Questions: Warm Rain Process and Collision-Coalescence

Question 1

Two cumulus clouds have equal depth and liquid water content. Cloud A formed over the open ocean (low CCN concentration), Cloud B over a large city (high CCN concentration). Which is more likely to produce warm rain, and why?

• A. Cloud B, because more CCN means more total droplets and therefore a higher collision rate
• B. Cloud A, because fewer CCN produce a broader droplet size spectrum with some larger collector drops, while Cloud B's many tiny uniform droplets have nearly the same fall speed and poor collision efficiency with each other
• C. Both equally, because both have the same liquid water content available for precipitation
• D. Cloud B, because the urban heat island effect raises temperatures and 'warm rain' requires high temperatures

Answer: (B)

The breadth of the droplet size spectrum — not the total number of droplets — controls warm rain initiation. Maritime clouds form on few, often larger CCN, creating a broad spectrum where some droplets grow noticeably larger than others. These larger drops fall faster, sweeping up smaller ones to initiate coalescence. Continental clouds form on abundant tiny CCN, activating many more droplets that share the available water as nearly uniform, very small drops. Uniform drops fall at nearly the same speed, producing minimal relative velocity and very low collision efficiency. This is why tropical oceanic clouds produce frequent brief downpours while continental clouds of similar depth often dissipate without raining.

Question 2

A 100 μm collector drop is falling through a cloud containing many 5 μm droplets. Despite the large size difference, collision efficiency is low. What is the most likely explanation?

• A. The 5 μm droplets are too cold to coalesce with the larger warm drop
• B. The 5 μm droplets are carried around the large drop by the airstream deflected around it, like dust particles flowing around a hand moving through air
• C. The 5 μm droplets are moving faster than the 100 μm drop due to updrafts
• D. Surface tension prevents 5 μm droplets from merging with a much larger drop

Answer: (B)

Very small droplets have so little inertia that they follow the airstream almost perfectly — they get swept around the falling collector drop rather than impacting it. Collision efficiency is the ratio of drops actually hit to drops in the geometric path, and it is lowest when the collected drops are very small relative to the collector. This is why warm rain is not efficient for a uniform population of very tiny droplets: even if a large collector exists, the smallest drops slip around it. The most efficient collection occurs when collected drops are around 10–20 μm, large enough to have some inertia but small enough relative to the collector to be swept up in large numbers.

Question 3

Warm rain can primarily occur in clouds where most levels remain above 0°C, since ice formation would interfere with the collision-coalescence mechanism.

• T. True
• F. False

Answer: False

'Warm rain' refers to the precipitation mechanism — liquid droplets growing through collision and coalescence — not to cloud temperature. Warm rain can occur in clouds that extend above the 0°C freezing level, provided the droplets remain supercooled liquid rather than freezing. Ice formation is not automatic at 0°C in clouds; supercooled liquid water commonly exists at temperatures well below freezing. The term 'warm rain' distinguishes the process from ice-based precipitation mechanisms (Bergeron process, riming), not from cold atmospheric conditions.

Question 4

A broader droplet size spectrum accelerates warm rain development compared to a spectrum of uniformly small droplets of the same total liquid water content.

• T. True
• F. False

Answer: True

Collision-coalescence requires relative motion between droplets, which requires size differences. In a broad spectrum, some droplets are larger and fall faster than others, creating the differential fall speeds needed for collisions. In a narrow, uniform spectrum, all droplets fall at nearly the same speed regardless of total water content — relative velocities are near zero, collision rates are negligible, and warm rain cannot initiate. Broad spectra arise in maritime clouds (few, larger CCN) or clouds with strong updraft variability that allows some drops to grow larger through preferential condensation.

Question 5

Explain why the warm rain process is described as 'self-accelerating' once a collector drop reaches a threshold size.

Model answer: Once a droplet grows large enough to fall noticeably faster than surrounding cloud droplets, it begins collecting smaller droplets through collision and coalescence. The growth from collection increases the drop's mass, which increases its fall velocity (since terminal velocity scales with size). The faster fall speed increases the volume of air swept per unit time and improves collision geometry (larger cross-section), causing the drop to collect even more smaller droplets per second. This positive feedback loop — size → speed → more collection → more growth → more speed — continues until the drop reaches raindrop size (~2 mm) or becomes large enough to break up from aerodynamic forces. The process can advance from initial coalescence to precipitation reaching the surface in roughly 15–20 minutes.

The self-accelerating nature explains both the rapid onset of warm rain and why it requires a trigger: once the chain reaction starts (when the droplet size distribution produces a collector large enough), it amplifies itself. Without the initial size advantage from a broad spectrum, the chain reaction never gets started, regardless of how much liquid water is available.