz. That is, alternatives to SPA must be found. One alternative was recently presented by Geer and Baordo (2014). They used a sub-set of the same reference data and simply selected as "shape model" the particle type giving the best overall agreement with observations. We present a way to perform the same selection of a representative particle shape, but without involving assumptions on particle size distribution and actual ice mass contents. Only an assumption on the occurrence frequency of different particle shapes is still required. Our analysis leads to the same selection of representative shape as found by Geer and Baordo (2014). In addition, we show that the selected particle shape has the desired properties also at higher frequencies as well as for radar applications. Finally, we demonstrate that in this context the assumption on particle shape is likely less critical when using mass equivalent diameter to characterise particle size, compared to using maximum dimension, but a better understanding of the variability of size distributions is required to fully characterise the advantage. Further advancements on these subjects are presently difficult to achieve due to a lack of reference data. One main problem is that most available databases of precalculated optical properties assume completely random particle orientation, while for certain conditions a horizontal alignment is expected. In addition, the only database covering frequencies above 340 GHz has a poor representation of absorption as it is based on outdated refractive index data, as well as only covering particles having a maximum dimension below 2 mm and a single temperature.

scholarly journals How small is a small cloud?

2008 ◽  
Vol 8 (2) ◽  
pp. 6379-6407 ◽  
Author(s):  
I. Koren ◽  
L. Oreopoulos ◽  
G. Feingold ◽  
L. A. Remer ◽  
O. Altaratz
Keyword(s):  
Optical Properties ◽  
Radiative Forcing ◽  
Climate Forcing ◽  
Radiation Budget ◽  
Free Atmosphere ◽  
Cumulus Clouds ◽  
Aerosol Forcing ◽  
The Individual ◽  
Aerosol Effects ◽  
Small Cloud

Abstract. The interplay between clouds and aerosols and their contribution to the radiation budget is one of the largest uncertainties of climate change. Most work to date has separated cloudy and cloud-free areas in order to evaluate the individual radiative forcing of aerosols, clouds, and aerosol effects on clouds. Here we examine the size distribution and the optical properties of small, sparse cumulus clouds and the associated optical properties of what is considered a cloud-free atmosphere within the cloud field. We show that any separation between clouds and cloud free atmosphere will incur errors in the calculated radiative forcing. The nature of small cumulus cloud size distributions suggests that at any resolution, a significant fraction of the clouds are missed, and their optical properties are relegated to the apparent cloud-free optical properties. At the same time, the cloudy portion incorporates significant contribution from non-cloudy pixels. We show that the largest contribution to the total cloud reflectance comes from the smallest clouds and that the spatial resolution changes the apparent energy flux of a broken cloudy scene. When changing the resolution from 30 m to 1 km (Landsat to MODIS) the average "cloud-free" reflectance at 1.65 μm increases more than 25%, the cloud reflectance decreases by half, and the cloud coverage doubles, resulting in an important impact on climate forcing estimations. The apparent aerosol forcing is on the order of 0.5 to 1 Wm−2 per cloud field.

scholarly journals Analysis of Aerosol Optical Properties due to a Haze Episode in the Himalayan Foothills: Implications for Climate Forcing

2018 ◽  
Vol 18 (5) ◽  
pp. 1331-1350 ◽  
Author(s):  
Khan Alam ◽  
Rehana Khan ◽  
Armin Sorooshian ◽  
Thomas Blaschke ◽  
Samina Bibi ◽  
...  
Keyword(s):  
Optical Properties ◽  
Climate Forcing ◽  
Aerosol Optical Properties ◽  
Himalayan Foothills ◽  
Haze Episode

Optical properties of tropospheric aerosols based on measurements of lidar, sun-photometer, and visibility at Chung-Li (25°N, 121°E)

2007 ◽  
Vol 41 (19) ◽  
pp. 4128-4137 ◽  
Author(s):  
Chih-Wei Chiang ◽  
Wei-Nai Chen ◽  
Wen-An Liang ◽  
Subrata Kumar Das ◽  
Jan-Bai Nee
Keyword(s):  
Optical Properties ◽  
Sun Photometer ◽  
Tropospheric Aerosols

scholarly journals Effect of Particle Shape, Density, and Inhomogeneity on the Microwave Optical Properties of Graupel and Hailstones

2017 ◽  
Vol 55 (11) ◽  
pp. 6366-6378 ◽  
Author(s):  
Guanglin Tang ◽  
Ping Yang ◽  
Patrick G. Stegmann ◽  
R. Lee Panetta ◽  
Leung Tsang ◽  
...  
Keyword(s):  
Optical Properties ◽  
Particle Shape
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