Many people have experienced the heavy rain and warm, moist air when hurricanes reach land. But several miles above the ground, where the air is very cold, hurricane clouds are made up of snowflakes and tiny ice particles called ice crystals. The computers that scientists use to forecast where hurricanes will go and how strong they will be make guesses about what these ice particles are like – how quickly they fall towards the ground, how many of them there are, and whether they are snowflakes or ice crystals. Scientists don’t have many complete measurements of the ice in hurricanes, so we don’t know how accurate these guesses are. If we knew this information, we might be able to use it to improve our computer forecasts.
When Hurricanes Irene (2011) and Arthur (2014) hit North Carolina, state-of-the-art weather radars measured the shapes, sizes, and amounts of rain and ice in the storms. We developed a new technique to use the measurements to estimate how much of the ice particles in hurricanes are snowflakes and how much are ice crystals, and how quickly they all fall towards the ground. This information can now be used to see how accurate these guesses are, and, if necessary, to improve them.
- In the eyewall (the strongest part of the hurricane surrounding the eye) and in the rainbands, around 90% of the total ice is made of snowflakes which fall towards the ground at about 4 mph. In between the eyewall and the rainbands, more ice crystals are present, and they fall more slowly towards the ground at about 2 mph.
- Strong showers and thunderstorms within the hurricane create more ice crystals and more snowflakes than weaker ones. Because the snowflakes are larger than the ice crystals, snowflakes make up most of the total ice by weight in these parts of the hurricane.
The paper can be accessed at http://journals.ametsoc.org/doi/abs/10.1175/JAMC-D-16-0300.1.