The dust flux is calculated by measuring the rate in which dust passes through a given window.
This calculation can be understood in a few steps.
First, the optical thickness is measured, for the dust aerosol particles which are checked.
The optical thickness is a parameter from which a scientist can calculate the size of an aerosol particle and the concentration of a group of particles. The result together with the knowledge of the dust caring velocity – the wind speed – determines the mass of a dust column.
Then, measurements are made again to determine how long was the dust emitted from it`s source in the Saharan desert.
These two parameters determine the dust rate of each dust storm that has been checked: how much dust per second was emitted.
Now in order to find the flux, all that is left is to pass the dust rate that was found through a fixed sized window within a fixed time-frame (a day, a month, a year etc.).The results are the statistical and numerical knowledge of the amount of dust which is transported and deposited over the Atlantic.
In this study dust transported from Africa was used at 15°W is calculated by applying the monthly average westward wind speed, W(m/s), to the monthly average dust concentration, M du (g/m 2 ), and the longitudinal length, L (m), of the segment through which the flux is being computed.
The dust particles are transported westward across the Atlantic Ocean by the middle level
easterly jet and sometimes North by the anticyclone over the Azores or Canaries Islands. The latitudinal variation of the dust belt is controlled by the movement of the West African midtropospheric jet which occupies its northernmost position (20°N) in the summer.
Click on image to view 2 different figures:
1. AOT - Aerosol optical thickness (see color bar on the right) as function of time (vertical axis),
and latitude . Annual dust transport westward from Africa to the Caribbean
and deposition in the Atlantic Ocean, observed as a reduction in the optical thickness.
2. Fine Fraction - high dust emissions in May -September, with its maximum moving North from 7°N
in February to 20°N in September. Contour line shows the value for optical thickness of 0.5.
This figure shows the fraction of the optical thickness attributed to the fine aerosol. Contour lines
of the fine fraction of 0.5 (solid line) corresponding to dust and 0.3 (dashed lines) corresponding to
maritime aerosol are dawn. Note that the maritime air at 20°-40°S has fine fraction of 0.1-0.5
(average of 0.3±0.1). Dust fine fraction is found at lat of 20°N in the May-Aug. as 0.5±0.05.
The figures are averaged over 15°-20°W respectively.
Aerosol optical thickness (see color bar on the right) as function of time (vertical axis), and longitude. The figure is averaged over 5°-20°N.
It shows the annual dust transport westward from Africa to the Caribbean
and deposition in the Atlantic Ocean, observed as a reduction in the optical thickness.
The
images are constructed from monthly average data on the MODIS on line web site:
http://lake.nascom.nasa.gov/movas/
