Saharan Dust

by Massimo Del Guasta and Alessio Baglioni - INO

Animation of GOES/Meteosat imagery of Saharan dust/dry air (split window IR technique)
Courtesy of Jason Dunion UW/CIMSS-NOAA/HRD

The plot evidences regions of mid-level dry air AND dust over land: this tecnique tracks dry and or dusty air in the low to mid-levels of the atmosphere. That means that sometimes it tracks the dust, while in other instances, it tracks mid-latitude dry air intrusions. Fortunately, Saharan Dust outbreaks often tend to be both dry and dusty. I find it very useful to look at these animations and HYPLIT trajectories to differentiate the air masses...

this is 15 min imagery...the animation shows the last 48 hours of data

Recent Saharan Dust events Florence (Italy) (2008-2011)

Here is a list of 2008-2011 Saharan Dust events observed by means of the INO LIDAR with 5 min time resolution:

(Click on the dates to get the 24-hours LIDAR color plot)

SAHARAN DUST: 2008

May 25-28
June 1, 24-25, 27-28
July 6-7, 12-13
August 2-3, 4-5
September 5-6, 9-12
October 15-17
November 2-3

SAHARAN DUST: 2009

April 1, 12,26-27
May 7, 11-14, 18, 27-28
June 8-9, 15-17
July 14-19, 22-24, 25-26, 27, 30
August 1-3, 10 ,10,21,22,24,26,27
October 3-4

SAHARAN DUST: 2010

April 7-11
June 6-16
July 12-17, 23
August 2,11-14,19,23-26
November 5-6

SAHARAN DUST: 2011

March 24-26
April 7-12
May 10-12
June 6-7
August 3-4,7,19,26-27
September 2-4
(Last update: 1 Marzo 2012)

Examples of Saharan Dust:
15-17 July 2009

In this HYSPLIT animation, the origin of airmasses at an altitude of 2000-4000 m is shown for the entire period from 15 to 17 July

In this MODIS NAAPS animation,. the (yellow-green) dust plume from Northern Africa is shown for the entire period from 15 to 17 July

On those days the LIDAR showed Saharan dust at an altitude of between 2000 and 3500 m. The relatively high depolarization of these aerosols, which is shown in the lower plot (depolarization up to 12%), made it very easy to distinguish between PBL and desertic aerosols.

PS Aerosol backscatter is represented in arbitrary units.

This plot, which was obtained from GRIB data, shows the low humidity at an altitude of ~ 1500 m in the Mediterranean basin at the time of the maximum LIDAR evidence of Saharan dust

Examples of Saharan Dust + Forest Fires:
22-24 July 2009

HYSPLIT trajectory analysis shows the origin of airmasses: from Africa, on 22-23 July, from the Atlantic, on July 24.

NAAPS MODIS shows the same phenomenon: dust (yellow-green) from N-Africa on 22-23 July, replaced by cleaner air on July 24. The forest fires are not shown here. Please see the 22-24 July INO Forest Fire page!

Desert dust reached our LIDAR site at an altitude of 2000-3000 m on July 22-23, as pointed out by a depolarization of ~10%. On July 23, the dust layer was enhanced by the arrival of ash from forest fires in Sardinia and Corsica. The high depolarization (up to 15%) was probably caused by fresh ash from the fires. low-depolarization wildfire aerosols were also observed in the early morning of July 24 up to an altitude of 3000 m.

....Please see our INO Forest Fire page for more details regarding the signal from forest fires

Mixing of Saharan Dust with Urban aerosols:

Two cases in which the the Saharan storm, the PBL structure and their mutual interaction were demonstrated thanks to High pressure conditions

A case of the absence of mixing of Saharan dusts with mixed-layer aerosols. In this case, the Saharan dust layer at an altitude of 2500-3000 m was not scavenged by the mixed layer developed above Florence. The mixed layer extended up to just 2000 m, in accordance with the aerosol backscatter plot.

A case of mixing of Saharan dusts with mixed layer aerosols. In this case, the Saharan dust layer at an altitude of 2000-3000 m was scavenged by the mixed layer on 6 September 2008. The mixed layer (ML) extended up to just 2500 m, in accordance with the aerosol backscatter plot. A mixing of Saharan dust with mixed layer aerosols was evidenced after 12 AM by an increased depolarization within the ML. The increased depolarization was particularly evident when observing the low depolarization of the mixed layer of the previous day (5 September). ML aerosols in Florence generally show a depolarization of less than 5%

LEGENDA: The typical daytime evolution of the atmospheric boundary layer in high pressure conditions over land

(.....copied from http://lidar.ssec.wisc.edu/papers/akp_thes/node6.htm)

The solar heating causes thermal plumes to rise, transporting moisture, heat and aerosols. The plumes rise and expand adiabatically until a thermodynamic equilibrium is reached at the top of the atmospheric boundary layer. The moisture transferred by the thermal plumes forms convective clouds. Drier air from the free atmosphere penetrates down, replacing rising air parcels. The part of the troposphere between the highest thermal plume tops and deepest parts of the sinking free air is called the entrainment zone. The convective air motions generate intense turbulent mixing. This tends to generate a mixed layer, which has potential temperature and humidity nearly constant with height. When buoyant turbulence generation dominates the mixed layer, it is called a convective boundary layer (CBL). The lowest part of the ABL is called the surface layer. In windy conditions, the surface layer is characterized by a strong wind shear caused by friction.
The boundary layer from sunset to sunrise is called the nocturnal boundary layer. It is often characterized by a stable layer, which forms when the solar heating ends and the radiative cooling and surface friction stabilize the lowest part of the ABL. Above that, the remnants of the daytime CBL form a residual layer.

Schematic fair-weather atmospheric boundary layer structure over land.
Under high pressure, divergence of air masses shallows the boundary layer. Typically, only fair-weather cumulus clouds are present. The analysis of the boundary layer structure is not usually that straightforward in the case of low pressure. The air parcels converge in low pressure in connection with updrafts, which transfer boundary layer air parcels high above the ground. The clouds may then grow to the top of the troposphere. This leads to extensive variations in the local boundary layer top. Thus, it becomes difficult to define a larger scale boundary layer depth.

Massimo Del Guasta - National Institute of Optics (INO) - National Research Council | Via Madonna del Piano, 10 - 50019 Sesto Fiorentino - Firenze, Italy | Tel (office): +39-055-5226423 - Tel (laboratory) +39-055-5226424 | Email: Massimo Del Guasta | Web: www.ino.it | sito ottimizzato per una risoluzione minima di 1024x768 e firefox