Dust Aerosol Optical Thickness Research Articles

Implications of North Atlantic warming for a possible increase of dust activity in northern East Asia

This study investigates variations of the March dust activities in northern East Asia (NEA) during 1980–2021 based on dust aerosol optical thickness (DAOT) from Moder-Era Retrospective analysis for Research and Applications version 2 dataset. In particular, the roles of the atmospheric circulation and remote sea surface temperature (SST) forcing in the long-term changes of dust activities are analyzed. Results show that the NEA dust impacts on the environment have a sign of increasing on downstream East Asia, North Pacific, and even western North America. The large-scale atmospheric circulation anomalies over Eurasia play an important role in the increased dust activities over northern East Asia. A quasi-barotropic wave train from the North Atlantic to southern Asia is identified as the key atmospheric circulation factor in the variations of dust activities over the NEA. The increased pressure gradient by the wave train in the NEA strengthens the northwesterly winds and facilitates the increase of intensity and frequency of dust activities there. Further analysis indicates that the anomalous wave train is forced by the North Atlantic sea surface temperature anomalies. Sensitivity experiments with a numerical model confirm the impact of the remote SST forcing in the North Atlantic Ocean on the formation of this anomalous wave train. A linear extrapolation of the long-term decadal North Atlantic SST anomalies provides a possibility for the near-term prediction of dust activities and it suggests that the frequency and intensity of dust activities may increase in the next decade.

Hotspot analysis and long-term trends of absorbing aerosol index from dust emissions measured by the Ozone Monitoring Instrument at different urban locations in India during 2005–2018

The dust storm is a natural hazard and knowledge of hotspots of dust emission is important to improve forecasting the dust events by numerical models of climate and air quality. The absorbing aerosol index (AAI) observations derived from the Ozone Monitoring Instrument (OMI) sensor are analyzed over India covering the period from 2005 to 2018. In this manuscript, the hotspot analysis of AAI was conducted using the spatial autocorrelation technique for different seasons. The results showed significant spatial and temporal variability in the observed AAI. During pre-monsoon, the hotspot region is widely distributed, though with noticed spatially varying intensities, from the Thar Desert to West Bengal through the Indo Gangetic Plain (IGP) region. It is revealed from the back trajectory analysis that the airmasses were originated from the arid desert regions located in the West of India transporting the dust particles during the pre-monsoon over the IGP regions. Further, the trend analysis of AAI during the pre-monsoon over selected metropolitan in India was done using the Mann–Kendall (MK) test and Sen's robust slope estimator. The results showed a significant increasing (0.02 yr−1) and decreasing (−0.01 yr−1) trend in AAI at Patna and Hyderabad, respectively, while it is noticed stable or no trend at other cities. MERRA-2 Dust aerosol optical thickness was found to be very well correlated with AAI and shows decreasing and increasing trends at Nagpur (0.0005 yr−1) and Jaisalmer (0.004 yr−1), respectively. A negative correlation is observed between rainfall and AAI, indicating the reduction of AAI with increasing rainfall.

Global aerosol simulations using NICAM.16 on a 14 km grid spacing for a climate study: improved and remaining issues relative to a lower-resolution model

Abstract. High-performance computing resources allow us to conduct numerical simulations with a horizontal grid spacing that is sufficiently high to resolve cloud systems on a global scale, and high-resolution models (HRMs) generally provide better simulation performance than low-resolution models (LRMs). In this study, we execute a next-generation model that is capable of simulating global aerosols using version 16 of the Nonhydrostatic Icosahedral Atmospheric Model (NICAM.16). The simulated aerosol distributions are obtained for 3 years with an HRM using a global 14 km grid spacing, an unprecedentedly high horizontal resolution and long integration period. For comparison, a NICAM with a 56 km grid spacing is also run as an LRM, although this horizontal resolution is still high among current global aerosol climate models. The comparison elucidated that the differences in the various variables of meteorological fields, including the wind speed, precipitation, clouds, radiation fluxes and total aerosols, are generally within 10 % of their annual averages, but most of the variables related to aerosols simulated by the HRM are slightly closer to the observations than are those simulated by the LRM. Upon investigating the aerosol components, the differences in the water-insoluble black carbon and sulfate concentrations between the HRM and LRM are large (up to 32 %), even in the annual averages. This finding is attributed to the differences in the aerosol wet deposition flux, which is determined by the conversion rate of cloud to precipitation, and the difference between the HRM and LRM is approximately 20 %. Additionally, the differences in the simulated aerosol concentrations at polluted sites during polluted months between the HRM and LRM are estimated with normalized mean biases of −19 % for black carbon (BC), −5 % for sulfate and −3 % for the aerosol optical thickness (AOT). These findings indicate that the impacts of higher horizontal grid spacings on model performance for secondary products such as sulfate, and complex products such as the AOT, are weaker than those for primary products, such as BC. On a global scale, the subgrid variabilities in the simulated AOT and cloud optical thickness (COT) in the 1∘×1∘ domain using 6-hourly data are estimated to be 28.5 % and 80.0 %, respectively, in the HRM, whereas the corresponding differences are 16.6 % and 22.9 % in the LRM. Over the Arctic, both the HRM and the LRM generally reproduce the observed aerosols, but the largest difference in the surface BC mass concentrations between the HRM and LRM reaches 30 % in spring (the HRM-simulated results are closer to the observations). The vertical distributions of the HRM- and LRM-simulated aerosols are generally close to the measurements, but the differences between the HRM and LRM results are large above a height of approximately 3 km, mainly due to differences in the wet deposition of aerosols. The global annual averages of the effective radiative forcings due to aerosol–radiation and aerosol–cloud interactions (ERFari and ERFaci) attributed to anthropogenic aerosols in the HRM are estimated to be -0.293±0.001 and -0.919±0.004 W m−2, respectively, whereas those in the LRM are -0.239±0.002 and -1.101±0.013 W m−2. The differences in the ERFari between the HRM and LRM are primarily caused by those in the aerosol burden, whereas the differences in the ERFaci are primarily caused by those in the cloud expression and performance, which are attributed to the grid spacing. The analysis of interannual variability revealed that the difference in reproducibility of both sulfate and carbonaceous aerosols at different horizontal resolution is greater than their interannual variability over 3 years, but those of dust and sea salt AOT and possibly clouds were the opposite. Because at least 10 times the computer resources are required for the HRM (14 km grid) compared to the LRM (56 km grid), these findings in this study help modelers decide whether the objectives can be achieved using such higher resolution or not under the limitation of available computational resources.

ON THE CONVERGENCE OF NUMERICAL COMPUTATIONS FOR BOTH EXACT AND APPROXIMATE SOLUTIONS FOR ELECTROMAGNETIC SCATTERING BY NONSPHERICAL DIELECTRIC PARTICLES (INVITED REVIEW)

We summarize the size parameter range of the applicability of four light-scattering computational methods for nonspherical dielectric particles. These methods include two exact methods - the extended boundary condition method (EBCM) and the invariant imbedding T-matrix method (II-TM) and two approximate approaches - the physical-geometric optics method (PGOM) and the improved geometric optics method (IGOM). For spheroids, the single-scattering properties computed by EBCM and II-TM agree for size parameters up to 150, and the comparison gives us confidence in using IITM as a benchmark for size parameters up to 150 for other geometries (e.g., hexagonal columns) because the applicability of II-TM with respect to particle shape is generic, as demonstrated in our previous studies involving a complex aggregate. This study demonstrates the convergence of the exact II-TM and approximate PGOM solutions for the complete set of single-scattering properties of a nonspherical shape other than spheroids and circular cylinders with particle sizes of ~ 48λ(size parameter ~150), specifically a hexagonal column with a length size parameter of kL = 300 where k = 2π/λ and L is the column length. IGOM is also quite accurate except near the exact 180°backscattering direction. This study demonstrates that a synergetic combination of the numerically-exact II-TM and the approximate PGOM can seamlessly cover the entire size parameter range of practical interest. To demonstrate the applicability of the approach, we compute the optical properties of dust particles and demonstrate a downstream application to the retrieval of dust aerosol optical thickness and effective particle size from satellite polarimetric observations.

Dust detection and AOT estimation using combined VIR and TIR satellite images in urban areas of Iran

This study introduces an empirical equation for estimation of Aerosol Optical Thickness (AOT) and visibility reduction based on three main dust indices of Normalized Di erence Dust Index (NDDI), Brightness Temperature Di erence (BTD), and Thermal infrared Dust Index (TDI). The implementation of NDDI, BTD, and TDI in dust enhancement over bright to dark background was evaluated. The thresholds of BTD over dust and cloud pixels revealed the capability of Moderate Resolution Imaging Spectroradiometer (MODIS) images in separating dust from underlying bright surfaces and clouds. The results indicated that solar reffective bands were insucient to precisely separate dust from clouds, but combination of solar reffective bands and thermal infrared bands synergistically improved the accuracy. The evaluation of results revealed the remarkable correlation of AOT with dust enhancement indices over 11 synoptic stations: BTD (R2 = 0:73), NDDI (R2 = 0:67), and TDI (R2 = 0:71). Hence, AOT and visibility reduction were obtained using multi-regression equation using NDDI and BTD as variables. The accuracy assessment indicated good correlation (R2 = 0:74) between both estimated AOT and the AOT reported by Air Quality Control stations. Moreover, the results con rmed the advantage of proposed AOT as a consistent index for dust enhancement over bright surfaces and dust classi cation as well.

New developments in the representation of Saharan dust sources in the aerosol–climate model ECHAM6-HAM2

Abstract. In the aerosol–climate model ECHAM6-HAM2, dust source activation (DSA) observations from Meteosat Second Generation (MSG) satellite are proposed to replace the original source area parameterization over the Sahara Desert. The new setup is tested in nudged simulations for the period 2007 to 2008. The evaluation is based on comparisons to dust emission events inferred from MSG dust index imagery, Aerosol Robotic Network (AERONET) sun photometer observations, and satellite retrievals of aerosol optical thickness (AOT).The model results agree well with AERONET measurements especially in terms of seasonal variability, and a good spatial correlation was found between model results and MSG-SEVIRI (Spinning-Enhanced Visible and InfraRed Imager) dust AOT as well as Multi-angle Imaging SpectroRadiometer (MISR) AOT. ECHAM6-HAM2 computes a more realistic geographical distribution and up to 20 % higher annual Saharan dust emissions, using the MSG-based source map. The representation of dust AOT is partly improved in the southern Sahara and Sahel. In addition, the spatial variability is increased towards a better agreement with observations depending on the season. Thus, using the MSG DSA map can help to circumvent the issue of uncertain soil input parameters.An important issue remains the need to improve the model representation of moist convection and stable nighttime conditions. Compared to sub-daily DSA information from MSG-SEVIRI and results from a regional model, ECHAM6-HAM2 notably underestimates the important fraction of morning dust events by the breakdown of the nocturnal low-level jet, while a major contribution is from afternoon-to-evening emissions.

Saharan dust as a causal factor of hemispheric asymmetry in aerosols and cloud cover over the tropical Atlantic Ocean

Previous studies showed that, over the global ocean, there is no noticeable hemispheric asymmetry in cloud fraction (CF). This contributes to the balance in solar radiation reaching the sea surface in the northern and southern hemispheres. In the current study, we focus on the tropical Atlantic (30° N–30° S), which is characterized by significant amounts of Saharan dust dominating other aerosol species over the North Atlantic. Our main point is that, over the tropical Atlantic, Saharan dust not only is responsible for the pronounced hemispheric aerosol asymmetry, but also contributes to significant cloud cover along the Saharan Air Layer (SAL). Over the tropical Atlantic in July, along the SAL, Moderate Resolution Imaging Spectroradiometer CF data showed significant cloud cover (up to 0.8–0.9). This significant CF along SAL together with clouds over the Atlantic Intertropical Convergence Zone contributes to the 20% hemispheric CF asymmetry. This leads to the imbalance in strong solar radiation, which reaches the sea surface between the tropical North and South Atlantic, and, consequently, affects climate formation in the tropical Atlantic. During the 10-year study period (July 2002–June 2012), NASA Aerosol Reanalysis (aka MERRAero) showed that, when the hemispheric asymmetry in dust aerosol optical thickness (AOT) was most pronounced (particularly in July), dust AOT averaged separately over the tropical North Atlantic was one order of magnitude higher than that averaged over the tropical South Atlantic. In the presence of such strong hemispheric asymmetry in dust AOT in July, CF averaged separately over the tropical North Atlantic exceeded that over the tropical South Atlantic by 20%. Both Multiangle Imaging Spectroradiometer measurements and MERRAero data were in agreement on seasonal variations in hemispheric aerosol asymmetry. Hemispheric asymmetry in total AOT over the Atlantic was most pronounced between March and July, when dust presence over the North Atlantic was maximal. In September and October, there was no noticeable hemispheric aerosol asymmetry between the tropical North and South Atlantic. During the season with no noticeable hemispheric aerosol asymmetry, we found no noticeable asymmetry in cloud cover.

Retrieval of dust storm aerosols using an integrated Neural Network model

Dust storms are known to have adverse effects on public health. Atmospheric dust loading is also one of the major uncertainties in global climatic modeling as it is known to have a significant impact on the radiation budget and atmospheric stability. This study develops an integrated model for dust storm detection and retrieval based on the combination of geostationary satellite images and forward trajectory model. The proposed model consists of three components: (i) a Neural Network (NN) model for near real-time detection of dust storms; (ii) a NN model for dust Aerosol Optical Thickness (AOT) retrieval; and (iii) the Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model to analyze the transports of dust storms. These three components are combined using an event-driven active geo-processing workflow technique. The NN models were trained for the dust detection and validated using sunphotometer measurements from the AErosol RObotic NETwork (AERONET). The HYSPLIT model was applied in the regions with high probabilities of dust locations, and simulated the transport pathways of dust storms. This newly automated hybrid method can be used to give advance near real-time warning of dust storms, for both environmental authorities and public. The proposed methodology can be applied on early warning of adverse air quality conditions, and prediction of low visibility associated with dust storm events for port and airport authorities.

Dust aerosol height estimation: A synergetic approach using passive remote sensing and modelling

In this paper, a simple methodology for deducing a proxy index inferring on the dust aerosol layer height (in the atmosphere) has been presented by exploiting the dependence of the relationship between Aerosol Index (AI) and aerosol optical thickness (AOT) on the aerosol altitude. For the purpose, OMI-AI and GOCART model simulated dust AOT values over northern Arabian Sea are used. Northern Arabian Sea has been chosen as the study grid due its frequent vulnerability to the Arabian dust storms. Temporal variations in the dust AOT and AI values over the study region are discussed. Linear regression to the scatter plots between the dust AOT and AI values revealed the qualitative information on the dust layer altitudes. Results indicated that dust aerosols over the study region attain their highest altitude during July, August months. CALIOP aerosol sub-type measurements are examined in order to support the results obtained. Expansion of the present technique on inferring the spatio-temporal variability in the aerosol altitude over dust storms prone regions can be highly advantageous in fine tuning the regional dust aerosol radiative forcing calculations.

Observation of low single scattering albedo of aerosols in the downwind of the East Asian desert and urban areas during the inflow of dust aerosols

AbstractWe analyzed data observed at Fukue‐jima (32.752°N, 128.682°E), the downwind of the East Asian desert and urban areas, during the spring season (March–April) of 2008–2011 aiming to understand the light‐absorption capacity of Asian dust aerosols, which is a topic of controversy. We observed the decreasing tendency of single‐scattering albedo (SSA) with the decrease of Ångström exponent and the increase of the ratio of dust aerosol optical thickness to total aerosol optical thickness, suggesting the important role of coarse‐mode dust aerosols on observed low SSAs. The observational data further indicated that the low SSAs during strong dust events were less likely due to the effect of only strong light‐absorbing carbonaceous aerosols, such as black carbon (BC), indicating the association of aerosol size distribution on modulating SSA. Such observational results are justified by numerical calculations showing that aerosol size distribution can be the key factor on modulating SSA even without any change in relative amount of light‐absorbing aerosol as well as total aerosol optical thickness. Therefore, the observed low SSAs in the downwind regions during dust events could be partially due to the dominance of coarse‐mode aerosols over fine‐mode aerosols, which are usual in dust events, along with the effect of mixed light‐absorbing aerosols. The study further suggests that such effect of aerosol size distribution on SSA can be one of the important reasons for the low SSAs of dust aerosols in the source region as reported by some studies, if coarse‐mode aerosols dominate fine‐mode aerosols.

The importance of the diurnal cycle of Aerosol Optical Depth in West Africa

High resolution atmospheric simulations with the AROME model coupled with a dust module over West Africa for the whole of June 2006 were used to calculate aerosol optical thickness (AOT). Simulations showed a significant diurnal cycle of 0.2 in the dust AOT that could not be inferred from the MODIS Deep Blue retrievals due to their timings. Dust sources are mainly driven by the breakdown of the early morning low‐level jet and by moist convection in the afternoon, leading to opposite diurnal cycles. Also, simulations show that cloud cover significantly prevents the observation of AOT. The under‐sampling of the diurnal cycle by satellites plus the impact of cloud masks on the space‐borne AOT retrievals induce an underestimation of 0.28 (~40%) over the convective regions and an overestimation of 0.1 (17%) over morning source areas like Bodélé. A combination of observations and high‐resolution models could constrain bias and give a more realistic AOT climatology.

Retrieving Asian dust AOT and height from hyperspectral sounder measurements: An artificial neural network approach

In order to examine potential use of infrared (IR) hyperspectral measurements for dust monitoring, a statistical artificial neural network (ANN) approach was taken as an inverse method of retrieving pixel‐level aerosol optical thickness (AOT) and dust height (zdust). The ANN model was trained by relating Atmospheric Infrared Sounder (AIRS) brightness temperatures across 234 channels, surface elevation, and relative air mass to collocated AOT derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) and zdust derived from Cloud Aerosol Lidar Infrared Pathfinder Satellite Observation (CALIPSO) observations for Asian dust cases. Results showing correlation coefficients of 0.84 and 0.79, and mean biases of 0.03 and about −0.02 km for AOT and zdust, respectively, suggest that dust retrievals from hyperspectral IR sounder measurements are comparable to MODIS‐derived AOT and CALIPSO‐measured zdust. The pixel‐level retrievals of AOT and zdust during both day and night from IR hyperspectral measurements may offer great potential to improve our ability to monitor and forecast the evolving features of Asian dust.

Maintenance of Lower Tropospheric Temperature Inversion in the Saharan Air Layer by Dust and Dry Anomaly

Abstract The role of Saharan dust and dry anomaly in maintaining the temperature inversion in the Saharan air layer (SAL) is investigated. The dust aerosol optical thickness (AOT) in the SAL is inferred from the measurements taken by Aqua Moderate Resolution Imaging Spectroradiometer (MODIS), and the corresponding temperature and specific humidity anomalies are identified using the National Centers for Environmental Prediction (NCEP) data in August–September over the North Atlantic tropical cyclone (TC) main development region (MDR; 10°–20°N, 40°–60°W). The authors also study the SAL simulated in the National Center of Atmospheric Research (NCAR) Community Atmosphere Model, version 3 (CAM3), coupled with dust radiative effect. It is found that higher AOT is associated with warmer and dryer anomalies below 700 hPa, which increases the atmospheric stability. The calculated instantaneous radiative heating anomalies from a radiative transfer model indicate that both the dust and low humidity are essential to maintaining the temperature structure in the SAL against thermal relaxation. At 850 hPa, heating anomalies caused by both the dust and dry anomalies (for AOT > 0.8) are 0.2–0.4 K day−1. The dust heats the atmosphere below 600 hPa, while the dry anomaly cools the atmosphere below 925 hPa, resulting in a peak of heating rate anomaly located at 700–850 hPa. In the eastern Atlantic, dust contributes about 50% of the heating rate anomaly. Westward of 40°W, when the dust content becomes small (AOT < 0.6), the heating rates are more sensitive to the water vapor profile used in the radiative transfer calculation. Retrieving or simulating correct water vapor profiles is essential to the assessment of the SAL heating budgets in regions where the dust content in the SAL is small.

Investigations of the March 2006 African dust storm using ground‐based column‐integrated high spectral resolution infrared (8–13 μm) and visible aerosol optical thickness measurements: 1. Measurement procedures and results

The infrared (IR) aerosol optical thickness (AOT) spectra of Saharan dust measured during the Portable Infrared Aerosol Transmission Experiment (PIRATE) are reported. Saharan dust optical thickness (extinction) spectra from 8 to 13 μm were obtained using column‐integrated solar transmission measurements in Puerto Rico in July 2005 and Senegal in January and March 2006 (during a dust plume) using a Fourier transform infrared (FTIR) spectrometer. The FTIR measured the solar spectral irradiance in the IR in the presence of Saharan dust, and the AOT was determined by comparing the measured spectra to modeled downwelling spectra without dust for the same atmospheric temperature profile, solar zenith angle, water vapor, and ozone concentrations. The modeled dust‐free spectra are generated using the Santa Barbara Disort Atmospheric Radiative Transfer (SBDART) program. The measured dust AOT is compared with modeled AOT spectra obtained using Mie theory with dust indices of refraction from Volz and Fouquart with assumed lognormal size distributions. When the visible AOT values from nearby Aerosol Robotic Network (AERONET) sensors are compared to the IR AOT values, results from various dust loadings show that the IR dust AOT at 9.5 μm is typically only one third that of the visible (670 nm) dust AOT, but there is some evidence that this ratio could increase for larger dust size distributions. The surface IR dust forcing is determined to be about −0.4 W/m2 by summing the dusty and clear irradiance differences.

Is the top of atmosphere dust net radiative effect different between Terra and Aqua?

We assess the difference in Top of Atmosphere (TOA) cloud‐free Net Radiative Effect (NRE) of dust aerosols between the Terra and Aqua satellites using three years of collocated Moderate Resolution Imaging SpectroRadiometer (MODIS) and the Clouds and the Earth's Radiant Energy System (CERES) data over the Atlantic Ocean [0–30°N, 10–60°W]. The dust aerosol optical thickness at 0.55 μm (τdust) was first separated from the total aerosol column aerosol optical thickness (τ) and our results indicate that the Terra minus Aqua difference for both τ and τdust is approximately 10%, with Terra values generally being slightly higher. The resulting difference in TOA NRE from dust aerosols is less than 1 Wm−2. The difference between Terra and Aqua NRE lies well within previously reported uncertainties indicating that data from either satellite can be used interchangeably if independent adjustments for diurnal effects and clear‐sky sample biases are made.

Satellite‐based assessment of cloud‐free net radiative effect of dust aerosols over the Atlantic Ocean

Using eighteen months (June–August, 2000–2005) of spatially and temporally collocated Moderate Resolution Imaging Spectroradiometer (MODIS) and the Clouds and the Earth's Radiant Energy System (CERES) data from the Terra satellite over the Atlantic Ocean [10W–60W, 0–30N], we first separate the dust aerosol optical thickness at 0.55 μm (AOT) from the total column MODIS AOT. We then calculate the cloud‐free TOA net radiative effect (NRE) of dust aerosols by accounting for diurnal effects and sample biases. The cloud‐free NRE is −6.31 ± 1.16 Wm−2 and nearly twenty percent of the shortwave radiative effect (−7.75 ± 0.86 Wm−2) is cancelled by the longwave radiative effect (+1.44 ± 0.57 Wm−2) indicating the importance of the dust aerosols in the thermal portion of the electromagnetic spectrum. This is the first multi‐year satellite‐based assessment of the NRE of dust aerosols indicating the importance of both the shortwave and longwave radiative effects of dust aerosols over the oceans unlike anthropogenic aerosols that have negligible TOA longwave radiative forcing effects.

Desert dust aerosol columnar properties over ocean and continental Africa from Lidar in‐Space Technology Experiment (LITE) and Meteosat synergy

The new generation of spaceborne backscatter lidar systems, prefigured by the Lidar in‐Space Technology Experiment (LITE) mission in September 1994, will give new insight on the vertical distribution of both aerosols and clouds in the atmosphere. This is especially of importance for aerosols over land, where retrievals from passive sensors are known to be more difficult because of the surface contribution. Here we analyze mineral dust aerosol transport events through a new approach coupling the active LITE and passive Meteosat‐5 spaceborne observations. The Meteosat‐derived aerosol optical thickness at 550 nm is shown to be a good boundary condition for the lidar inversion in order to retrieve both the aerosol backscatter to extinction ratio (BER) and the aerosol extinction vertical profile above the Tropical Atlantic Ocean (TAO) and the Mediterranean Sea. Sensitivity tests indicate that the aerosol scattering coefficient is retrieved within a 20% relative uncertainty. Air mass trajectories allow us to further retrieve the vertical profile of aerosol optical properties over the continent in the Saharan Heat Low (SHL) region using the BER determined over the ocean. Results confirm a large dispersion of the BER which is not attributed to errors in the method. This shows the need to account for such dispersion in the retrieval of dust aerosol optical thickness and aerosol impact on the earth radiative budget. The coupling between LITE and Meteosat‐5 made here is shown to be interesting for an improvement of the direct dust aerosol forcing, and results should be improved by the CALIOP‐MODIS synergy.

Diurnal variability of dust aerosol optical thickness and Angström exponent over dust source regions in China

Using 22 months of Sunphotometer Aerosol Optical Thickness (AOT) data collected near the Taklamakan and Gobi dust source regions (Dunhuang, 40.09°N, 94.41°E) in China; we examine the diurnal and seasonal change of dust aerosol properties. Most dust events are during the spring through early summer months with a season‐invariant diurnal change of more than ±10% for AOT and ±30% for Angström exponent, with larger AOT and smaller Angström exponent values late in the afternoon. These values are much larger when compared to recent studies that have reported a much smaller (±5%) diurnal variability of dust AOT over various AERONET sites where dust is a major contributor to AOT. The differences are largely due to the geographical locations and meteorological conditions and such large diurnal changes of aerosol properties at or near dust source regions may be significant enough for consideration in regional radiative forcing, air quality and numerical modeling studies.

Analysis of measurements of Saharan dust by airborne and ground‐based remote sensing methods during the Puerto Rico Dust Experiment (PRIDE)

For 26 days in mid‐June and July 2000, a research group comprised of U.S. Navy, NASA, and university scientists conducted the Puerto Rico Dust Experiment (PRIDE). In this paper we give a brief overview of mean meteorological conditions during the study. We focus on our findings on African dust transported into the Caribbean utilizing a Navajo aircraft and AERONET Sun photometer data. During the study midvisible aerosol optical thickness (AOT) in Puerto Rico averaged 0.25, with a maximum >0.5 and with clean marine periods of ∼0.08. Dust AOTs near the coast of Africa (Cape Verde Islands and Dakar) averaged ∼0.4, 30% less than previous years. By analyzing dust vertical profiles in addition to supplemental meteorology and MPLNET lidar data we found that dust transport cannot be easily categorized into any particular conceptual model. Toward the end of the study period, the vertical distribution of dust was similar to the commonly assumed Saharan Air Layer (SAL) transport. During the early periods of the study, dust had the highest concentrations in the marine and convective boundary layers with only a weak dust layer in the SAL being present, a state usually associated with wintertime transport patterns. We corroborate the findings of Maring et al. [2003] that in most cases, there was an unexpected lack of vertical stratification of dust particle size. We systematically analyze processes that may impact dust vertical distribution and speculate that dust vertical distribution predominately influenced by flow patterns over Africa and differential advection coupled with fair weather cloud entrainment, mixing by easterly waves, and regional subsidence.

GOES 8 retrieval of dust aerosol optical thickness over the Atlantic Ocean during PRIDE

Using 30 days of half‐hourly, high temporal resolution GOES 8 imager data and radiative transfer calculations, dust aerosol optical thickness (AOT) was retrieved over the Atlantic Ocean (14°N ∼ 26°N, 73°W–63°W) during the Puerto Rico Dust Experiment (PRIDE). Dust aerosol size distributions and complex index of refraction inferred from ground‐based measurements (1.53–0.0015i at 0.55 μm), which were used in Mie calculations and a plane‐parallel discrete ordinate radiative transfer model (DISORT) to compute look up tables for AOT retrievals. Using a combination of spectral, spatial, and temporal tests, a dust detection algorithm was developed from the GOES 8 imager data. The degradation of the signal response relative to the prelaunched calibration of the GOES 8 visible channel was 39% in July 2000 and the GOES 8 AOT detection limit was estimated to be 0.04 in AOT (0.67 μm). The satellite‐retrieved AOT were then compared with AOT values derived from ground‐based Sun photometer (SP) sites. The comparison showed that GOES 8 retrieved AOT are in good agreement with the SP derived values, with linear correlation coefficient of 0.91 and 0.80 for the two sites. The GOES 8 monthly mean 0.67 μm AOT (0.19 ± 0.13, 0.22 ± 0.12) over the two SP sites matched the monthly mean SP AOT values (0.23 ± 0.13, 0.22 ± 0.10). The linear correlation between the GOES 8 retrieved AOT and the aircraft derived values from particle probe data and airborne Sun photometer AATS‐6 measurements were 0.88 and 0.83, respectively. Besides the uncertainties from the nonspherical effect of dust aerosols, sensitivity studies showed that the uncertainties (Δτ) of the GOES 8 retrieved AOT values were mainly from the uncertainties due to the imaginary part of refractive index (Δτ = ±0.05) and surface reflectance [Δτ = ±(0.02 ∼ 0.04)]. This paper demonstrates the application of geostationary satellites to detect and retrieve dust AOT even at low to moderate AOTs. The GOES 8 imager with high temporal resolutions also captures aerosol diurnal variation in this study that can further reduce the uncertainties in the current aerosol forcing estimations caused by the high temporal variations of AOT, thereby playing a complementary role with global AOT retrievals from polar orbiting satellites.