Scientists Accuse Dendritic Snowflakes of Causing Bright Radar Signatures in Winter Storms
Image of a six-sided branched snowflake, called a dendrite.
Source: Shutterstock
Have you ever checked the weather app on your phone, saw that it wasn’t going to snow, only for there to be inches of snow piled up outside your doorstep a few hours later? Weather forecasts are not always perfect. This is especially the case when it comes to predicting where and how much snow will fall during a winter storm. Part of the struggle of getting snow predictions right stems from forecasts assuming that all snowflakes are round ice balls despite their unique shapes. In fact, weather forecasts know very little about the iconic six-sided snowflake, called a dendrite, and how they grow in snowstorms.
This ignorance becomes a bigger problem for snow prediction when scientists assume that dendrites produce heavy snowfall. Starting from what we know for certain, studies growing snowflakes in a lab have identified the -4 ̊F to 14 ̊F temperature range to be the “sweet spot” for growing dendrites if and only if the air is extremely moist. While conditions that humid would be unbearably sticky for us on the ground, this is actually a common occurrence 15-kilofeet above the ground in snowstorms. Oftentimes bright signatures in radar images are also seen at this same height level in snowstorms, suggesting that something must be growing 15-kilofeet above ground. Knowing what we know about dendrite growth in the lab, scientists were quick to blame the bright radar signatures 15-kilofeet above ground on dendrites. However, while dendrites have been grown in the lab, they have never been observed high up in real, live snowstorms.
It goes without saying that, if forecasts assume that all snowflakes are round ice balls, their predictions may be wildly incorrect, especially if six-sided branched snowflakes are affecting snowfall. So, are dendrites responsible for greater snowfall? The only way to know for sure was to fly planes directly into snowstorms and see for ourselves. My research involves flying through snowstorms in order to directly scan and sample the snowflakes in winter storms 15-kilofeet above the ground by using radar and cloud probes. So far, no conclusions can be made yet, but, after looking at two different storms over the Midwest and Northeast, I have found that dendrites and the bright radar signatures aren’t always seen 15-kilofeet above ground level. They are seen when there is upward air motion in the storm to carry up water droplets and moisten the air, allowing snowflakes to grow into dendrites. That’s not all that’s growing though. Pictures taken in-cloud show other snowflakes 15-kilofeet above ground with complicated, messy-looking shapes that are very different from dendrites. If there’s one thing these results show it’s that snowflake growth in snowstorms is more complicated than meets the eye. More storms need to be analyzed before a final verdict can be reached about whether dendrites enhance snowfall. Until then, dendrites still remain front and center of this investigation, but the scientific search for answers is far from over.
When it comes to where and how much snow falls in winter storms, weather forecasts do not always get it right. Snow prediction accuracy is jeopardized when forecasts take shortcuts in how snow is represented, assuming that all snowflakes are round ice balls despite their different shapes. As such, weather forecasts know very little about how the iconic six-sided snowflake, called a dendrite, grows in snowstorms. Scientists assume that snowflakes grow 15-kilofeet above the ground in 26-kilofeet snowstorms, with the growth of large dendrites leading to heavy snowfall. However, no evidence inside snowstorms has been shown to prove these assumptions true. Without a clear understanding of how and where dendrites grow in snowstorms, forecasters cannot hope to improve the accuracy of their forecasts.
Are dendrites really responsible for most snowfall? Getting down to the bottom of this wintery mystery, Valeria Garcia flies in aircraft directly through winter storms 15 kilofeet above the ground in order to sample the snowflakes within. Her work will uncover the truth about what is growing inside snowstorms, paving the way for improving the accuracy of winter weather forecasts.