Strong Dust Outbreak Research Articles

Synergistic Use of Remote Sensing and Modeling to Assess an Anomalously High Chlorophyll-a Event during Summer 2015 in the South Central Red Sea

An anomalously high chlorophyll-a (Chl-a) event (>2 mg/m3) during June 2015 in the South Central Red Sea (17.5° to 22°N, 37° to 42°E) was observed using Moderate Resolution Imaging Spectroradiometer (MODIS) data from the Terra and Aqua satellite platforms. This differs from the low Chl-a values (<0.5 mg/m3) usually encountered over the same region during summertime. To assess this anomaly and possible causes, we used a wide range of oceanographical and meteorological datasets, including Chl-a concentrations, sea surface temperature (SST), sea surface height (SSH), mixed layer depth (MLD), ocean current velocity and aerosol optical depth (AOD) obtained from different sensors and models. Findings confirmed this anomalous behavior in the spatial domain using Hovmöller data analysis techniques, while a time series analysis addressed monthly and daily variability. Our analysis suggests that a combination of factors controlling nutrient supply contributed to the anomalous phytoplankton growth. These factors include horizontal transfer of upwelling water through eddy circulation and possible mineral fertilization from atmospheric dust deposition. Coral reefs might have provided extra nutrient supply, yet this is out of the scope of our analysis. We thought that dust deposition from a coastal dust jet event in late June, coinciding with the phytoplankton blooms in the area under investigation, might have also contributed as shown by our AOD findings. However, a lag cross correlation showed a two- month lag between strong dust outbreak and the high Chl-a anomaly. The high Chl-a concentration at the edge of the eddy emphasizes the importance of horizontal advection in fertilizing oligotrophic (nutrient poor) Red Sea waters.

The footprints of Saharan air layer and lightning on the formation of tropical depressions over the eastern Atlantic Ocean

The roles of the Saharan Air Layer (SAL) and lightning during genesis of Tropical Depression (TD) 8 (2006) and TD 12 (2010) were investigated in relation to the interaction of the dust outbreaks with each system and their surrounding environment. This study applied data collected from the 2006 NASA African Monsoon Multidisciplinary Analysis and 2010 Genesis and Rapid Intensification Processes projects. Satellite observations from METEOSAT and Moderate Resolution Imaging Spectroradiometer (MODIS)—Aerosol Optical Depth (AOD) were also employed for the study of the dust content. Lightning activity data from the Met Office Arrival Time Difference (ATD) system were used as another parameter to correlate moist convective overturning and a sign of cyclone formation. The AOD and lightning analysis for TD 8 demonstrated the time-lag connection through their positive contribution to TC-genesis. TD 12 developed without strong dust outbreak, but with lower wind shear (2 m s−1) and an organized Mesoscale Convective System (MCS). Overall, the results from the combination of various data analyses in this study support the fact that both systems developed under either strong or weak dust conditions. From these two cases, the location (i.e., the target area) of strong versus weak dust outbreaks, in association with lightning, were essential interactions that impacted TC-genesis. While our dust footprints hypothesis applied under strong dust conditions (i.e., TD 8), other factors (e.g., vertical wind shear, pre-existing vortex and trough location, thermodynamics) need to be evaluated as well. The results from this study suggest that the SAL is not a determining factor that affects the formation of tropical cyclones (i.e., TD 8 and TD 12).

Record heavy mineral dust outbreaks over Korea in 2010: Two cases observed with multiwavelength aerosol/depolarization/Raman‐quartz lidar

We report on two strong events of transport of mineral dust from Central Asia across Korea. The events took place in March and November 2010. The November case is important as fall is not a typical time for strong dust outbreaks in East Asia. We observed the dust with a multiwavelength aerosol/depolarization/Raman quartz lidar. The record PM‐10 concentration of nearly 1600μg/m3 in March 2010 exceeds the record value of 1470 μg/m3 measured in Seoul in March 2002. The event in November was the strongest case of dust transport ever observed over Korea in fall. We find up to 360 μg/m3dust in heights above 250 m which is significantly different from the ground‐based PM‐10 observations.

Regional modelling of Saharan dust and biomass-burning smoke

The spatio-temporal evolution of the Saharan dust and biomass-burning plume during the SAMUM-2 field campaign in January and February 2008 is simulated at 28 km horizontal resolution with the regional model-system COSMOMUSCAT. The model performance is thoroughly tested using routine ground-based and space-borne remote sensing and local field measurements. Good agreement with the observations is found in many cases regarding transport patterns, aerosol optical thicknesses and the ratio of dust to smoke aerosol. The model also captures major features of the complex aerosol layering. Nevertheless, discrepancies in the modelled aerosol distribution occur, which are analysed in detail. The dry synoptic dynamics controlling dust uplift and transport during the dry season are well described by the model, but surface wind peaks associated with the breakdown of nocturnal low-level jets are not always reproduced. Thus, a strong dust outbreak is underestimated. While dust emission modelling is a priori more challenging, since strength and placement of dust sources depend on on-line computed winds, considerable inaccuracies also arise in observation-based estimates of biomass-burning emissions. They are caused by cloud and spatial errors of satellite fire products and uncertainties in fire emission parameters, and can lead to unrealistic model results of smoke transport.

Response of the eastern subtropical Atlantic SST to Saharan dust: A modeling and observational study

Retrievals of aerosol optical depth (AOD) from MODIS, and of sea surface temperature (SST) from TMI are analyzed jointly with the output of a numerical model for the period 2000–2006 to determine the impact of Saharan dust on the eastern subtropical North Atlantic SST. Simultaneously with, or shortly after strong dust outbreaks, a decrease in SST of 0.2° to 0.4°C can be observed in the microwave SST data set, which is consistent with an independent estimate of SST decrease simulated here by a local mixed layer model. However, low wind conditions and a shallow mixed layer are required to reach this response, and it is therefore unlikely that a clear response of SST to dust lasting more than a few days can be seen in the microwave SST observations. An inspection of microwave SST observations suggests that about 30% of SST variance could be explained by dust‐induced cooling in our study region that is not represented in existing AVHRR SST fields nor represented in reanalysis centers‐provided surface heat fluxes. On longer time scales, a comparison between observed SST fields and simulated SST, using an eddy‐permitting model of the North Atlantic, suggests a cooling of about 0.5°C on the local SST on sub‐seasonal to interannual time scales which is significantly correlated and consistent with a dust‐induced cooling. However, while supportive of the hypothesis that Saharan dust lead to a reduction in SST, the eddy‐resolving model results are not by themselves conclusive. Moreover, the effects of dust‐induced cooling on simulations of the ocean circulation, on atmospheric forecasts and on climate simulations remains to be investigated in future studies.

Aerosol Characterization and Direct Radiative Forcing Assessment over the Ocean. Part II: Application to Test Cases and Validation

Abstract A method based on the synergistic use of low earth orbit and geostationary earth orbit satellite data for aerosol-type characterization and aerosol optical thickness (AOT: τa) retrieval and monitoring over the ocean is presented in Part I of this paper. The method is now applied to a strong dust outbreak over the Atlantic Ocean in June 1997 and to two other relevant transport events of biomass burning and desert dust aerosol that occurred in 2000 over the Atlantic and Indian Oceans, respectively. The retrievals of the aerosol optical properties are checked against retrievals from sun and sky radiance measurements from the ground-based Aerosol Robotic Network (AERONET). The single-scattering albedo values obtained from AERONET are always within the error bars presented for Global Ozone Monitoring Experiment (GOME) retrievals, resulting in differences lower than 0.041. The retrieved AOT values are compared with the independent space–time-collocated measurements from the AERONET, as well as to the satellite aerosol official products of the Polarization and Directionality of the Earth Reflectances (POLDER) and the Moderate Resolution Imaging Spectroradiometer (MODIS). A first estimate of the AOT accuracy derived from comparisons with AERONET data leads to ±0.02 ± 0.22τa when all AOT values are retained or to ±0.02 ± 0.16τa for aerosol transport events (AOT > 0.4). The upwelling flux at the top of the atmosphere (TOA) was computed with radiative transfer calculations and used to estimate the TOA direct shortwave aerosol radiative forcing; a comparison with space–time-collocated measurements from the Clouds and the Earth's Radiant Energy System (CERES) TOA flux product was also done. It was found that more than 90% of the values differ from CERES fluxes by less than ±15%.

Radiative properties and direct effect of Saharan dust measured by the C‐130 aircraft during Saharan Dust Experiment (SHADE): 2. Terrestrial spectrum

In situ measurements of terrestrial radiation from the C‐130 aircraft during the Saharan Dust Experiment (SHADE) are used to quantify the effect of a strong dust outbreak on radiance and brightness temperatures. The dust gives a distinct spectral signature in upwelling and downwelling terrestrial radiation when high spectral resolution data for a dusty day is compared to data from a clear day. A radiative transfer model is used together with a size distribution retrieved from Sun photometers and atmospheric profiles from dropsondes to simulate the radiance data and provide a constraint on the refractive indices of Saharan dust in the terrestrial part of the spectrum. The degree of agreement between observed and simulated brightness temperatures is dominated by the choice of refractive index, the mass loading, and the altitude of the dust layer. The uncertainties in size distribution appear to have less of an effect so long as large particles (radius greater than 1 μm) are included. In the terrestrial spectrum the dust produced a relative warming rate of up to 0.5 K/day below the dust and a relative cooling of up to 0.5 K/day within the dust layer itself. The effect on irradiance due to this dust outbreak was a decrease in upwelling terrestrial radiation at the top of the atmosphere of 6.5 Wm−2 and an increase in downwelling terrestrial radiation at the surface of 11.5 Wm−2. The dust led to decreases in brightness temperature of 2–4 K in the window region, consistent with apparent features in the sea surface temperature retrieved from the advanced very high resolution radiometer.