Bénard cells as seen by Meteosat-7 off the west coast of Africa, 23 August at 11:00 UTC.
21 October 2020
23 August 2001
The increased reporting of gravity waves in the literature is no doubt partly due to improvements in observations using techniques such as weather radars and meteorological satellites. Many decades ago the identification of such waves were revealed by the analysis of conventional surface microbarograph records.
Today, radars and satellites provide direct visual evidence of the waves, particularly those occurring in the mid and upper troposphere. When clouds are present the satellite pictures may reveal a whole train of gravity waves, such as those shown in Figure 1 (marked by black arrows).
The main characteristics of mesoscale travelling gravity waves are wavelengths in the 10–30 km range and periods of five–30 minutes. Phase speeds vary from 10 to 50 ms-1. Mesoscale gravity waves frequently occur in situations where there is stable stratification of the atmosphere and marked vertical wind shear.
In the case shown below the gravity waves developed within a field of closed Bénard cells that occur frequently in the cold Southern Atlantic Ocean. Bénard cells (also called Rayleigh-Bénard convection cells) are hexagonal convection cells, caused by surface tension gradients, that are formed when a thin layer of fluid is heated from below. The cells are named after the French scientist H. Bénard, who first described them in his doctoral thesis in 1900.
Figure 1: Low-level gravity waves
Low-level gravity waves within a layer of Bénard cells off the west coast of Africa, seen by Meteosat-7, 23 August 2001, 11:00 UTC. The coordinates of the image are: rows 1500-2200 and columns 1800-3000.
Figure 2: Geographical area
the graphic shows the selected area of the full Meteosat image.
Figure 3: Gravity Waves in Meteosat-7 Visible Image
23 August 2001, 11:00 UTC
The two photos below illustrate the strides made by Dr E Lothar Koschmieder in creating and photographing Bénard convection cells. Two kinds of Bénard convection cells can frequently be distinguished in satellite images, open cells and closed cells. The characteristics are described below (Courtesy Environmental Technology Laboratory, Boulder, Colorado ).
Photo taken in 1978.
Photo taken in 1990. Source: E. Lothar Koschmieder "Benard Cells and Taylor Vortices", Cambridge University Press.
Warm air rises on the sides of a cell, water vapour condenses to form clouds, and air sinks in the middle of the cell. This leaves clear sky surrounded by a ring of clouds.
Warm air rises in the middle of a cell, water vapour condenses to form a cloud, and air sinks down the sides of the cell. This leaves clear sky around the cloud.
It is not uncommon for both type of Bénard cell to be observed together within the satellite image. An example, taken on the same day (23 August) and just a few hundred kilometres to the west of Figure 1, is shown below. See also this animation .
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