NASA’s new study shows that updrafts are more important than previously understood in determining what makes clouds produce drizzle.
Scientists from NASA’s Jet Propulsion, UCLA and the University of Tokyo have found that low-lying clouds over the ocean produce more drizzle droplets than the same type of cloud over land.
According to the study, water droplets in clouds initially form on microscopic airborne particles or aerosols. The team studied the role of aerosols in clouds and rain for decades. There are more aerosols over land than over the ocean. Scientists initially thought the additional aerosols would tend to form more drizzle over land as well. The new study shows that the presence of aerosols alone can’t explain where drizzle occurs.
Scientists also observed plumes of warm air rising from the solar-heated Earth. Within tall thunderclouds, strong updrafts play a role in rain formation.
In low-lying clouds, however, updrafts are known to be much weaker. They haven’t received much scientific attention in connection with rain. As the droplets floated within clouds, they continued to grow until the updrafts couldn’t hold them up any longer. Then they fell as full-sized raindrops.
In similar clouds that formed over the ocean, updrafts were even weaker than over land. As a result, droplets fell out of the clouds as drizzle, before they had the opportunity to grow into full-sized raindrops. This helps explain the preponderance of drizzle over the ocean.
This finding, published online in the Quarterly Journal of the Royal Meteorological Society, gives new insight into the basic atmospheric process of rain formation. This is helpful in both weather forecasting and climate modeling. This study also offers a pathway for improving accuracy of the forecasts.
Airborne water vapor molecules condense on aerosol particles called cloud condensation nuclei and grow into droplets of different sizes. Here are some relevant diameters:
— A typical cloud condensation nucleus is 0.0002 millimeters, or mm (about 1,000 times bigger than a water molecule).
— A typical cloud droplet is around 0.02 mm (100 times bigger than the cloud condensation nucleus). Cloud droplets don’t have enough mass to fall.
— A typical drizzle droplet is 0.5 mm (25 times bigger than a cloud droplet). Drizzle is just heavy enough to fall.
— A typical raindrop is about 2 mm (100 times bigger than a cloud droplet and 4 times bigger than drizzle).