Doppler lidar studies of heat island effects on vertical mixing of aerosols during SAMUM–2

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145

139

A B S T R A C TIn the framework of the Saharan Mineral Dust Experiment (SAMUM) airborne High Spectral Resolution Lidar and in situ measurements of the particle size, aerosol mixing state and absorption coefficient were conducted. Here, the properties of mineral dust and tropical biomass burning layers in the Cape Verde region in January/February 2008 are investigated and compared with the properties of fresh dust observed in May/June 2006 close the Sahara. In the Cape Verde area, we found a complex stratification with dust layers covering the altitude range below 2 km and biomass burning layers aloft. The aerosol type of the individual layers was classified based on depolarization and lidar ratios and, in addition, on in situ measured Ångström exponents of absorptionå ap . The dust layers had a depth of 1.3 ± 0.4 km and showed a medianå ap of 3.95. The median effective diameter D eff was 2.5 μm and the dust layers over Cape Verde yielded clear signals of aging: large particles were depleted due to gravitational settling and the accumulation mode diameter was shifted towards larger sizes as a result of coagulation. The tropical biomass layers had a depth of 2.0 ± 1.1 km and were characterized by a medianå ap of 1.34. They always contained a certain amount of large dust particles and showed a median D eff of 1.1 μm and a fine mode D eff,fine of 0.33. The dust and biomass burning layers had a median aerosol optical depth (AOD) of 0.23 and 0.09, respectively. The median contributions to the AOD of the total atmospheric column below 10 km were 75 and 37%, respectively.

Section: Resultsmentioning

confidence: 81%

“…11, while the two HSRL profiles are shown in blue and green. A detailed study of the island effects is presented in Engelmann et al (2011). They find that on this day the wind near the surface came from northeast to east (i.e.…”

Section: Resultsmentioning

confidence: 99%

Microphysical and optical properties of dust and tropical biomass burning aerosol layers in the Cape Verde region—an overview of the airborne in situ and lidar measurements during SAMUM-2

Weinzierl¹,

Sauer

Esselborn

et al. 2011

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145

139

“…The dominance of smoke aerosol increases with height and leads to very low SSA of 0.66 retrieved for the 2.8‐km height level. The measurements of 25–26 January 2008 are also discussed in detail by Ansmann et al (2011), Engelmann et al (2011), and Lieke et al (2011).…”

Section: Observations and Resultsmentioning

confidence: 99%

“…The SSA decreased to 0.92–0.95 during dust‐dominated days and showed lowest values of 0.84–0.88 when the authors claim that smoke might have reached down to the surface between 6 and 9 February 2008. Downward mixing as described by Engelmann et al (2011) may have caused the episodically occurring high smoke concentrations at ground level. The imaginary part of the refractive index at 550 nm was found to be 0.02–0.03 during this period.…”

Section: Comparison To Further Samum‐2 In Situ Observationsmentioning

confidence: 96%

Optical and microphysical properties of smoke over Cape Verde inferred from multiwavelength lidar measurements

Tesche

Müller

Groß

et al. 2011

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112

104

Citing articles: 2 View citing articles Tellus (2011), 63B, 677-694 C A B S T R A C T Lidar measurements of mixed dust/smoke plumes over the tropical Atlantic ocean were carried out during the winter campaign of SAMUM-2 at Cape Verde. Profiles of backscatter and extinction coefficients, lidar ratios, and Ångström exponents related to pure biomass-burning aerosol from southern West Africa were extracted from these observations. Furthermore, these findings were used as input for an inversion algorithm to retrieve microphysical properties of pure smoke. Seven measurement days were found suitable for the procedure of aerosol-type separation and successive inversion of optical data that describe biomass-burning smoke. We inferred high smoke lidar ratios of 87 ± 17 sr at 355 nm and 79 ± 17 sr at 532 nm. Smoke lidar ratios and Ångström exponents are higher compared to the ones for the dust/smoke mixture. These numbers indicate higher absorption and smaller sizes for pure smoke particles compared to the dust/smoke mixture. Inversion of the smoke data set results in mean effective radii of 0.22 ± 0.08 µm with individual results varying between 0.10 and 0.36 µm. The single-scattering albedo for pure biomass-burning smoke was found to vary between 0.63 and 0.89 with a very low mean value of 0.75 ± 0.07. This is in good agreement with findings of airborne in situ measurements which showed values of 0.77 ± 0.03. Effective radii from the inversion were similar to the ones found for the fine mode of the in situ size distributions.

“…There are indications of a diurnal cycle with a general tendency of stronger winds during midday and weaker winds during the night and morning. This is possibly related to the deepening of the boundary layer and a sea‐breeze circulation during the day (Engelmann et al, 2011).…”

Section: Other Types Of Aerosols: Sea Salt and Biomass‐burning Smokementioning

confidence: 99%

Dust mobilization and aerosol transport from West Africa to Cape Verde—a meteorological overview of SAMUM-2

Knippertz

Tesche

Heinold

et al. 2011

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118

The second field campaign of the SAharan Mineral dUst experiMent (SAMUM‐2) was performed between 15 January and 14 February 2008 at the airport of Praia, Cape Verde, and provided valuable information to study the westward transport of Saharan dust and the mixing with biomass‐burning smoke and sea‐salt aerosol. Here lidar, meteorological, and particle measurements at Praia, together with operational analyses, trajectories, and satellite and synoptic station data are used to give an overview of the meteorological conditions and to place other SAMUM‐2 measurements into a large‐scale context. It is demonstrated that wintertime dust conditions at Cape Verde are closely related to the movement and intensification of mid‐latitude high‐pressure systems and the associated pressure gradients at their southern flanks. These cause dust emission over Mauritania, Mali, and Niger, and subsequent westward transport to Cape Verde within about 1–5 d. Dust emissions often peak around midday, suggesting a relation to daytime mixing of momentum from nocturnal low‐level jets to the surface. The dust layer over Cape Verde is usually restricted to the lowest 1.5 km of the atmosphere. During periods with near‐surface wind speeds about 5.5 m s−1, a maritime aerosol layer develops which often mixes with dust from above. On most days, the middle levels up to about 5 km additionally contain smoke that can be traced back to sources in southern West Africa. Above this layer, clean air masses are transported to Cape Verde with the westerly flow at the southern side of the subtropical jet. The penetration of extra‐tropical disturbances to low latitudes can bring troposphere‐deep westerly flow and unusually clean conditions to the region.