Impact of Cloud Microphysics on Idealized Hurricane Intensity
Simulating hurricanes in a regional numerical weather model typically means using grid sizes on the order of 1 kilometer. Getting accurate cloud behavior, then, can be very challenging, especially because particles like ice, snow, hail (called graupel when it is beginning to form), and rain droplets, which make up clouds, are less than 1 millimeter in size. It would take more computer power than is favorable for a computer model to resolve those itty, bitty particles while also resolving phenomenal storms, like hurricanes.
In order to conserve computing energy, we must use what are called “microphysical schemes”. This means that instead of literally simulating every single particle inside our models, then following each one as they evolve in time, we instead represent them mathematically by packing up (or “bulking”) similar particles with each other. However, this method is also limited, as the scientists who develop mathematical microphysical schemes must make certain assumptions about the average particle’s size, density, and distribution. Those assumptions certainly make it easier for our computers to run faster, but they also add uncertainty into our models.
So, for the type of research that I do currently–improving computer models of hurricanes–this means that we don’t always know how hurricane growth and intensity depend on the various particles in all of the clouds that make up the storm’s system. In order to better tease out those dependencies, my current research investigates 6 different hurricanes. These 6 hurricanes were simulated with 3 different microphysics schemes at 2 different intensities (Category 3 and Category 5).
My preliminary analysis of those 6 hurricane simulations shows that all of the storms grew and reached maximum intensity at about the same timing. However, the weaker (category 3) hurricanes experienced greater differences than did the stronger storms. 2 microphysical schemes produced more ice and snow than the third scheme and in those 2 schemes, the storms that were produced were always bigger. Those four hurricanes also experienced lower altitude light-to-moderate rain. Conversely, The 3rd microphysical scheme produced smaller hurricanes that had more hail distributed throughout the simulated storms.
As I continue my research, I will be using a radar simulator developed by Brookhaven National Laboratory in Long Island, NY that will help me to better analyze how different types of particles behave in deep cloud systems like hurricanes.