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Tyrrhenian Sea Tyrrhenian Sea

Latitude: 38° 26' N - Longitude: 15° 30' E

The cellular features visible in most parts of this image are sea surface manifestations of such convective cells. On this day the water was 9 °C warmer than the air and thus giving rise to an unstable sea-air interface. The quasi-regular features are absent in a coastal sector adjacent to the northern Sicilian coast. There the convective cells are destroyed by the katabatic wind blowing from the northern Sicilian mountains through the valleys onto the sea.
Korea Bay Korea Bay

Latitude: 39° 15' N - Longitude: 122° 48' E

A cold wind is blowing in January from the mountainous Liaodong Peninsula, North East China, over the warm water of the Korea Bay and generates convective cells which have diameters of the order of a few kilometres. Behind the small islands to the south, the cellular structures are disturbed by island wakes.
Sea of Japan Sea of Japan

Latitude: 41° 49' N - Longitude: 130° 44' E

Convective cells generated by a cold northeasterly wind blowing from the mountains of Sikhote-Alin northeast of Vladivostok (Russia) over the Sea of Japan. The river visible in the left-hand section of the image is the border between Russia and Korea.
Coast of Angola Coast of Angola

Latitude: 14° 12' S - Longitude: 11° 51' E

Convective cells over the Atlantic Ocean near the coast of Angola. The dark area at the bottom of the image is very likely caused by upwelling. Here the water is cooler than west giving rise to a stable air-sea interface.
# Orbit Frame(s) Satellite Date Time Location
1 6104 765 ERS-1 08-Sep-1992 21:13
2 07935 2817 ERS-1 21-Jan-1993 02:33
3 20455 2763 ERS-2 20-Mar-1999 02:03
4 10704 6903 ERS-2 07-May-1997 22:02

If you have any comments on these images please write an e-mail to alpers@ifm.uni-hamburg.de.

Introduction

Atmospheric convective cells over the ocean can be formed when a negative air-sea temperature difference gives rise to an unstable stratification of the marine atmospheric boundary layer and thus to a pronounced energy exchange in vertical direction (Agee, 1982, 1984, 1987; Mitnik, 1992; Atkinson and Zhang, 1996). The air is heated from below and warm air bubbles move upwards giving rise to cellular structures (see Fig. 1). Typical convective cells are characterized by a cylindrical flow pattern which is superimposed upon the ambient wind field. The direction of the cellular air flow directly above the sea surface is either radially outward from the center of the cells ("open cells") or from the rim toward the centre ("closed cells"). Open-cell circulation has downward motion and clear sky in the center, and is surrounded by cloud associated with upward motion (Atkinson and Zhang, 1996). Open-cell mesoscale cellular convection is more frequently observed than close-cell mesoscale convection (Busack et al., 1985). Both types of mesoscale convective cells become visible on SAR images because they are associated with a variable wind velocity at the sea surface which modulates the sea surface roughness and thus the NRCS (Mitnik, 1992; Ufermann and Romeiser, 1999).

Usually mesoscale atmospheric convective cells are defined as organised cellular structures in the planetary boundary layer which have cell diameters ranging from 10 to 40 km (Atkinson and Zhang, 1996). However, on ERS SAR images often kilometre-scale backscatter patterns ("mottles") are visible which are attributed to "convective boundary-spinning eddies" (Sikora et al., 1995). According to Kaimal et al. (1976) the horizontal wavelength of the convective boundary-spinning eddies should be a factor of 1.5 larger than the height of the boundary layer. In an investigation carried out by Sikora et al. (1995) over the northwestern edge of the Gulf Stream North Wall the averaged measured wavelength of a mottle was determined to be 1.33 km with a standard deviation of 0.5 km. The aspect ratio (the ratio between the wavelength of the mottled pattern and the boundary layer height) was determined to be 1.98 plus or minus 0.5. This result suggests that it is possible to estimate the height of the marine boundary layer from the characteristic length scale of the kilometre-scale backscatter patterns, associated with convective boundary-spinning eddies.

Figure 1

Fig. 1: Schematic diagram illustrating the convective process where warm air bubbles ascend through the colder atmosphere. The uplifting is caused by heating from below and by mixing with the surrounding colder air (adapted from Liljequist and Cehak, 1979).

References

  • Agee, E.M., An introduction to shallow convective systems, in Cloud Dynamics, edited by E.M. Agee, and T. Asai, 3-30, D. Reidel, Norwell, Mass. (1982).
  • Agee, E.M., Observations from space and thermal convection, Bull. Am. Meteorol. Soc., 65, 938-949 (1984).
  • Agee, E.M., Meso-scale cellular convection over the oceans, Dyn. Atmos. Ocean., 10, 317-341 (1987).
  • Atkinson, B.W. & Zhang, J.W., Mesoscale shallow convection in the atmosphere, Reviews of Geophysics, 34, 403-431 (1996).
  • Bakan, S. & Schwarz, E., Cellular convection over the northeastern Atlantic, Int. J. Climatol., 12, 353-367 (1992).
  • Boppe, R.S. & Neu, W.L., Quasi-coherent structures in the marine atmospheric surface layer, J. Geophys. Res., 100, 20635-20648 (1995).
  • Busack, B., Bakan, S. & Luthardt, H., Surface conditions during meso-scale cellular convection, Contrib. Atmos. Phys., 58, 4-10 (1985).
  • Liljequist, G.H. & Cehak, K., Allgemeine Meteorologie, Friedr. Vieweg & Sohn, Braunschweig/Wiesbaden (1979).
  • Mitnik, L.M., Mesoscale coherent structures in the surface wind field during cold air outbreaks over the far eastern seas from the satellite side looking radar, Mer., 30, 297-314 (1992).
  • Savtchenko, A., Effect of large eddies on atmospheric surface layer turbulence and the underlying wave field, J. Geophys. Res., 104, 3149-3157 (1999).
  • Sikora, T.D., Young, G.S., Beal, R.C. & Edson, J.B., Use of spaceborne synthetic aperture radar imagery of the sea surface in detecting the presence and structure of the convective marine atmospheric boundary layer, Monthly Weather Rev., 123, 3623-3632 (1995).
  • Ufermann, S. & Romeiser, R., Numerical study on signatures of atmospheric convective cells in radar images of the ocean, J. Geophys. Res., 104, 25707-25719 (1999).
  • Young, G.S. & Sikora, T.D., Distinguishing boundary layer signatures from mesoscale, IGARSS'98.