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The Anatomy of a Bow Echo

The form of a bow echo is somewhat variable because the storm changes through its development, but all bow echoes share several common characteristics.

A view of a bow echo from above shows that the storm is shaped like a bow (hence the name) anywhere from 20 to 120 km (12-75 miles) long. The bow typically travels in an easterly direction with cyclonic winds (turning counter-clockwise) on the north end of the bow and anticyclonic winds (turning clockwise) on the south end. The Coriolis force strengthens the cyclonic winds and the bow shape evolves to be increasingly pronounced over the life of the storm.

Looking at a cross section of the storm explains why the winds become so fierce. Air flows from the front of the storm to the rear, ascending to higher altitudes as it travels. This strong inflow of warm, moist air fuels heavy rain, and in time the rain-cooled air descends. As it does so, it pulls energy from powerful upper-level winds toward the ground, forming a strong downdraft from the back to the front of the bow. If this wind descends to ground level, it causes strong surface winds that blow down forests, destroy telephone poles, and remove roofs. Smaller bow echoes are not necessarily less of a threat, as the cyclone and anticyclone are closer together producing stronger surface winds.

NCAR researchers Morris Weisman and Christopher Davis have been seeking to improve our understanding of bow echoes through computer modeling. Models have been vital tools for understanding all sorts of meteorological phenomena, including bow echoes. For instance, computer modeling has allowed researchers to understand how bow echoes would evolve in the absence of the Coriolis force. This helps us understand that the Coriolis force is responsible for enlarging the northern cyclonic end of the squall line as is observed in actual bow echoes.

In their models, Weisman and Davis found that the cyclonic movement on the north end of large bow echoes can develop into balanced mesoscale convective vortices (MCV). However, larger and more long-lived MCVs typically are produced by MCSs. Read on to learn more about MCVs and the impact they can have on their parent storms.