ATSC 113 Weather for Sailing, Flying & Snow Sports

Mesoscale Cellular Convection

Learning Goal 10c: Identify areas of mesoscale cellular convection (open and closed cells) and horizontal roll vortices in satellite imagery and describe how they are formed.

Convection is the upward movement of warm air. Mesoscale cellular convection occurs in the boundary layer between the Earth’s surface and the troposphere. It is most common over the oceans behind cold fronts, where colder air from the continents blows out over the warmer ocean air. This process is known as cold-air advection.

As warm air converges on the ground, it rises due to buoyancy a few kilometers and then hits a temperature inversion causing the air to spread sideways. An adjacent cell does the same and air aloft spreads until it hits the air spreading from adjacent cells. The converging air aloft between the two cells is then forced to sink back toward the sea surface. This pattern of convergence and divergence creates a honeycomb pattern of convective clouds that are recognized as open or closed cells.

Open cells have clear centres with surrounding by clouds, often in a honeycomb shape (adjacent hexagons of cloud rings).  Closed cells have convective clouds in the centre surrounded by clear sky. Both are sketched below. Typical diameters of these cells are 10 to 50 km (which is a size called mesoscale), while their depth is only 1 or 2 km.

Cell diagram

Source: image with permission from

When looking at satellite imagery, it is easy to spot the honeycomb-like convective clouds trailing behind a cold front. The satellite photo below shows open and closed MCC over the Pacific Ocean just southwest of Vancouver Island in February.  The winds on this day were blowing from the northeast toward the southwest; namely, from the cold ground to over the warmer (unfrozen) ocean.

open and closed cells
Open- and closed-cell Mesoscale Cellular Convection (MCC) over the Pacific Ocean just southwest of Vancouver Island, BC, Canada.  UBC is located at the white-filled black circle.

Horizontal Roll Vortices

Horizontal roll vortices happen often when the following conditions are met:

  1. the synoptic-scale winds are light to moderate
  2. the earth's surface (or ocean surface) is warmer than the air, and
  3. cold-air is advecting under a strong temperature inversion

Often these gentle roll vortices are invisible unless there are clouds in the air.  The gentle roll vortices sweep the rising thermals and their associated cumulus clouds into parallel rows called  cloud streets.  They are called cloud streets because of the long rows of cumulous clouds that look like parallel streets or highways.

These are similar to the open and closed convective cells; however, rather than taking a honeycomb shape, the synoptic-scale wind causes them to form long lines of cloud that are roughly parallel to the wind direction.  Typical vertical thickness of each roll vortex is 1 or 2 km, and each pair of counter-rotating roll vortices has a pair width about 3 times the vortex depth. 

In the sketch below, the synoptic scale moderate wind speed (yellow arrows) is blowing from bottom left to upper right in the figure; namely, parallel to the horizontal rolls.  In addition to that dominant wind direction, sailors would notice the gentle left and right crosswind components associated with the winds on the undersides of the roll vortices. 

Cloudstreet diagram

Source: Daniel Tyndall, Departmant of Meteorology, University of Utah. - User created schematic, Public Domain, Annotated by R. Stull 2019.

Note the long rows of clouds that form as cold air is blown off the continent from the northwest toward the southeast over Hudson’s Bay, Canada:


Source: NASA Goddard Space Flight Center from Greenbelt, MD, USA - NASA Satellite "Drove Over" the Cloud Streets of Hudson Bay, CC BY 2.0,

cloud streets lake superior

The cold air from Wisconsin is blowing (yellow arrows) across the unfrozen Lake Superior (dark blue colour).  The lake is warmer than the cold air, so as the air travels further away from the Wisconsin shoreline it picks up more moisture via evaporation from the unfrozen lake.  Eventually there is enough moisture in the air for cloud streets start to form about halfway across the lake.  These cloud streets continue a short distance into Ontario just north of Lake Superior.  Source: NASA Worldview polar orbiting satellites.

Additional Resources: (non-required material)

Open Cell Convection and Closed Cell Convection:

CIMSS Satellite Blog – Open-cell vs closed-cell convection over the Pacific Ocean:

Meteorology Glossary:

Wikipedia – Horizontal Convective Rolls:

Keywords: mesoscale cellular convection, cold-air advection, convective clouds, inversion, open cell, closed cell, cyclone, anticyclone, horizontal roll vortices, cyclones, anticyclones

Image credits: are given near the images.