Questions & explanations
1. How do the charge separation mechanisms explain the typical tripole charge structure of a thunderstorm (positive at top, negative in middle, small positive at bottom)?
The tripole structure has a main positive charge region at the top, a main negative region in the middle, and a small positive region at the bottom. The Takahashi and Saunders-Peck mechanisms produce charge separation mostly in the mixed-phase region (between -10 and -20°C). At these altitudes, graupel falling from above becomes negatively charged (in the main negative region) while ice crystals are lifted and carry positive charge upward, creating the top positive layer. The small positive region at the bottom comes from different processes: it may be due to graupel that gets positive charge at warmer temperatures (above -10°C) or from other ions. The models explain the main charge regions but the bottom positive region is less well understood.
2. In the Schumacher-Johnson classification, what distinguishes 'training line' storms from 'backbuilding' storms?
Both 'training line' and 'backbuilding' storms can cause flash floods. A training line storm consists of a line of thunderstorms that repeatedly pass over the same area, like railroad cars on a track. Each cell moves along the line, but new cells form at the upwind end, so the entire line remains stationary relative to the ground. In contrast, a backbuilding storm is a single cell that builds continuously on its upwind side while the downwind side decays, so it stays in place. Both produce extreme rainfall, but training lines involve multiple cells, while backbuilding is a stationary cluster. Schumacher-Johnson classify these based on rainfall efficiency, which is the fraction of rainwater that falls to the ground versus that carried away.
3. How does the MacGorman-Rust model explain the propagation of positive lightning (CG+), and how does it differ from negative lightning?
The MacGorman-Rust model notes that positive cloud-to-ground lightning usually comes from the upper part of the storm, where positive charge is stored. Unlike negative lightning, positive lightning often has only a single stroke. The leader in positive lightning is typically continuous rather than stepped, and it moves faster. Positive lightning also carries a larger peak current, sometimes over 200,000 amperes, and a longer continuing current. Because it has a single stroke, the total charge transferred can be higher. Positive lightning is less common but more damaging. The differences arise because the leader propagates through regions of different charge density and electric field conditions.
4. Both the Rakov-Uman and MacGorman-Rust models describe the initiation of lightning inside the thundercloud. What conditions are required for the initial breakdown to begin?
For lightning to initiate, the electric field inside the thundercloud must exceed the dielectric breakdown strength of air, about 3 million volts per meter at sea level. However, such high fields are rarely observed directly. Models suggest that hydrometeors like graupel and ice crystals interact to create local field enhancements. The Rakov-Uman model assumes that a small region of charge concentration triggers the initial breakdown. The MacGorman-Rust model emphasizes the role of hydrometeor alignment and small-scale discharges that serve as starters. Both agree that a large charge separation has to exist, typically several tens of millions of volts, created by graupel-ice collisions.
5. What is meant by 'rainfall efficiency' in the Schumacher-Johnson classification? Give an example of a high-efficiency storm.
Rainfall efficiency is the ratio of total rainfall at the surface to the total water vapor that enters the storm. A high-efficiency storm produces a lot of rain from the available moisture. For example, a storm with weak outflow and slow movement can recycle water vapor efficiently. The Schumacher-Johnson classification uses efficiency as a key factor: storms with efficiency >50% are more likely to produce flash floods. High efficiency often occurs in storms with weak wind shear and very moist air. In such storms, updrafts are strong but not too strong, so raindrops do not evaporate much. The resulting heavy rainfall can cause flash floods even if the storm is not particularly large.
6. According to the Davis-Bosart classification, what are the two main types of derechoes, and what causes each?
The Davis-Bosart classification identifies two main types: progressive derechoes and serial derechoes. A progressive derecho is a long-lived bow echo that moves rapidly, often over hundreds of miles, producing a swath of damaging straight-line winds. It typically forms in an environment with strong wind shear and instability. A serial derecho consists of a series of bow echoes that develop along a longer line, often associated with a strong cold front. Serial derechoes can cover a wider area but may have less intense winds than progressive ones. The distinction is based on the organization of the convective system and its parent synoptic pattern. Both cause widespread wind damage.
7. Compare the wet growth and dry growth modes in the Knight-Knight model. Which mode produces larger hail and why?
In the Knight-Knight model, dry growth happens when the hailstone surface is below freezing and all collected liquid water freezes instantly. Wet growth occurs when the hailstone surface warms to 0°C, so some water stays liquid and forms a spongy layer. In dry growth, the hailstone grows by collecting supercooled droplets that freeze immediately, making it dense and clear. In wet growth, the hailstone can collect more water because it stays liquid longer, leading to larger but less dense hail. However, if wet growth continues too long, water may shed off, limiting size. Generally, dry growth produces more compact hail, but careful conditions of wet growth can lead to giant hail.
8. How does the Schumacher-Johnson classification help forecasters distinguish between storms that will produce flash flooding versus merely heavy rain?
The classification provides a framework for forecasters to evaluate storms based on organizational mode, rainfall efficiency, and motion. By identifying whether a storm is a training line, backbuilding, or other type (like a pulse storm), forecasters can estimate the duration of heavy rain over a watershed. They also consider the soil moisture and terrain. A storm with high efficiency and slow motion (e.g., a training line) is likely to produce flash flooding, while a fast-moving storm with moderate efficiency may only cause urban ponding. The classification helps focus warnings on the most dangerous storms, especially those that are quasi-stationary and have intense rain rates.
9. Why can satellite data alone not give the exact surface pressure in mid-latitude storms?
Satellite sensors measure radiative properties (temperature, moisture) from the atmosphere and surface, but not pressure directly. Surface pressure must be inferred from other variables, which requires assumptions or models. For example, a deep low-pressure system has a distinct cloud spiral, but the exact central pressure is estimated from cloud-top temperatures and wind estimates, with large uncertainty. Ground stations and radiosondes provide direct pressure readings. Therefore, satellite data is combined with other observations to analyze the storm's intensity. Numerical weather models use satellite data to improve initial conditions, but the pressure field is still derived.
10. Compare how satellite data is used for winter storm forecasting in mid-latitudes versus tropical cyclone forecasting.
For winter storms, satellites help identify the location of fronts, the extent of cloud cover, and areas of heavy snowfall by estimating cloud liquid water and ice. However, snow cover complicates the interpretation because snow looks similar to clouds in some bands. For tropical cyclones, satellites focus on the eyewall structure, warm core, and rainbands, using microwave to see through high clouds. In mid-latitudes, the storm structure is more asymmetric and influenced by the jet stream, so water vapor imagery is heavily used. Both rely on satellite data for areas without radar, but mid-latitude storms also require frequent manual analysis of fronts and pressure patterns.
11. Both the Knight-Knight and Atlas-Ulbrich models assume a certain updraft structure. How does a multiple updraft tower environment (pulse storms) affect hail growth compared to a steady updraft?
In a steady updraft, hailstones follow a smooth trajectory and can grow uniformly. In a multiple updraft tower environment, like in a supercell, the updraft can have pulses of varying strength. This can cause hailstones to experience different growth stages: they may be lifted rapidly, then fall slightly, then be lifted again. This repeated cycling can allow hailstones to grow larger because they can collect water at different heights and times. The Knight-Knight model can be extended to include such trajectories, known as 'zigzag' growth. Such models show that storm pulsations can produce very large hail by allowing stones to spend more total time in the growth zone.
12. Why can hurricanes sometimes show more thunderstorms at night than during the day?
Hurricanes often have a diurnal cycle: their thunderstorms are stronger and more widespread at night. During the day, sunlight warms the top of the clouds, which can make the atmosphere more stable and suppress some convection. At night, the cloud tops cool, increasing instability and allowing more upward motion. Also, at night, the temperature difference between the sea surface and the upper atmosphere is larger, which helps thunderstorms grow. This means that rainfall rates usually peak in the early morning hours. Satellite images often show brighter, colder cloud tops at night, indicating stronger storms. Forecasters consider this when predicting rain trends.