6 Impacts of Saharan Dust on the Marine Environment in the Area . . .
121
be in the order of pure water absorption and the scattering can be twenty times higher
than the pure seawater scattering (Stramski et al. 2004).
Atmospheric dust can disturb satellite measurements of surface wind speeds because of the attenuation of the microwave signal. Recent investigations established
that the attenuation depends on the operating satellite frequency (Comparetto 1993),
the dust particle radius, the density of dust particles, the permittivity (Altshuler 1983)
and the moisture content (e.g. Ansari and Evans 1982).
One goal of the present investigation was the study of the relationships between
Saharan dust deposition, upwelling process and phytoplankton on the basis of statistical analysis of remotely sensed data and event statistics of dust storms. Another
aim was the determination of the influence of atmospheric dust on PAR below the
water surface on the basis of an optical model along with parameterized spectral
effects. These studies were restricted to the dust effect on PAR if the dust is in the
atmosphere and not in the water column. The present investigation also contributed
to the validation of satellite microwave sensors by the quantification of the impact
of Saharan dust on satellite derived ocean surface wind speeds.
6.2 Methodology
The investigations in this chapter based on satellite observations of different sensors,
radiation measurements below and above the water surface in the area of NW Africa
and modelling of optical processes in the water column. The following remarks of
the used methods were given in detail in different papers (Ohde and Siegel 2010,
2012a, b, 2013).
6.2.1 Methods in Relation to Dust Impact on Phytoplankton
A time series of 8-day (8d) means of geophysical data of Sea-Wide Field-of-view
Sensor (SeaWiFS), microwave scatterometer SeaWinds (QuikScat) and Moderate
Resolution Imaging Spectroradiometer (MODIS) were the basis of the investigations
of the dust impact on phytoplankton, in the investigation period from 2000 to 2008
(Ohde and Siegel 2010).
Remotely sensed chlorophyll-a (Chl-a) concentration
1 was used as a proxy for
phytoplankton biomass. As a proxy for the strength of the coastal upwelling, the
satellite-derived alongshore wind stress (τ v ) was determined according to Yelland
and Taylor (1996) based on satellite wind data
2 . The dust component of the Aerosol
Optical Depth (AOD dust ) was used as a proxy for the atmospheric dust column load
3 .
1 From http://daac.gsfc.nasa.gov.
2 From http://www.remss.com.
3 From http://daac.gsfc.nasa.gov.
121
be in the order of pure water absorption and the scattering can be twenty times higher
than the pure seawater scattering (Stramski et al. 2004).
Atmospheric dust can disturb satellite measurements of surface wind speeds because of the attenuation of the microwave signal. Recent investigations established
that the attenuation depends on the operating satellite frequency (Comparetto 1993),
the dust particle radius, the density of dust particles, the permittivity (Altshuler 1983)
and the moisture content (e.g. Ansari and Evans 1982).
One goal of the present investigation was the study of the relationships between
Saharan dust deposition, upwelling process and phytoplankton on the basis of statistical analysis of remotely sensed data and event statistics of dust storms. Another
aim was the determination of the influence of atmospheric dust on PAR below the
water surface on the basis of an optical model along with parameterized spectral
effects. These studies were restricted to the dust effect on PAR if the dust is in the
atmosphere and not in the water column. The present investigation also contributed
to the validation of satellite microwave sensors by the quantification of the impact
of Saharan dust on satellite derived ocean surface wind speeds.
6.2 Methodology
The investigations in this chapter based on satellite observations of different sensors,
radiation measurements below and above the water surface in the area of NW Africa
and modelling of optical processes in the water column. The following remarks of
the used methods were given in detail in different papers (Ohde and Siegel 2010,
2012a, b, 2013).
6.2.1 Methods in Relation to Dust Impact on Phytoplankton
A time series of 8-day (8d) means of geophysical data of Sea-Wide Field-of-view
Sensor (SeaWiFS), microwave scatterometer SeaWinds (QuikScat) and Moderate
Resolution Imaging Spectroradiometer (MODIS) were the basis of the investigations
of the dust impact on phytoplankton, in the investigation period from 2000 to 2008
(Ohde and Siegel 2010).
Remotely sensed chlorophyll-a (Chl-a) concentration
1 was used as a proxy for
phytoplankton biomass. As a proxy for the strength of the coastal upwelling, the
satellite-derived alongshore wind stress (τ v ) was determined according to Yelland
and Taylor (1996) based on satellite wind data
2 . The dust component of the Aerosol
Optical Depth (AOD dust ) was used as a proxy for the atmospheric dust column load
3 .
1 From http://daac.gsfc.nasa.gov.
2 From http://www.remss.com.
3 From http://daac.gsfc.nasa.gov.