Meteorological Physical Background

Open Cell Convection and Closed Cell Convection are the result of processes involving air mass modification. Cold, dry and stable air is advected from a cold surface, e.g. an icecap or snow-covered land, over a warm water surface (an ocean, sea or lake). The cold air is advected in the rear flank of a mid-latitude cyclone. The water body supplies heat and moisture to the air by means of heating and evaporation. Due to this supply from below, the air mass becomes unstable and shallow convection occurs. The difference between air temperature and water temperature decreases with increasing distance from the source area. Also, the average potential temperature of the mixed layer increases and heat, moisture and momentum are mixed over an increasingly deeper layer. This causes a gradually increasing depth of the mixed layer, due to convective processes. The air mass in which the cellular convection occurs is characterized by quite a low inversion in the vertical stratification. That's why the convection is limited to a shallow layer.

Satellite images frequently show large areas of mesoscale cellular convection. These areas are predominantly observed over water surfaces. This mesoscale cellular convection is characterized by honeycomb-like patterns of shallow (generally 1-2 km but in some occasions larger) convective cloudiness, arranged in either 'open' or 'closed' cells. The former have cloudy borders and cloud free centers whereas the latter have cloud free borders and cloudy centers. Open and closed cells are part of a distinct lifecycle in Cold Air Outbreaks from continents or ice-surfaces over warmer sea surfaces. In this lifecycle cloudiness develops from Stratus/Sea-Fog via Cloud Streets into Open and Closed Cells.

Open and Closed Cells mostly occur over water surfaces, but Open Cells can also develop over land, when cold air is advected over a land surface heated by solar insolation. A favorable period for this is springtime. Open and Closed Cells can also be advected from sea onto land by the mean flow. Over land the cellular patterns are somewhat disturbed due to the increased surface friction over land.

Rayleigh-Bénard cells:
The organisation of clouds into a hexagonal pattern can be regarded as an atmospheric manifestation of Rayleigh-Bénard cells. These cells are formed when convection occurs in a fluid which is confined between an upper and a lower boundary and is heated from below. The heating from below causes convection in the fluid, and rising motions (in the manifestation of 'updraft plumes') will occur. When the rising fluid meets the upper boundary it has to spread sideways. If this happens at an uniform speed, the boundary of the flow will have the shape of a circle. When there are more spots with rising fluids, the boundaries will grow towards each other and where 2 boundaries meet the converging fluid is forced to flow downwards. At the bottom of the layer the fluid has to flow sideways again and in this way the circulation is closed. The hexagonal pattern as observed from above, is caused by the convergence of the sideward motions originating from many 'updraft plumes'. When the updraft plumes are of uniform strength a regular hexagonal pattern will be visible.

In the case of Open and Closed Cellular Convection, the lower boundary is the sea or land surface and the upper boundary is an inversion (often a subsidence inversion). However, in the case of the theoretical Rayleigh-Bénard cells the only forcing factor is the temperature difference, in the atmosphere there are a number of extra factors and processes which influence the convective circulation. These factors and processes interfere with each other and resulting effects can be enhanced or dampened.

  1. One of these factors and processes is the large scale vertical motion in the boundary layer imposed by synoptic scale systems. When cellular convection occurs, the large scale vertical flow has a marked influence on the orientation of the circulation in the cells. Open cells will develop if the large-scale vertical motions in the fluid layer are downwards, closed cells will develop when the large-scale vertical motions are upwards. However the large-scale vertical motions are not strong enough to overcome the inversion at the top of the layer.
  2. Another important factor in the atmosphere is the release of latent heat due to condensation. Initially the convection is only thermally induced, but when condensation occurs latent heat release adds to the thermal forcing. In the development of a cold air outbreak condensation becomes quickly a factor of importance when clouds develop such as sea-fog, stratus or convective clouds.
  3. Closed Cells have a larger cloud deck and therefore cloud radiative processes and entrainment are also factors which influence the direction of the circulation in the cells.

Synoptic Environment and Characteristics

In the following paragraphs the typical synoptic environment and other characteristics will be discussed.

Open Cellular Convection

Open Cellular Convection is mostly seen in cold air outbreaks, when cold and dry air from continents is transported over a relatively warm (water) surface. This flow often occurs behind a Cold Front (see Cold Front ). The air mass is modified by the input of heat and moisture from the water surface. Near the water surface a mixed layer will be formed, which is capped by an inversion. This inversion stems from the period that the air resided in its source area. The depth of the mixed layer will increase with increasing distance from shore due to the continuing input of heat and moisture from the water surface. Clouds develop during the transformation of the air mass. At first Sea-fog or Stratus clouds will occur. Further downstream they will develop into Cloud Streets (see Cloud Streets ) and eventually cellular convection will occur (Open and Closed Cells). The appearance of cellular convection can be considered as the mature stage of the air mass transformation. In this stage the mixed layer has reached its final depth and is not deepening anymore with increasing distance from the original source area.
Cellular convection is confined to a shallow layer, and dynamical forcing is not a factor. However, when dynamical forcing (e.g. PVA near upper-troughs) occurs in a cold air outbreak, the convection will be enhanced and EC (see Enhanced Cumulus ) and Comma (see Comma ) will form.

Open cells tend to develop in a cyclonic flow at the rear side of a mature cyclone, while closed cells preferably exist in an anti cyclonic flow with subsidence in the lower troposphere (see scheme below).
Whether Open or Closed cells form chiefly depends on the intensity of cold air, and the boundary between areas of Open and Closed cells is said to coincide with the strong wind axis of the upper level (Images in Weather Forecasting M. J. Bader )

11 March 2006/12.00 UTC - Meteosat 8 IR image; blue: isobars

As was mentioned in the subchapter Cloud Structure In Satellite Images, the centers of Open Cells are cloud free and the borders are cloudy. This is an indication for the circulation in the cells, which indeed consists of rising motions at the borders and sinking motions in the centers. The rising motions lead to the formation of convective clouds at the borders.

Scheme of vertical circulation and cloudiness in Open Cell Convection

These clouds are relatively shallow (cloud top at 2-3 km), but they are capable of producing precipitation. The highest cloud tops are often situated at the vertices of the hexagons, because at the vertices there is extra convergence in the circulation in the convective cell. Open cells occur in a cold air outbreak and are driven by heating from below (from the relatively warm sea water). This means that the driving process is the same as in classic Bénard convection, however there are some additional factors. These factors are responsible for the different type of circulation in Open Cell Convection and a different vertical scaling of the convection cells compared to the classical B?ard convection. The large scale vertical motion in cold air outbreaks is generally downwards. As mentioned before, downward vertical motions lead to an open cell structure in a fluid heated from below.

11 March 2006/12.00 UTC - Meteosat 8 HRVIS image
11 March 2006/12.00 UTC - Tephigram Ekofisk

In the tephigram it can be seen that the lowest 3 km (about 1000-700 hPa) have unstable stratification. This unstable layer is capped by a weak inversion. The winds back with height in the unstable layer, which is an indication for Cold Air Advection. The wind speed increases only slightly with height.

Height (hPa) Winddirection (Deg) Windspeed (kt)
1007 010 17
1000 015 17
925 020 23
850 350 25
772 295 23
700 295 23

Closed Cellular Convection

Closed Cellular Convection can occur in two different situations: in the aforementioned cold air outbreaks and in a subtropical anticyclone. The first case is a somewhat mature or final stage in the developments in a cold air outbreak. The closed cells are often situated a very far downstream and immediately in the rear of a Cold Front (see Cold Front ) or near the anticyclone which flanks the cold air outbreak to the west. The second case is a typical marine stratocumulus situation, within a subtropical anticyclone and often also situated over relatively cool ocean currents. In both situations there is a well mixed and moist shallow boundary layer situated underneath quite a strong subsidence inversion. The subsidence inversion functions as a strong cap for convection by hindering the vertical growth of the clouds and forcing them to spread out horizontally

Scheme of vertical circulation and cloudiness in Closed Cell Convection

The circulation in Closed Cells is the opposite of that in Open Cells: in Closed Cells the air rises in the centres and sinks at the borders. The rising motions in Closed Cells are therefore less intense than those in Open Cells and hence the cloudiness in closed cells is vertically less developed. The clouds are shallower than those in Open Cell Convection, and due to their shallow depth and their relatively large area in the cells they look more like stratocumulus than like the convective clouds observed in Open Cellular Convection. The borders are cloud free. It is considered that radiative cooling of the cloud tops is the key responsible process in Closed Cellular Convection. This means that while the circulation of the cells is the same as in classical Bénard cells, the driving mechanism is different. The cooling of the cloud tops causes instability and sinking motions in the cloud layer, while entrainment of dry air from above the inversion and subsequent evaporative cooling also contributes to the generation of downward flows.

18 July 2005/12.00 UTC - Meteosat 8 HRVIS image
18 July 2005/12.00 UTC - Tephigram Valentia, SW Ireland

The tephigram of Valentia shows the vertical stratification in a region of Closed Cell Convection. There is a saturated layer of about 650 m depth directly underneath the inversion at 800 hPa, indicating a cloud layer. The wind is veering slightly with height, which is an indication for Warm Air Advection, while the wind speed is nearly constant. Above the inversion the wind is backing with height. At the inversion wind speed decreases, but above the inversion it increases again.

Height (hPa) Winddirection (Deg) Windspeed (kt)
1028 060 12
1000 065 25
925 080 25
850 075 35
700 065 45

The change from Open Cell Convection into Closed Cell Convection is not clearly understood yet. Possible factors which could play a role are: the temperature difference between sea and air, the strength of the inversion, large-scale vertical motions in the boundary layer and forces/processing governing them, and cloud deck processes (radiative cooling, entrainment). It's an interplay between the instability in the mixed layer, the strength of the inversion and vertical motions in the mixed layer. When the upward motions (either purely convective or induced by large scale vertical motions) are not strong enough to overcome (or penetrate into) the inversion layer, the flow will spread horizontally just under the inversion. Accompanying cloudiness will then spread also and cloudiness will increase. Due to the increased cloudiness, which grows into a deck under the inversion, cloud deck processes will become more important. Eventually this can lead to a change in the driving mechanism of the cellular convection and hence the change from Open into Closed Cells. Mostly the transition between cell types is quite abrupt, which infers that there is some kind of 'switch mechanism' (in the forcings) governing the direction of the circulation.