Dennis Weaver on hail meteorology and mapping

Scientists predict that large hail will become more common in future climate scenarios. To understand why, RETC spoke to Dennis Weaver, a meteorologist who has researched and helped map the societal impacts of hail risk.

What are the key ingredients of a severe convective storm?

DW: Severe convective storm generation requires four primary ingredients. The first key ingredient is high moisture content in the air. Higher dewpoints are more conducive to thunderstorm formation. Another key ingredient is lift, which is a forcing mechanism of some sort that pushes air upward. The lift mechanism could occur at a convergence boundary where large air masses meet, such as along a warm or cold front, or at a physical feature, such as a mountain range that forces air vertically upward. A third key ingredient is atmospheric instability. In the meteorological context, instability refers to a state where the relative temperature of a pocket—or parcel—of air is warmer, less dense, and more buoyant than its surroundings, meaning it can rise freely. The fourth key ingredient is vertical wind shear, which is a change in wind direction or speed with height. Imagine you have 35-mph [56-kph] surface winds out of the west and 100-mph [161-kph] winds out of the north at a height of 10,000 ft [3,048 m]. That wind shear is not only relevant to tornado genesis but also impacts thunderstorm longevity. Wind shear separates the updraft region of a severe convective storm from the downdraft, where the precipitation is happening. Without wind shear, sinking cool air in the downdraft effectively knocks energy out of the vertically moving upward air, resulting in a short-lived storm event.

How does hail form in a severe thunderstorm environment?

DW: When you have these four ingredients, conditions are favorable for the formation of longer-lived supercell storm systems. As these storms are growing, there’s a region within the cloud where you find supercooled liquid water—water that remains in a liquid state even though its temperature is below freezing. Supercooled liquid water is crucial for hail formation and usually occurs at an altitude where temperatures range from around 0°C [32°F] to -20°C [-4°F]. Hail forms when supercooled liquid water collides with other small particles, which could be dust, sand, or small ice pellets, and freezes upon contact. If you have enough of that supercooled liquid water and increasing numbers of collisions within the cloud, hail will continue to grow. Moisture content is important to hail growth because it supplies the supercooled liquid water. Stronger updrafts, associated with high levels of atmospheric instability, can hold growing hail embryos in the primary growth region for longer and lead to large hail formation.

Is there scientific consensus about the impact of anthropogenic warming on hail?

DW: Part of what makes this area of study so interesting is that climate scientists and models need to account for competing effects. On the one hand, a warming climate will increase convective instability. For many locations across the United States, climate models highly project an increase in warm moist air near the Earth’s surface. Because stronger updrafts support the transport of moisture into the cloud and increase supercooled liquid water content available for hail formation, this is a recipe for stronger storm systems and larger hail. On the other hand, increasing the depth of the warm surface air layer raises the melting layer height—the height below which hail starts to melt. If you increase the depth of the warm surface air layer by a couple thousand feet, falling hail will spend more time traveling through the melting layer, increasing the degree to which it melts as it falls. Interestingly, larger hailstones are less impacted than smaller ones by an increase in melting potential because larger hailstones have greater mass and fall through the melting layer more quickly. Accounting for these competing effects, future climate scenarios generally predict a decrease in small-diameter hail events but an increase in the probability of large-diameter hail. It’s important to note that these are general predictions, and specific changes will vary on a regional basis.

Can solar project stakeholders screen sites based on hail meteorology?

DW: Yes, but all hail maps are not created equal. As an example, FEMA [Federal Emergency Management Agency] publishes maps that characterize hail risk across the continental United States, ranging from very low hail risk to very high hail risk. If you look at these maps, you see a patchwork of colors rather than smooth contours. The problem with patchwork-patterned hail risk maps is that these invariably exhibit population bias, meaning the exposure values are influenced by the availability of actuarial data or infield hail spotter reports. Mapping hail risk based only on hail claims and observations, which tend to be concentrated around population centers and areas with agricultural activities, misrepresents risk exposure in remote or undeveloped locations. In other words, hail severity maps with population bias tend to underestimate meteorological risk in prime locations for utilityscale solar development.

What is the best practice for mapping hail risk during project development?

DW: To eliminate population bias, you need to map hail risk based on models that fully account for atmospheric phenomena. For the VDE Hail Risk Atlas, a set of ArcGIS-based maps that I help develop and maintain, we map hail risk across the continental United States based on a strategic distribution of approximately 2,500 grid points. At each of these locations, we compute the probability of hail at different sizes based on historical hail observation datasets that include both NEXRAD [Next Generation Weather Radar] and ground observations. The benefit of using the NEXRAD network is it allows for hail identification in these remote areas where we may not be capturing a full set of hail events from ground observations alone. Once we have the full distribution of naturally occurring hail for each of these locations, we use spatial interpolation to estimate risk at unsampled areas of the map. This process allows us to map hail risk as a series of smooth contours. The data sets that VDE offers via subscription in Esri’s ArcGIS environment include both meteorological hail risk maps, which characterize the return interval in years for different hail sizes, and financial hail risk maps, which estimate probable maximum loss and average annual loss values based on fielded technology. These advanced ArcGIS maps are very powerful tools for supporting early-stage solar project development activities.