scholarly journals Characterization of aerosol size properties from measurements of spectral optical depth: a global validation of the GRASP-AOD code using long-term AERONET data

2021 ◽  
Vol 14 (6) ◽  
pp. 4471-4506
Author(s):  
Benjamin Torres ◽  
David Fuertes
Keyword(s):  
Size Distribution ◽  
Optical Depth ◽  
Volume Concentration ◽  
Effective Radius ◽  
Retrieval Algorithm ◽  
Aerosol Retrieval ◽  
Coarse Mode ◽  
Fine Mode ◽  
Aerosol Types

Abstract. A validation study is conducted regarding aerosol optical size property retrievals from measurements of the direct sun beam only (without the aid of diffuse radiation). The study focuses on using real data to test the new GRASP-AOD application, which uses only spectral optical depth measurements to retrieve the total column aerosol size distributions, assumed to be bimodal lognormal. In addition, a set of secondary integral parameters of aerosol size distribution and optical properties are provided: effective radius, total volume concentration and fine-mode fraction of aerosol optical depth (AOD). The GRASP-AOD code is applied to almost 3 million observations acquired over 20 years (1997–2016) at 30 AERONET (Aerosol Robotic Network) sites. These validation sites have been selected based on known availability of an extensive data record, significant aerosol load variability throughout the year, wide worldwide coverage and diverse aerosol types and source regions. The output parameters are compared to those coming from the operational AERONET retrievals. The retrieved fine-mode fractions at 500 nm (τf(500)) obtained by the GRASP-AOD application are compared to those retrieved by the spectral deconvolution algorithm (SDA) and by the AERONET aerosol retrieval algorithm. The size distribution properties obtained by the GRASP-AOD are compared to their equivalent values from the AERONET aerosol retrieval algorithm. The analysis showed the convincing capacity of the GRASP-AOD approach to successfully discriminate between fine- and coarse-mode extinction to robustly retrieve τf(500). The comparisons of 2 million results of τf(500) retrieval by the GRASP-AOD and SDA showed high correlation with a root mean square error (RMSE) of 0.015. Also, the analysis showed that the τf(500) values computed by the AERONET aerosol retrieval algorithm agree slightly better with the GRASP-AOD (RMSE = 0.018, from 148 526 comparisons) than with the SDA (RMSE = 0.022, from 127 203 comparisons). The comparisons of the size distribution retrieval showed agreement for the fine-mode median radius between the GRASP-AOD and AERONET aerosol retrieval algorithm results with an RMSE of 0.032 µm (or 18.7 % in relative terms) for the situations when τ(440)>0.2 occur for more than 80 000 pairs of the study. For the cases where the fine mode is dominant (i.e., α>1.2), the RMSE is only of 0.023 µm (or 13.9 % in relative terms). Major limitations in the retrieval were found for the characterization of the coarse-mode details. For example, the analysis revealed that the GRASP-AOD retrieval is not sensitive to the small variations of the coarse-mode volume median radius for different aerosol types observed at different locations. Nonetheless the GRASP-AOD retrieval provides reasonable agreement with the AERONET aerosol retrieval algorithm for overall coarse-mode properties with with RMSE = 0.500 µm (RMSRE = 20 %) when τ(440)>0.2. The values of effective radius and total volume concentration computed from the GRASP-AOD retrieval have been compared to those estimated by the AERONET aerosol retrieval algorithm. The RMSE values of the correlations were 30 % for the effective radius and 25 % for the total volume concentration when τ(440)>0.2. Finally, the study discusses the importance of employing the assumption of bimodal lognormal size distribution. It also evaluates the potential of using ancillary data, in particular aureole measurements, for improving the characterization of the aerosol coarse-mode properties.

scholarly journals Characterisation of aerosol size properties from measurements of spectral optical depth: a global validation of the GRASP-AOD code using long-term AERONET data

2020 ◽  
Author(s):  
Benjamin Torres ◽  
David Fuertes
Keyword(s):  
Size Distribution ◽  
Optical Depth ◽  
Volume Concentration ◽  
Effective Radius ◽  
Aerosol Size Distribution ◽  
Retrieval Algorithm ◽  
Aerosol Retrieval ◽  
Coarse Mode ◽  
Fine Mode ◽  
Log Normal

Abstract. A validation study is conducted regarding aerosol optical size property retrievals from only measurements of the direct Sun beam (without the aid of diffuse radiation). The study focuses on testing with real data the new GRASP-AOD application which uses only spectral optical depth measurements to retrieve the total column aerosol size distributions, assumed as bimodal log-normal. In addition, a set of secondary integral parameters of aerosol size distribution and optical properties are provided: effective radius, total volume concentration and fine mode fraction of aerosol optical depth. The GRASP-AOD code is applied to almost three million observations acquired during twenty years (1997–2016) at thirty AERONET (Aerosol Robotic Network) sites. These validation sites have been selected based on known availability of an extensive data record, significant aerosol load variability along the year, wide worldwide coverage and divers aerosol types and source regions. The output parameters are compared to those coming from the operational AERONET retrievals. The retrieved fine mode fractions at 500 nm (τf(500)) obtained by GRASP-AOD application are compared to those retrieved by the Spectral Deconvolution Algorithm and by AERONET aerosol retrieval algorithm. The size distribution properties obtained by GRASP-AOD are compared to their equivalent values from the AERONET aerosol retrieval algorithm. The analysis showed the convincing capacity of GRASP-AOD approach to successfully discriminate between fine and coarse mode extinction to robustly retrieve τf(500). The comparisons of 2 million results of τf(500) retrieval by GRASP-AOD and SDA showed high correlation with a root-mean-square-error of 0.015. Also, the analysis showed that the τf(500) values computed by AERONET aerosol retrieval algorithm agree slightly better with GRASP-AOD (RMSE = 0.018, from 148526 comparisons) than with SDA (RMSE = 0.022, from 127203 comparisons). The comparisons of the size distribution retrieval showed the agreement for fine mode median radius between GRASP-AOD and AERONET aerosol retrieval algorithm results with RMSE of 0.032 μm (or 18.7 % in relative terms) for the situations when τ(440) > 0.2 that occurs for more than eighty thousand pairs of the study. For the cases where fine mode is dominant (i.e. α  0.2. The values of effective radius and total volume concentration computed from GRASP-AOD retrieval have been compared to those estimated by AERONET aerosol retrieval algorithm. The RMSE values of the correlations were of 30 % for the effective radius and 25 % for the total volume concentration when τ(440) > 0.2. Finally, the study discusses the importance of employing the assumption of bimodal log-normal size distribution. It also evaluates the potential of using ancillary data, in particular aureole measurements, for improving the characterization of the aerosol coarse mode properties.

scholarly journals Advanced characterisation of aerosol size properties from measurements of spectral optical depth using the GRASP algorithm

2017 ◽  
Vol 10 (10) ◽  
pp. 3743-3781 ◽  
Author(s):  
Benjamin Torres ◽  
Oleg Dubovik ◽  
David Fuertes ◽  
Gregory Schuster ◽  
Victoria Eugenia Cachorro ◽  
...  
Keyword(s):  
Optical Depth ◽  
Atmospheric Aerosols ◽  
Volume Concentration ◽  
A Priori ◽  
Western Mediterranean ◽  
Dominant Mode ◽  
Coarse Mode ◽  
Fine Mode ◽  
The Difference ◽  
Log Normal

Abstract. This study evaluates the potential of using aerosol optical depth (τa) measurements to characterise the microphysical and optical properties of atmospheric aerosols. With this aim, we used the recently developed GRASP (Generalized Retrieval of Aerosol and Surface Properties) code for numerical testing of six different aerosol models with different aerosol loads. The direct numerical simulations (self-consistency tests) indicate that the GRASP-AOD retrieval provides modal aerosol optical depths (fine and coarse) to within 0.01 of the input values. The retrieval of the fine-mode radius, width and volume concentration are stable and precise if the real part of the refractive index is known. The coarse-mode properties are less accurate, but they are significantly improved when additional a priori information is available. The tests with random simulated errors show that the uncertainty in the bimodal log-normal size distribution parameters increases as the aerosol load decreases. Similarly, the reduction in the spectral range diminishes the stability of the retrieved parameters. In addition to these numerical studies, we used optical depth observations at eight AERONET locations to validate our results with the standard AERONET inversion products. We found that bimodal log-normal size distributions serve as useful input assumptions, especially when the measurements have inadequate spectral coverage and/or limited accuracy, such as moon photometry. Comparisons of the mode median radii between GRASP-AOD and AERONET indicate average differences of 0.013 µm for the fine mode and typical values of 0.2–0.3 µm for the coarse mode. The dominant mode (i.e. fine or coarse) indicates a 10 % difference in mode radii between the GRASP-AOD and AERONET inversions, and the average of the difference in volume concentration is around 17 % for both modes. The retrieved values of the fine-mode τa(500) using GRASP-AOD are generally between those values obtained by the standard AERONET inversion and the values obtained by the AERONET spectral deconvolution algorithm (SDA), with differences typically lower than 0.02 between GRASP-AOD and both algorithms. Finally, we present some examples of application of GRASP-AOD inversion using moon photometry and the airborne PLASMA sun photometer during the ChArMEx summer 2013 campaign in the western Mediterranean.

scholarly journals Column-integrated aerosol microphysical properties from AERONET Sun photometer over Southwestern Spain

2011 ◽  
Vol 11 (6) ◽  
pp. 18349-18384 ◽  
Author(s):  
N. Prats ◽  
V. E. Cachorro ◽  
A. Berjón ◽  
C. Toledano ◽  
A. M. De Frutos
Keyword(s):  
Particle Size ◽  
Particle Size Distribution ◽  
Size Distribution ◽  
Volume Fraction ◽  
Effective Radius ◽  
Desert Dust ◽  
Microphysical Properties ◽  
Coarse Mode ◽  
Fine Mode ◽  
Sun Photometer

Abstract. The aim of the present work is to carry out a detailed analysis of columnar microphysical properties obtained from Cimel sun-photometer measurements in the Southwest of Spain within the frame of the AERONET-RIMA network. AERONET inversion products are analysed, in particular the particle size distribution together with their associated microphysical parameters for both fine and coarse modes: concentration, effective radius and the fine mode volume fraction. This work complements previous works based on aerosol optical depth (AOD) and the Ångström exponent (AE) for a global characterization of atmospheric aerosol in this representative area of Spain and Europe. The analysed dataset spans between February 2000 and October 2008. Time series and statistical analysis has been carried out for these parameters in order to assess their typical values and seasonality together with their relationships with the AOD and AE. Mean values of volume particle concentration are 0.06 ± 0.07 μm3 μm−2 for total, 0.019 ± 0.015 μm3 μm−2 for fine and 0.04 ± 0.06 μm3 μm−2 for coarse mode; and of effective radius are 0.040 ± 0.19 μm for total, 0.14 ± 0.02 μm for fine and 1.96 ± 0.41 μm for coarse mode. The most relevant features are the clear bimodality of the volume particle size distribution, with a slight dominance of the coarse mode for the total climatology and under the prevailing atmospheric conditions of the site (coastal marine). There is a clear prevalence of the coarse mode in summer months, September and March in coincidence with the occurrence of desert dust intrusions and highest AOD values. During aerosol desert dust arrivals, the particle size distribution is practically mono-modal with strong prevalence of the coarse mode which also shows a shift of the modal radius to lower values. The size particle predominance defines the characteristic of the site and it has been analysed under two different approaches: with respect to particle number, using the Ångström exponent and with respect to particle volume, where the fine mode volume fraction Vf/Vt is taken.

scholarly journals Sensitivity study on polarized aerosol retrievals of PARASOL in Beijing and Kanpur

2011 ◽  
Vol 4 (5) ◽  
pp. 5773-5806 ◽  
Author(s):  
X. F. Gu ◽  
S. P. Wang ◽  
L. Fang ◽  
T. Yu ◽  
J. Gao
Keyword(s):  
Size Distribution ◽  
Sensitivity Study ◽  
Relative Difference ◽  
Aerosol Size Distribution ◽  
Retrieval Algorithm ◽  
Aerosol Retrieval ◽  
Different Seasons ◽  
Fine Mode ◽  
Retrieval Errors ◽  
Autumn And Winter

Abstract. Sensitivity study on the PARASOL aerosol retrieval algorithm over land is presented for aerosol mixtures composed of fine mode pollution particles combined with coarse mode desert dust. First an assessment of the PARASOL aerosol products during the period of 2005–2009 is investigated by comparison with AOD derived by AERONET (Aerosol Robotic Network) at Beijing and Kanpur. Validation against AERONET fine mode AOD shows an overall high correlation of R2 = 0.79 for Beijing and R2 = 0.83 for Kanpur. However, the PARASOL retrievals are found to underestimate aerosol optical depth by about 27% and 34% for Beijing and Kanpur, respectively. The AOD agreement is obviously poorer as compared to AERONET total AOD, showing underestimation by 60% and 67%. At both sites, the PARASOL retrieval algorithm performs better in autumn and winter seasons with the best appearing in autumn. As PARASOL aerosol algorithm is sensitive to the accumulation mode of the aerosol size distribution, we conduct study on the threshold radius of this fraction of size distribution, named as sensitive radius, for different seasons at both Beijing and Kanpur. The results show that the sensitive radius for polarized aerosol retrieval is 0.35 μm for all seasons. And the agreement is significantly improved by employing comparison against the AERONET AOD recomputed for radius <0.35 μm, showing a correlation coefficient (R2) of 0.82 with relative difference being 12% for Beijing and 0.87 with relative difference being 19% for Kanpur. The sensitivity study on uncertainty of PARASOL aerosol retrieval demonstrates that uncertainties caused by the algorithm-assumed refractive index and size distribution are significantly higher in spring than those of autumn and winter seasons. The aerosol retrieval errors caused by aerosol polarized phase function qa(Θ) for spring are found to be higher at Kanpur, due to the obviously higher content of coarse dust particles. For all seasons the aerosol retrieval errors contributed by uncertainty in qa(Θ) are much close to the total retrieval errors (accounts for about 65% to 94% in different seasons), indicating that the overestimate of qa(Θ) in PARASOL algorithm accounts for most of the underestimate of retrieved AOD at both sites. Investigation on the uncertainty of surface contribution shows that the surface model overestimates surface polarization from about 20% to 50% with the maximum uncertainties occurring in winter.

scholarly journals Column-integrated aerosol microphysical properties from AERONET Sun photometer over southwestern Spain

2011 ◽  
Vol 11 (24) ◽  
pp. 12535-12547 ◽  
Author(s):  
N. Prats ◽  
V. E. Cachorro ◽  
A. Berjón ◽  
C. Toledano ◽  
A. M. De Frutos
Keyword(s):  
Particle Size ◽  
Particle Size Distribution ◽  
Size Distribution ◽  
Volume Fraction ◽  
Effective Radius ◽  
Desert Dust ◽  
Microphysical Properties ◽  
Coarse Mode ◽  
Fine Mode ◽  
Sun Photometer

Abstract. The aim of the present work is to carry out a detailed analysis of columnar microphysical properties obtained from Cimel sun-photometer measurements in the Southwest of Spain within the frame of the Aerosol Robotic Network (AERONET) – Iberian Network for aerosol measurements (RIMA). AERONET level 2 inversion products are analysed, in particular the particle size distribution together with their associated microphysical parameters for both fine and coarse modes: volume concentration, effective radius and the fine mode volume fraction. This work complements previous works based on aerosol optical depth (AOD) and the Ångström exponent (AE) for a global characterization of atmospheric aerosol in this area of southwestern Spain. The analysed dataset spans between February 2000 and October 2008. Time series and statistical analysis has been carried out for these parameters in order to assess their typical values and seasonality together with their relationships with the AOD and AE. Mean values of volume particle concentration are 0.06±0.07 μm3 μm−2 for total, 0.019±0.015 μm3 μm−2 for fine and 0.04±0.06 μm3 μm−2 for coarse mode; mean effective radii are 0.40±0.19 μm for total, 0.14±0.02 μm for fine and 1.96±0.41 μm for coarse mode. The most relevant features are the clear bimodality of the volume particle size distribution, with a slight dominance of the coarse mode in the overall climatology given the prevailing atmospheric conditions at the site (coastal marine). There is a clear prevalence of the coarse mode in summer months plus September and March, in coincidence with the occurrence of desert dust intrusions and highest AOD values. During desert dust outbreaks, the particle size distribution is practically monomodal with strong prevalence of the coarse mode which also shows a shift of the modal radius toward lower values. The size particle predominance defines the characteristic of the site and it has been analysed under two different parameters: the Ångström exponent and the fine mode volume fraction Vf/Vt. We investigated the relationship between them and also their relationship with the effective radius of the size distribution.

scholarly journals Aerosol optical depth and fine-mode fraction retrieval over East Asia using multi-angular total and polarized remote sensing

2012 ◽  
Vol 5 (3) ◽  
pp. 501-516 ◽  
Author(s):  
T. Cheng ◽  
X. Gu ◽  
D. Xie ◽  
Z. Li ◽  
T. Yu ◽  
...  
Keyword(s):  
East Asia ◽  
Aerosol Optical Depth ◽  
Optical Depth ◽  
Lookup Table ◽  
Retrieval Algorithm ◽  
Spatial And Temporal Variability ◽  
Aerosol Retrieval ◽  
Fine Mode ◽  
Sun Photometer ◽  
The Impact

Abstract. A new aerosol retrieval algorithm using multi-angular total and polarized measurements is presented. The algorithm retrieves aerosol optical depth (AOD), fine-mode fraction (FMF) for studying the impact of aerosol on climate change. The retrieval algorithm is based on a lookup table (LUT) method, which assumes that one fine and one coarse lognormal aerosol modes can be combined with proper weightings to represent the ambient aerosol properties. To reduce the ambiguity in retrieval algorithm, the key characteristics of aerosol model over East Asia are constrained using the cluster analysis technique based on the AERONET sun-photometer observation over East Asia, and the fine and coarse modes are not fixed but can vary. A mixing model of bare soil and green vegetation spectra and the Nadal and Breon model for the bidirectional polarized reflectance factor (BPDF) were used to simulate total and polarized surface reflectance of East Asia. By applying the present algorithm to POLDER measurements, three different aerosol cases of clear, polluted and dust are analyzed to test the algorithm. The comparison of retrieved aerosol optical depth (AOD) and fine-mode fraction (FMF) with those of AERONET sun-photometer observations show reliable results. Preliminary validation is encouraging. Using the new aerosol retrieval algorithm for multi-angular total and polarized measurements, the spatial and temporal variability of anthropogenic aerosol optical properties over East Asia, which were observed during a heavy polluted event, were analyzed. Exceptionally high values of aerosol optical depth contributed by fine mode of up to 0.5 (at 0.865 μm), and high values of fine-mode fraction of up to 0.9, were observed in this case study.

scholarly journals Aerosol optical depth and fine-mode fraction retrieval over East Asia using multi-angular total and polarized remote sensing

2011 ◽  
Vol 4 (5) ◽  
pp. 5689-5716
Author(s):  
T. Cheng ◽  
X. Gu ◽  
D. Xie ◽  
Z. Li ◽  
T. Yu ◽  
...  
Keyword(s):  
East Asia ◽  
Aerosol Optical Depth ◽  
Optical Depth ◽  
Lookup Table ◽  
Retrieval Algorithm ◽  
Spatial And Temporal Variability ◽  
Aerosol Retrieval ◽  
Fine Mode ◽  
Sun Photometer ◽  
The Impact

Abstract. A new aerosol retrieval algorithm using multi-angular total and polarized measurements is presented. The algorithm retrieves aerosol optical depth (AOD), fine-mode fraction (FMF) for studying the impact of aerosol on climate change. The retrieval algorithm is based on a lookup table (LUT) method, which assumes that one fine and one coarse lognormal aerosol modes can be combined with proper weightings to represent the ambient aerosol properties. To reduce the ambiguity in retrieval algorithm, the key characteristics of aerosol model over East Asia are constrained using the cluster analysis technique based on the AERONET sun-photometer observation over East Asia. A mixing model of bare soil and green vegetation spectra and the Nadal and Breon model for the bidirectional polarized reflectance factor (BPDF) were used to simulate total and polarized surface reflectance of East Asia. By applying the present algorithm to POLDER measurements, three different aerosol cases of clear, polluted and dust are analyzed to test the algorithm. The comparison of retrieved aerosol optical depth (AOD) and fine-mode fraction (FMF) with those of AERONET sun-photometer observations show reliable results. Preliminary validation is encouraging. Using the new aerosol retrieval algorithm for multi-angular total and polarized measurements, the spatial and temporal variability of anthropogenic aerosol optical properties over East Asia, which were observed during a heavy polluted event, were analyzed. Exceptionally high values of aerosol optical depth contributed by fine mode of up to 0.5 (at 0.865 μm), and high values of fine-mode fraction of up to 0.9, were observed in this case study.

scholarly journals Indirect estimation of absorption properties for fine aerosol particles using AATSR observations: a case study of wildfires in Russia in 2010

2015 ◽  
Vol 8 (8) ◽  
pp. 3075-3085 ◽  
Author(s):  
E. Rodríguez ◽  
P. Kolmonen ◽  
T. H. Virtanen ◽  
L. Sogacheva ◽  
A.-M. Sundström ◽  
...  
Keyword(s):  
Optical Depth ◽  
Continuous Variation ◽  
Radiative Transfer Model ◽  
Retrieval Algorithm ◽  
Transfer Model ◽  
Retrieval Process ◽  
Aerosol Model ◽  
Coarse Mode ◽  
Fine Mode ◽  
Aerosol Properties

Abstract. The Advanced Along-Track Scanning Radiometer (AATSR) on board the ENVISAT satellite is used to study aerosol properties. The retrieval of aerosol properties from satellite data is based on the optimized fit of simulated and measured reflectances at the top of the atmosphere (TOA). The simulations are made using a radiative transfer model with a variety of representative aerosol properties. The retrieval process utilizes a combination of four aerosol components, each of which is defined by their (lognormal) size distribution and a complex refractive index: a weakly and a strongly absorbing fine-mode component, coarse mode sea salt aerosol and coarse mode desert dust aerosol). These components are externally mixed to provide the aerosol model which in turn is used to calculate the aerosol optical depth (AOD). In the AATSR aerosol retrieval algorithm, the mixing of these components is decided by minimizing the error function given by the sum of the differences between measured and calculated path radiances at 3–4 wavelengths, where the path radiances are varied by varying the aerosol component mixing ratios. The continuous variation of the fine-mode components allows for the continuous variation of the fine-mode aerosol absorption. Assuming that the correct aerosol model (i.e. the correct mixing fractions of the four components) is selected during the retrieval process, also other aerosol properties could be computed such as the single scattering albedo (SSA). Implications of this assumption regarding the ratio of the weakly/strongly absorbing fine-mode fraction are investigated in this paper by evaluating the validity of the SSA thus obtained. The SSA is indirectly estimated for aerosol plumes with moderate-to-high AOD resulting from wildfires in Russia in the summer of 2010. Together with the AOD, the SSA provides the aerosol absorbing optical depth (AAOD). The results are compared with AERONET data, i.e. AOD level 2.0 and SSA and AAOD inversion products. The RMSE (root mean square error) is 0.03 for SSA and 0.02 for AAOD lower than 0.05. The SSA is further evaluated by comparison with the SSA retrieved from the Ozone Monitoring Instrument (OMI). The SSA retrieved from both instruments show similar features, with generally lower AATSR-estimated SSA values over areas affected by wildfires.

scholarly journals Sub-Mode Aerosol Volume Size Distribution and Complex Refractive Index from the Three-Year Ground-Based Measurements in Chengdu China

Atmosphere ◽  
2019 ◽  
Vol 10 (2) ◽  
pp. 46 ◽  
Author(s):  
Chi Zhang ◽  
Ying Zhang ◽  
Zhengqiang Li ◽  
Yongqian Wang ◽  
Hua Xu ◽  
...  
Keyword(s):  
Refractive Index ◽  
Size Distribution ◽  
Aerosol Optical Depth ◽  
Optical Depth ◽  
Complex Refractive Index ◽  
Observation Data ◽  
Aerosol Model ◽  
Coarse Mode ◽  
Fine Mode ◽  
Volume Size

Chengdu is a typical basin city of Southwest China with rare observations of remote sensing measurements. To assess the climate change and establish a region aerosol model, a deeper understanding of the separated volume size distribution (VSD) and complex refractive index (CRI) is required. In this study, we employed the sub-mode VSD and CRI in Chengdu based on the three years observation data to investigate the sub-mode characteristics and climate effects. The annual average fraction of the fine-mode aerosol optical depth (AODf) is 92%, which has the same monthly tendency as the total AOD. But the coarse-mode aerosol optical depth (AODc) has little variation in different months. There are four distinguishing modes of VSD in Chengdu; the median radii are 0.17 μm ± 0.05, 0.31 μm ± 0.12, 1.62 μm ± 0.45, 3.25 μm ± 0.99, respectively. The multi-year average and seasonal variations of fine- and coarse-mode VSD and CRI are also analyzed to characterize aerosols over this region. The fine-mode single scattering albedos (SSAs) are higher than the coarse-mode ones, which suggests that the coarse-mode aerosols have a stronger absorbing effect on solar light than the small-size aerosol particles in Chengdu.

scholarly journals Models transport Saharan dust too low in the atmosphere: a comparison of the MetUM and CAMS forecasts with observations

2020 ◽  
Vol 20 (21) ◽  
pp. 12955-12982
Author(s):  
Debbie O'Sullivan ◽  
Franco Marenco ◽  
Claire L. Ryder ◽  
Yaswant Pradhan ◽  
Zak Kipling ◽  
...  
Keyword(s):  
Optical Properties ◽  
Vertical Distribution ◽  
Size Distribution ◽  
Optical Depth ◽  
Global Scale ◽  
Saharan Dust ◽  
Atmospheric Measurements ◽  
Coarse Mode ◽  
Fine Mode

Abstract. We investigate the dust forecasts from two operational global atmospheric models in comparison with in situ and remote sensing measurements obtained during the AERosol properties – Dust (AER-D) field campaign. Airborne elastic backscatter lidar measurements were performed on board the Facility for Airborne Atmospheric Measurements during August 2015 over the eastern Atlantic, and they permitted us to characterise the dust vertical distribution in detail, offering insights on transport from the Sahara. They were complemented with airborne in situ measurements of dust size distribution and optical properties, as well as datasets from the Cloud–Aerosol Transport System (CATS) spaceborne lidar and the Moderate Resolution Imaging Spectroradiometer (MODIS). We compare the airborne and spaceborne datasets to operational predictions obtained from the Met Office Unified Model (MetUM) and the Copernicus Atmosphere Monitoring Service (CAMS). The dust aerosol optical depth predictions from the models are generally in agreement with the observations but display a low bias. However, the predicted vertical distribution places the dust lower in the atmosphere than highlighted in our observations. This is particularly noticeable for the MetUM, which does not transport coarse dust high enough in the atmosphere or far enough away from the source. We also found that both model forecasts underpredict coarse-mode dust and at times overpredict fine-mode dust, but as they are fine-tuned to represent the observed optical depth, the fine mode is set to compensate for the underestimation of the coarse mode. As aerosol–cloud interactions are dependent on particle numbers rather than on the optical properties, this behaviour is likely to affect their correct representation. This leads us to propose an augmentation of the set of aerosol observations available on a global scale for constraining models, with a better focus on the vertical distribution and on the particle size distribution. Mineral dust is a major component of the climate system; therefore, it is important to work towards improving how models reproduce its properties and transport mechanisms.

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