Cloud streets

Although there are no polar lows in this picture, I thought it was too nice not to share. It was captured by NASA’s Terra satellite and shows cloud streets off the coast of the eastern US [click here for their article]. These are cloud features that form during cold air outbreaks, when you get really cold air streaming off a cold continent (or sea ice) onto a warm ocean surface. Much in the same way that you get steam when you blow on hot soup, this leads to evaporation from the sea, and cloud formation. The most common cloud type during cold air outbreaks is Stratocumulus or Cumulus, but sometimes you get Cumulonimbus thunderstorms as well.

The clouds in the cloud streets are often called roll clouds. Here’s what Liu et al. (2004, Geophysical Research Letters) [read their paper here] had to say about them:

Convective roll clouds are one of the most common forms of shallow boundary layer convection. They typically develop during cold-air outbreaks that are characterized by large air-sea temperature and humidity differences and high surface wind speeds. These characteristics provide favorable conditions for the development of convection that is organized into long quasi two-dimensional rolls. This organization results in cloud streets or roll clouds oriented along the direction of the mean low-level wind. Typically, the roll clouds are observed to evolve into cellular convection as one moves downstream from the coastline or ice edge.

In NASA’s story you’ll find a high-resolution version of the same image. I’ve cropped the image to show some details:

Click the image for even more detail. What interests me here is that there is clearly a narrow band near the coast with no clouds. Liu et al. managed to capture that with their numerical model:

This means that “we” (not I) have a certain understanding of the physics behind roll clouds and cloud streets. And that’s important, because polar lows always form in cold air outbreaks.


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