Synthetic aperture radar observations of sea surface signatures of atmospheric gravity waves over Southeast Asian coastal waters Werner Alpers Institute of Oceanography, University of Hamburg, Hamburg, Germany Weigen Huang, Gan Xilin State Key Lab. of Satellite Ocean Environment Dynamics, Second Institute of Oceanography,State Oceanic Administration, Hangzhou, China
It is often not easy to decide whether wave patterns visible on SAR images of the sea surface are sea surface signatuures of oceanic internal waves or of atmospheric gravity waves (AGWs). Examples of wave patterns visible on SAR images of the sea surface:
ASAR IM, 9 Nov 2007, 13:43 UTC, Yellow Sea, SW of Qingdao
ASAR IM, 26 May 2007, 13:48 UTC, Yellow Sea
ASAR AP, Yellow Sea, 2 Sept. 2007 13:37 UTC Atmospheric gravity waves Oceanic internal waves
Strait of Taiwan south of Shantou 1 ASAR AP, VVHH 9 March 2006, 1408 UTC
Atmospheric internal waves (gravity waves) are waves in the atmosphere propagating along the interface of air layers of different densities Oceanic internal waves are waves are in the interior of the ocean propagating along the interface of water layers of different densities.
The SAR imaging mechanisms of both types of internal waves are different. 1) The radar signature of an atmospheric gravity wave results from the variation of the sea surface wind velocity U. σ = B ( U + ΔU) α σ = normalized radar cross section, α = wind speed exponent 2) The radar signature of an oceanic internal wave results from the gradient of the surface current u. σ = σ 0 ( 1 + A du x /dx )
a a Wave propagation direction Wind direction Streamlines associated with a nonlinear atmospheric gravity wave (lee wave). In the shadowed region the airflow associated with the wave is opposite to the ambient wind direction (adapted from Doyle, and Durran,2002). σ b Variation of the normalized radar σ cross section σ caused by this 0 nonlinear atmospheric gravity wave. σ 0 is the undisturbed (background) value. On the SAR SAR image the atmospheric gravity wave appears as a broad bright band bordered by two narrow dark bands.
SAR imaging of nonlinear atmospheric gravity waves and of nonlinear oceanic internal waves 1) atmospheric internal waves ΔU can be large in large regions. A nonlinear atmospheric internal wave appears on SAR images as a broad bright band sandwiched-in between two narrow dark bands. 2) oceanic internal waves du x /dx can be large only in small regions A nonlinear ocanic internal waves appears on SAR images as a bright band in front followed by a dark band.
Analysis of ASAR images showing sea surface signatures of atmospheric gravity waves
ASAR IM, 9 Nov 2007, 13:43 UTC, Yellow Sea, SW of Qingdao MTSAT-IR IR1 9 November 2005, 13 UTC, China area infrared image, IR1 (10.3-11.3 μm)
MERIS image,9 November 2005 at 0228 UTC Atm. gravity waves Qingdao Wavelength: 10 km
σ 0 ASAR IM, 9 Nov 2007, 13:43 UTC, SW of Qingdao Sea surface wind speed variation induced by the AGWs calculated along the transect inserted in ASAR image as a white line. The wavelength of the AGW is approximately 10 km.
Liaoning Peninsula Duzhua Shan Dao islands ASAR, IM 27 May 2007, 0230 UTC, Bohai Sea courtesy: Knut- Frode Dagestad, NERSC, Bergen
ASAR, IM, 27 May 2007, 0230 UTC, Bohai Sea Wind fields along the profile according to the NCEP wind direction.
Vertical profile of the wind component in the east-west direction calculated from the radiosonde data of Dalian on 27 May 2007 at 0000 UTC. Height 700 m -6 m/s 0 Wind speed 6 m/s
Upstream atmospheric gravity wave Hangzhou Bay (Wangpang Yang) south of Shanghai ASAR, APM, 27 Febr. 2007 at 0202 UTC MODIS, 27 Febr. 2007, 0325 UTC ASAR APM image, 27 February 2007 at 0202 UTC over the Hangzhou Bay (Wangpang Yang)
900 m Vertical temperature (right curve) and dew point temperature (left curve) profiles measured by a radiosonde launched at Shanghai on 27 February 2007 at 0000 UTC. These profiles are plotted as skew-t diagrams. Note that the very strong inversion at a height of 837 m. Streamlines along the wave propagation direction in x-z space based on the NCEP wind field. The airflow from the north (to the right) is blocked by the mountains on the peninsula (blue area to the left).
Atmospheric gravity waves over the Strait of Taiwan near Raoping (between Shantou and Xiamen) Sub-rotors? Wind from SE 4 m/s Wavelength of atm. gravity waves: 10 km ASAR APM 9 April 2005 at 1406 UTC
A prime criterion for a sea surface signature to originate from solitary atmospheric gravity waves is that usually it consists of a narrow dark band in front followed by a broad bright and again a narrow dark band. A prime criterion for a sea surface signature to originate from solitary oceanic internal waves is that, in general, it consists of a broad bright band in front followed by a weak dark band.
Conclusions In order to identify wave patterns visible on SAR images of the sea surface as sea surface manifestations of oceanic or atmospheric internal waves one needs, in general, auxiliary information on the topography (above ground and under water), on the state of the ocean and the state of the atmosphere. But it many cases one can infer its origin with a high confidence level already from the shape and the location of the wave pattern.
Conclusions cont d Spaceborne SAR images can be used to derive quantitative information on the sea surface wind fluctuations caused by the secondary airflow associated with the AGWs. These wind fluctuations can be quite strong, in particular in the lee of mountains, where they can become hazardous to small aircraft. Since atmospheric gravity waves, which leave fingerprints the sea surface and thus become detectable by SAR, require a specific state of the lower troposphere, which leads to trapping of the waves, spaceborne SAR images contain valuable information on the state of the marine boundary layer.
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