Example: Initiation of a squall line

The birth of a squall line

The story behind this case is a complex but fascinating one, as it demonstrates the synergy between the instruments and between the parameters being measured.

At the initial time (top image), we can observe a few weak disorganized showers on the reflectivity image. This had been the case for the past several hours, and nothing suggests that it should change. However, the refractivity data reveal the presence of a front moving from the north-west, with moist air (blue colors) to the south and drier air (yellow-red colors) to the north. Fronts are generally associated with regions of potential storm development, and it was the case that time: within 30 min, a full line of thunderstorms (squall line) developed along the front.

Reflectivity at 2.5 km (left) and refractivity near the surface (right) before the squall line formed.

Reflectivity at 2.5 km (left) and refractivity near the surface (right) after the squall line formed.

Other hints that this front was present were available from other instruments. Consider the following image from our vertically pointing radar:

Reflectivity fineline on VPR

On the reflectivity image (above), we observe insects populating the bottom 1.5 km of the atmosphere (1). Around 2:36, when the front passed over the radar, there is a concentration of insects (2) along the convergence line. These concentrations of insects have been observed for a few decades on scanning radars where they appear as fine lines. Over and behind the convergence line, cumulus clouds have formed (3). On the vertical velocity image (below), we observe that these clouds form there because the front (blue line) forces air the warm air up (6). The result is a series of clouds in different stages of formation (7): two initial "hot" bubbles (a,b), a fully developed cumulus (c), a mature cumulus (d), and a dissipating one (e). This activity observed on the vertically pointing radar confirms the presence of the front and testify of its ability to lift air and result in the formation of convective cumulus clouds. A few minutes after, the squall line would form.

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