Chapter 4
Remote Sensing of African Coastal Waters Using
Active Microwaves Instrument
Werner Alpers
Abstract Active microwave instruments flown on satellites to remotely sense the
ocean are the radar altimeter, the scatterometer, and the Synthetic Aperture Radar
(SAR). While radar altimeter data are mainly used as input for general ocean circulation models and for wind and global wave forecast, scatterometer and SAR data
are well suited to investigate singular events in the marine boundary layer and in the
ocean. Examples of atmospheric and oceanic phenomena observed by scatterometers and SARs over African coastal waters are presented. Concerning atmospheric
phenomena, first near-surface wind fields derived from scatterometer data acquired
over a cylone in the Mozambique Channel and over a wind front west of Morocco
are presented. Then SAR images showing sea surface signatures of an atmospheric
front, atmospheric gravity waves and a wind jet are presented. Quantitative information on the near-surface wind field is derived from two of the four SAR images
shown. Concerning oceanic phenomena, three SAR images are presented. The first
two SAR images show sea surface signatures of internal waves generated in the Strait
of Gibraltar and at the Atlantic shelf west of Morocco and the third one shows sea
surface signatures of biogenic surface films in the upwelling region off the coast of
Senegal.
4.1 Introduction
Microwaves are electromagnetic waves with wavelengths between one millimeter
and 1 m, or equivalently, with frequencies between 300 MHz (0.3 GHz) and 300 GHz.
However, microwave instruments used for remote sensing of the ocean from satellites
use only microwaves with wavelengths above 2 cm (below 15 GHz) because above
this wavelength the microwaves are very little affected by clouds and (ligth) rain. Thus
microwave instruments can provide information on the ocean (almost) independent of
weather conditions. Depending on their mission objective, microwave instruments
use different frequency bands. Their names are L-band (1–2 GHz, 30–15 cm), Sband (2–4 GHz, 15–7.5 cm), C-band (4–8 GHz, 7.5–3.8 cm), X-band (8–12 GHz,
W. Alpers ()
Institut für Meereskunde, Universität Hamburg,
Hamburg, Germany
e-mail: werner.alpers@zmaw.de
V. Barale, M. Gade (eds.), Remote Sensing of the African Seas,
75
DOI 10.1007/978-94-017-8008-7_4,
© Springer Science+Business Media Dordrecht 2014
Remote Sensing of African Coastal Waters Using
Active Microwaves Instrument
Werner Alpers
Abstract Active microwave instruments flown on satellites to remotely sense the
ocean are the radar altimeter, the scatterometer, and the Synthetic Aperture Radar
(SAR). While radar altimeter data are mainly used as input for general ocean circulation models and for wind and global wave forecast, scatterometer and SAR data
are well suited to investigate singular events in the marine boundary layer and in the
ocean. Examples of atmospheric and oceanic phenomena observed by scatterometers and SARs over African coastal waters are presented. Concerning atmospheric
phenomena, first near-surface wind fields derived from scatterometer data acquired
over a cylone in the Mozambique Channel and over a wind front west of Morocco
are presented. Then SAR images showing sea surface signatures of an atmospheric
front, atmospheric gravity waves and a wind jet are presented. Quantitative information on the near-surface wind field is derived from two of the four SAR images
shown. Concerning oceanic phenomena, three SAR images are presented. The first
two SAR images show sea surface signatures of internal waves generated in the Strait
of Gibraltar and at the Atlantic shelf west of Morocco and the third one shows sea
surface signatures of biogenic surface films in the upwelling region off the coast of
Senegal.
4.1 Introduction
Microwaves are electromagnetic waves with wavelengths between one millimeter
and 1 m, or equivalently, with frequencies between 300 MHz (0.3 GHz) and 300 GHz.
However, microwave instruments used for remote sensing of the ocean from satellites
use only microwaves with wavelengths above 2 cm (below 15 GHz) because above
this wavelength the microwaves are very little affected by clouds and (ligth) rain. Thus
microwave instruments can provide information on the ocean (almost) independent of
weather conditions. Depending on their mission objective, microwave instruments
use different frequency bands. Their names are L-band (1–2 GHz, 30–15 cm), Sband (2–4 GHz, 15–7.5 cm), C-band (4–8 GHz, 7.5–3.8 cm), X-band (8–12 GHz,
W. Alpers ()
Institut für Meereskunde, Universität Hamburg,
Hamburg, Germany
e-mail: werner.alpers@zmaw.de
V. Barale, M. Gade (eds.), Remote Sensing of the African Seas,
75
DOI 10.1007/978-94-017-8008-7_4,
© Springer Science+Business Media Dordrecht 2014