Severe Dust Event Research Articles

The Relationship between Occurrence of Dust Events and Synoptic Climatological Condition in East Asia, 1999-2003

Regional differences in occurrence frequency of dust events (dust storms, sand storms and blowing dust) observed in spring (March, Apri1, and May) in 1999-2003 in the northern half of China (north of 35°N), and Mongolia are studied from the viewpoint of the relationship with strong wind and precipitation occurrence. In order to categorize the weather conditions, World Surface Data were used.Dust events are frequently observed in the regions of (I) West Tarim Basin, (II) East Tarim Basin and East Xinjiang, (III) the Hexi Corridor, (IV) southern Mongolia to Inner Mongolia, (V) a part of southern Dongbei Plain (Northeast China Plain), and (VI) the Loess Plateau. However, dust event occurrence frequency under strong wind condition, is not the same among regions and these 6 areas are divided into 2 groups as follows. In areas (I), (II), (III), and (VI), dust event frequency under strong wind conditions is high, even though the number of strong wind occurrences is small. In contrast, in areas (IV) and (V), the number of dust event occurrences is large, but the dust event frequency under strong wind conditions is low.Dust events were observed not only in arid, or semi arid region. At almost all of observatories in the study area, dust events were reported at least once in 1999-2003. However, there are areas where both the number of dust events, and dust event frequency under strong wind conditions are low, even close to arid/semi-arid land, such as Dzungaria Basin. It is attributed to high frequency of precipitation events under strong wind and land surface characteristics.We made composite maps of sea level pressure, surface air temperature, and surface wind for 10 severest “Widespread Dust Event Cluster” (WDE Clusters) by using NCEP/NCAR reanalysis data for the above 6 areas respectively. It is found that severe dust events are observed in these 6 areas on the condition that anti-cyclones develop over Kazakhstan to the Altai Mountains and western Mongolia, associated with strong cold wind over each area. In areas (I), (II), (III), and (VI), dust events occurred only under the influence of these anti-cyclones. On the other hand, in areas (IV) and (V), a clean contrast was found between these anti-cyclones and cyclones located around 47.5°N, 120°E, and the role of these cyclones seems to be important. Only in area (V), surface air temperature tends to increase at the time of the 10 strongest WDE Clusters. This is caused by southerly wind associated with the cyclone.It is suggested that the possible causes of low dust event frequencies, under strong wind condition in areas (IV) and (V), are precipitations and land surface characteristics (e.g., snow cover) caused by the cyclone development.

Seasonal variability of optical properties of aerosols in the Eastern Mediterranean

The aerosol optical properties (scattering and absorption coefficients) were investigated at two remote locations in the Eastern Mediterranean in conjunction with aerosol ion composition measurements: Finokalia in the Crete Island in Greece (March 2001–June 2002) and Erdemli in Turkey (July 1999–June 2000). Ambient light-scattering coefficient ( σ sp−532 nm ) at Finokalia had a mean value of 50±23 Mm −1 while at Erdemli this value was 90±160 Mm −1, due to a severe dust event that occurred from 17 to 19 April 2000. Scattering coefficients up to 5000 Mm −1 were encountered during the transition periods (spring and autumn) and were associated with dust storm events. During these events significant correlations were observed between dust and σ sp and mass scattering efficiencies of 0.21 and 0.96 m 2g −1 were calculated for dust for Finokalia and Erdemli, respectively. Significant correlations were also observed at both locations between non-sea-salt sulphate (nss-SO 4 2−); σ sp and mass scattering efficiencies of 5.9±1.8 and 5.7±1.4 m 2g −1 were calculated for the nss-SO 4 2− at Finokalia and Erdemli, respectively. At Finokalia absorption measurements were also performed at the same time and the mean absorption coefficient ( σ ap−565 nm ) was found to be 5.6±3.6 Mm −1. Maxima of absorption coefficient were associated with two distinct meteorological situations indicative of pollution transported from northern Europe and Saharan dust events. Saharan dust can therefore significantly contribute to both scattering and absorption of solar radiation, the latter due to its hematite content. Based on scattering and absorption measurements, an annual mean single-scattering albedo ( ω) adjusted at 550 nm of 0.89±0.04 was calculated for Finokalia. Finally, radiative forcing efficiency (RFE) over the sea at 550 nm induced by aerosols has been calculated for Finokalia. RFE follows a clear seasonal variation, with the lowest mean values during summer (−73W m −2) and the highest during winter (−30W m 2). Using aerosol optical thickness measurements in the area, we obtain radiative forcing estimates at the top of the atmosphere (TOA) ranging from −12.6 to −2.3 W m 2 for summer and winter, respectively. These values are up to five times higher than that induced by the greenhouse gases (2.4 Wm −2) but opposite in sign.