Desert Dust Determines Aerial Spread of Thunderstorm Clouds

Bhattacharya, A., Pacific Northwest National Laboratory

Cloud Distributions/Characterizations

Cloud Life Cycle

Zeng X, W Tao, SW Powell, RA Houze, P Ciesielski, N Guy, H Pierce, and T Matsui. 2013. "A Comparison of the Water Budgets between Clouds from AMMA and TWP-ICE." Journal of the Atmospheric Sciences, 70(2), 10.1175/jas-d-12-050.1.

The sun, seen through a dusty atmosphere, sets at Niamey, the capital of Niger, which is located in the African Sahara.

Anvil clouds that accompany thunderstorms.

The sun, seen through a dusty atmosphere, sets at Niamey, the capital of Niger, which is located in the African Sahara.

Anvil clouds that accompany thunderstorms.

Contrasts often provide unique perspectives, and scientists seize any such opportunity—when it arises.

In a new research paper, published in the Journal of Atmospheric Sciences, scientists compared weather and climate observations from two field campaigns that took place around the same time, but in environments that could not offer any greater contrast.

They found that the amount of dust in thunderstorm clouds determines how far the clouds spread and how much they affect the Earth’s radiation budget.

One of the two campaigns, the Tropical Warm Pool-International Cloud Experiment (TWP-ICE), took place in 2006 near the tropical northern coast of Darwin in Australia. The African Monsoon Multidisciplinary Analysis (AMMA), on the other hand, took place in the same year but in an arid environment—near Niamey in Niger, bordering the Sahara desert.

The coincidence of these two field campaigns presented a rare opportunity: to study contrasts in atmospheric convection—the starting point of thunderstorms and all related weather events that occur in most parts of the world.

Scientists have asked for a quite a while: what drives atmospheric convection? Is there a common pattern in how these events evolve? Why do these events have such starkly different characteristics in different environments? The last question, particularly, has eluded scientific understanding for a long time. Finally though, scientists are able to address some of these issues.

Using detailed observations from the two campaigns, a science team funded by the United States Department of Energy’s Atmospheric System Research program simulated large convection systems in the tropics as well as in arid regions. The exercise led to the discovery of quite a few common characteristics associated with the initiation and development of these well-observed phenomena.

Clouds composed of ice particles and having large aerial extent usually accompany convection cells in both environments. Known as anvil clouds, these have a unique shape: resembling an anvil, as the name suggests, or like mushrooms with their heads chopped off. Anvil clouds get their shape from rising air during thunderstorms that spreads out against the lower stratosphere; the air in the stratosphere is warmer than the rising air, preventing it from going up any further and forcing it to spread laterally. The aerial extent of the anvil clouds dramatically changes their impact on the Earth’s temperature and climate by altering the radiation budget of the Earth.

The study, led by Xiping Zeng of the NASA Goddard Space Flight Center, reported that dust content (seeds for ice nuclei in clouds) in the rising air mass, which forms these anvil clouds, actually determines how far these clouds spread.

In West Africa, the air is dirtier, meaning that the dust content of the clouds is higher than in tropical Australia. The dirty air in West Africa causes the anvil clouds to spread for almost 400 km, whereas the cleaner, tropical clouds in Australia extend only for about 100 kilometers.

New information about cloud properties such as these holds keys to understanding atmospheric convection systems and improving Earth’s climate model, the team suggests.