CL4.27

This study investigates the underlying atmospheric dynamics associated with intense dust storms in summer 2018 over the Arabian Peninsula (AP); a major dust source at global scale. It reports, for the first time, on the formation of cyclone over the Empty Quarter Desert as important mechanism for intense dust storms over this source region. The dust direct and semi-direct radiative forcings are observed, for the first time over this source region, using high-resolution in-situ and CERES-SYN satellite observational data. The three-dimensional structure and evolution of the dust storms are inferred from state-of-the-art satellite products such as SEVIRI, AEROIASI and CALIPSO. The dynamics and thermodynamics of the boundary layer during this event are thoroughly analyzed using ERA5 reanalysis and ground based observations.

We found that a large dust storm by Shamal winds led up, through radiative forcing, to cyclone development over the Empty Quarter Desert, subsequent dust emissions, development of convective clouds and rain. The cyclogenesis over this region initiated a second intense dust storm which developed and impacted the AP for 3 consecutive days. The uplifted dust by the cyclone reached 5 km in altitude and altered the radiative budget at the surface, inducing both significant warming during night and cooling during day. The dust load uplifted by the cyclone was estimated by the mesoscale model Meso-NH to be in the order of 20 Tg, and the associated aerosol optical depth was higher than 3. The model simulates reasonably the radiative impact of the dust in the shortwave but highly underestimated its impact in the LW.

Our study stresses the importance of the dust radiative forcing in the longwave and that it should be accurately accounted for in models to properly represent the impact of dust on the Earth system especially near source areas. Missing the warming effect of dust aerosols would impact both the weather and air quality forecast, and the regional climate projections.

These results were published in November 2020 in the journal Atmospheric Research doi.org/10.1016/j.atmosres.2020.105364.

How to cite: Francis, D., Chaboureau, J.-P., Nelli, N., Cuesta, J., and Alshamsi, N.: Summertime dust storms over the Arabian Peninsula and impacts on radiation and atmospheric circulation., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-439, https://doi.org/10.5194/egusphere-egu21-439, 2021.

Atmospheric aerosols considered as one of the main concerns in the ongoing climate change. Meteorological changes have a significant role in the inter-decadal Aerosols variation. In this talk, long term (2000-2019) aerosol optical depth (AOD) and metrological factors data from the reanalysis-based Modern Era Retrospective Analysis for Research and Applications (MERRA-2) were used provide deep insight into the relationship between meteorological factors and AOD variability over Iran during the dusty season (MJJA: May, June, July, and August). Prior to regression analyses, Iran was divided into three parts based on the climatological conditions (west part: dusty area, north part: wetter, and center: dry area). Using a multiple linear regression model, AOD variability over Iran was significantly linked to sea level pressure and soil moisture. Winter surface temperatures and relative humidity are the main contributors to MJJA AOD variability over the western and northern parts.  AOD was not affected by precipitation. Our results suggested that climatic variations strongly affect the dust cycle, with a strong dependence on wintertime conditions in the region.

How to cite: Yousefi, R., Wang, F., Ge, Q., and Shaheen, A.: Contribution of meteorological factors to AOD variability during the dusty season over Iran, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1349, https://doi.org/10.5194/egusphere-egu21-1349, 2021.

Dust storms are frequent phenomenon in arid and semi arid regions of Iran which cover near 60 percent of the entire country in the center of Iran. Due to geographic and climatic conditions of prolonged dry conditions as well as poor land use management, dust storms occur in almost all seasons across the region. Drought is a major fator affecting the likelihood of dust storm occurrence across arid regions of Iran. We develop copula functions to investigate the effect of drought on dust storm frequency.The standardized precipitation Index (SPI) was caluclated and drought condition was defined based on SPI< -0.5. Dought severity and duration for each drought event were ca;culated and the number of dust days in each drought event ws also identified. The Archimedean copula families shoed that the probability of dust occurrence has a significant relationship to extreme drought conditions. The joint probability is then used to derive the joint return period of dust storms in relation to drought condition.

How to cite: modarres, R. and Mohit Esfahani, P.: Dust storm frequency in relation to extreme droughts in arid regions of Iran, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1810, https://doi.org/10.5194/egusphere-egu21-1810, 2021.

Several studies have associated high dust years over South Asia to warming of the central or eastern equatorial Pacific Ocean (El Nino conditions) and the resulting weakening of the summer monsoon. Using satellite aerosol data for 2001-2018, we show that there has been a departure from this relation since the second decade of the 21st century with the North Atlantic Ocean emerging as a major driver of interannual variability of dust over South Asia. This change in relation coincides with the end of the global warming hiatus and a shift towards persistent positive phase of the winter North Atlantic Oscillation (NAO). Positive phase of the NAO induces cold phase of the spring/summer North Atlantic sea surface temperature (SST) tripole pattern. We show here that high dust activity during 2011-2018 is associated with positive SST anomaly over the mid-latitude North Atlantic and negative SST anomaly over the sub-tropical North Atlantic: the two southern arms of the SST tripole pattern. Interestingly, the relation between NAO and these two southern arms of the SST tripole has undergone changes in recent years, which has impacted the South Asian monsoon. The result is general drying over South Asia and an increase in the strength of the dust-carrying northwesterlies. Simulations with the Community Earth System Model (CESM) shows that SST tripole-like anomalies recorded during 2011-2018 over the North Atlantic can generate mid-latitude wave train that weakens the South Asian monsoon circulation, leads to surface high pressure anomalies and increase in dust emission and transport over northwest India and Pakistan. Most of the increase in the dust load can be attributed to enhanced transport at 800 hPa pressure level during May-June, which can lead to ~40-50% increase in dust concentrations at this level.

How to cite: Banerjee, P., Satheesh, S. K., and Krishna Moorthy, K.: Recent dust variability over South Asia controlled by North Atlantic SST anomalies, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12649, https://doi.org/10.5194/egusphere-egu21-12649, 2021.

Desert dust is the most abundant aerosol by mass residing in the atmosphere. It plays a key role in the Earth’s system by influencing the radiation balance, by affecting cloud formation and cloud chemistry, and by acting as a fertilizer for the growth of phytoplankton and for soil through its deposition over the ocean and land.

Due to the nature of its emission and transport, atmospheric dust concentrations are highly variable in space and time and, therefore, require a continuous monitoring by measurements. Dust observations are best exploited by being combined with model simulations for the production of analyses and reanalyses, i.e., complete and consistent four dimensional reconstructions of the atmosphere. Existing aerosol (and dust) reanalyses for the global domain have been produced by total aerosol constraint and at relatively coarse spatial resolution, while regional reanalyses exclude some of the regions containing the major sources of desert dust in Northern Africa and the Middle East.

We present here a 10-year reanalysis data set of desert dust at a horizontal resolution of 0.1°, and which covers the domain of Northern Africa, the Middle East and Europe. The reanalysis has been produced by assimilating in the MONARCH chemical weather prediction system (Di Tomaso et al., 2017) satellite retrievals over dust source regions with specific dust observational constraint (Ginoux et al., 2012; Pu and Ginoux, 2016).

Furthermore, we describe its evaluation in terms of data assimilation diagnostics and comparison against independent observations. Statistics of analysis departures from assimilated observations prove the consistency of the data assimilation system showing that the analysis is closer to the observations than the first-guess. Temporal mean of analysis increments show that the assimilation led to an overall reduction of dust with pattern of systematic corrections that vary with the seasons, and can be linked primarily to misrepresentation of source strength.

Independent evaluation of the analysis with AERONET observations indicates that the reanalysis data set is highly accurate, and provides therefore a reliable historical record of atmospheric desert dust concentrations in a recent decade.

References

Di Tomaso, E., Schutgens, N. A. J., Jorba, O., and Pérez García-Pando, C. (2017): Assimilation of MODIS Dark Target and Deep Blue observations in the dust aerosol component of NMMB-MONARCH version 1.0, Geosci. Model Dev., 10, 1107-1129.

Ginoux, P., Prospero, J. M., Gill, T. E., Hsu, N. C. and Zhao, M. (2012): Global-Scale Attribution of Anthropogenic and Natural Dust Sources and Their Emission Rates Based on Modis Deep Blue Aerosol Products. Rev Geophys 50.

Pu, B., and Ginoux, P. (2016). The impact of the Pacific Decadal Oscillation on springtime dust activity in Syria. Atmospheric Chemistry and Physics, 16(21), 13431-13448.

Acknowledgements

The authors acknowledge the DustClim project which is part of ERA4CS, an ERA-NET initiated by JPI Climate, and funded by FORMAS (SE), DLR (DE), BMWFW (AT), IFD (DK), MINECO (ES), ANR (FR) with co-funding by the European Union (435690462); PRACE (eDUST/eFRAGMENT1/eFRAGMENT2), RES (AECT-2020-3-0013/AECT-2019-3-0001/AECT-2020-1-0007) for awarding access to MareNostrum at BSC and for technical support.

How to cite: Di Tomaso, E., Escribano, J., Ginoux, P., Basart, S., Macchia, F., Barnaba, F., Castrillo, M., Formenti, P., Jorba, O., Mona, L., Montané, G., Mytilinaios, M., Obiso, V., Schutgens, N., Votsis, A., Werner, E., and PérezGarcía-Pando, C.: A 10-year regional reanalysis of desert dust aerosol at high spatial resolution, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11644, https://doi.org/10.5194/egusphere-egu21-11644, 2021.

The magnitude of the climate forcing associated with mineral dust aerosols remains uncertain, due in part to a lack of direct observations on dust source areas. While modeling and satellite studies provide spatially extensive constraints, these studies must be supported by surface-validating, in situ dust monitoring. Our study focuses on Patagonia, the main source of dust to the southern oceans (>45ºS), a region of low biological productivity potentially susceptible to increased micronutrient fertilization through dust deposition and associated atmosphere‐to‐ocean CO₂ flux. This mechanism is hypothesized to have contributed significantly to the last interglacial‐to‐glacial climatic transition. However, the dust‐phytoplankton biomass connection remains contentious for the present‐day climate system.

We analysed multi-year time series of surface dust-related visibility reduction (DRVR) and dust mass sampling at four downwind coastal monitoring sites, along with key meteorological time series at these same sites. We find that local DRVR across Patagonia is partly controlled by long-term water deficit (i.e., months), while same-day conditions play a smaller role, reflective of water retention properties of fine-grained dust-emitting soils in low-moisture conditions. This conclusion is supported independently by reanalysis data showing that large-scale dust outbreaks are usually associated with anomalously high long-term water deficit. By combining visibility data with surface dust sampling we were able to derive emission rates associated with regional patches of dust-emitting surfaces and test the skill of dust emission schemes to simulate dust activity close to the sources. Our results suggest that the inclusion of long-term hydrologic soil balance may improve the performance of dust emission schemes in climate models.

We also analyzed the impact of southernmost Patagonian dust emissions on southwestern Atlantic Ocean continental shelf and proximal open ocean satellite chlorophyll‐a concentration. We used the DRVR and mass flux time series of the southernmost site to model dust emission, transport, and deposition to the ocean. We then performed a dust event‐based analysis of chlorophyll‐a time series, using a novel approach by which time series are corrected for post‐depositional particle advection due to ocean currents. Finally, we performed total iron determinations, release experiments and iron solid speciation analysis in dust samples. Iron is a key micronutrient limiting phytoplankton biomass in high‐nutrient, low‐chlorophyll oceans such as offshore of the 200‐m isobath off Patagonia. We find no compelling evidence for an influence of dust as an enhancer of phytoplankton biomass either on shelf or proximal open ocean waters of the southwestern Atlantic Ocean. For open ocean waters this is consistent with a lack of source‐inherited bioavailable iron in dust samples. Future case studies addressing similar questions should concentrate on dust sources with identified high contents of bioavailable iron, particularly in the Southern Hemisphere where atmospheric processing of iron-bearing particles is weak.

How to cite: Cosentino, N., Gaiero, D., Ruiz-Etcheverry, L., Lambert, F., Bia, G., Simonella, L., Coppo, R., Torre, G., Saraceno, M., and Tur, V.: Present-day Patagonian dust emissions: Mass flux constraints, meteorological triggers and the effect on phytoplankton biomass, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-156, https://doi.org/10.5194/egusphere-egu21-156, 2021.

In this study, the Weather Research and Forecasting model coupled with chemistry (WRF-Chem) is employed to simulate a dust process in Northwest China during May 2018. The model's ability to simulate the dust process in Northwest China is firstly evaluated using various satellite-retrieved and observational data. The four-dimensional assimilation method is also used to optimize meteorological data and effectively improve the simulation of the dust process.

Fig. 1. Differences of wind field (unit: m/s) between the simulations (a-d: unassimilated; e-h: assimilated) and the observations at 03:00 UTC on 20–23 May.

The comparisons between the simulations based on five dust emission schemes within WRF-Chem and the observations show that, the Shao01 scheme overall has good performance in simulating the emission flux, the spatial pattern of source region, as well as the spatiotemporal variation of dust mass concentration, during this dust process. In comparison to Shao01, the GOCART AFWA and Shao04 schemes can also produce quite similar spatial pattern of dust source region, but tend to overestimate or underestimate dust emission and mass concentration. The Shao11 scheme fails to simulate the dust process since the importance of the fully disturbed particle size distribution is omitted. It is also noted that the GOCART scheme can well reproduce dust emission processes under weak wind erosion but underestimate dust emission flux under strong wind erosion. In addition, the GOCART scheme has produced some spurious emissions and thus blurred the distribution of dust source region.

Fig. 2. The averaged dust emission flux (unit: μg/m²/s) from the GOCART (a), GOCART AFWA (b), Shao01 (c), Shao04 (d) and Shao11 (e) schemes during 17–23 May.

Fig. 3. Variations of daily (a) and hourly (b) surface PM₁₀ concentrations, friction velocity (c) and 10 m wind speed (d) at the Turpan station during 17–23 May.

Northwest China is covered by mountains, basins, deserts and other landforms, thus the complex terrain is one of the key factors to influence the dust process over the region. Our study shows that after being emitted, the airborne dust transported toward the east and west. The dust to the east was diffused rapidly, but the portion toward the west was blocked and accumulated at the edges of the mountains and thus produced dust weather characterized by high dust concentration and long lifetime. The dust accumulated at the edges of the mountains could reach an altitude of more than 6 km due to wind and thermal effect, and finally arrive at Tibetan Plateau and eastern China.

Fig. 4. The simulated (with Shao01 scheme) dust (unit: μg /m³) transport path (a-c) during 20–23 May and vertical profiles of 38.5°N (d) and 85°E (g) at 05:00 UTC on 21 May, 36°N (e) and 95° E (h) at 06:00 UTC on 22 May and 36°N (f) and 98°E (i) at 05:00 UTC on 23 May.

How to cite: Zhao, J., Ma, X., Wu, S., and Sha, T.: Dust emission and transport in Northwest China: WRF-Chem simulation and comparisons with multi-sensor observations , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4677, https://doi.org/10.5194/egusphere-egu21-4677, 2021.

Mineral dust plays a significant role in climate change and air quality, but large uncertainties remain in terms of dust emission prediction. In this study, we improved the treatments of dust emission process in a Global 3-D Chemical Transport model (GEOS-Chem) v12.6.0, by incorporating the geographical variation of aerodynamic roughness length (Z₀), smooth roughness length (Z_0s), soil texture, introducing Owen effect and Lu and Shao (1999) formulation of sandblasting efficiency α. To investigate the impact of the modifications incorporated in the model, several sensitivity simulations were performed for a severe dust storm during March 27, 2015 to April 2, 2015 over northern China. Results show that simulated threshold friction velocity is very sensitive to the updated Z₀ and Z_0s field, with the relative difference ranging from 10% to 60% compared to the original model with uniform value. An inclusion of Owen effect leads to an increase in surface friction velocity, which mainly occurs in the arid and semi-arid regions of northwest China. The substitution of fixed value of α assumed in original scheme with one varying with friction velocity and soil texture based on observations reduces α by 50% on average, especially over regions with sand texture. Comparisons of sensitivity simulations and measurements show that the revised scheme with the implement of updates provides more realistic threshold friction velocities and PM₁₀ mass concentrations. The performance of the improved model has been evaluated against surface PM₁₀ observations as well as MODIS aerosol optical depth (AOD) values, showing that the spatial and temporal variation of mineral dust are better captured by the revised scheme. Due to the inclusion of the improvement, average PM₁₀ concentrations at observational sites are more comparable to the observations, and the average mean bias (MB) and normalized mean bias (NMB) values are reduced from -196.29μg m^-3 and -52.79% to -47.72μg m^-3 and -22.46% respectively. Our study suggests that the erodibility factor, sandblasting efficiency and soil-related properties which are simply assumed in the empirical scheme may lack physical mechanism and spatial-temporal representative. Further study and measurements should be conducted to obtain more realistic and detailed map of these parameters in order to improve dust representation in the model. 

How to cite: Tian, R., Ma, X., and Zhao, J.: A revised mineral dust emission scheme in GEOS-Chem: improvements in dust simulations over China, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9167, https://doi.org/10.5194/egusphere-egu21-9167, 2021.