5.3 Precipitation types and formation mechanisms

Precipitation comes in many forms, from gentle rain to hailstones. These different types result from complex processes in the atmosphere, involving temperature, moisture, and air movement. Understanding how precipitation forms is key to predicting weather patterns.

Measuring precipitation accurately is crucial for meteorologists and hydrologists. They use various tools, from simple rain gauges to advanced radar systems and satellites, to track precipitation across different scales. This data helps forecast weather and manage water resources.

Precipitation Types and Formation Mechanisms

Types of precipitation

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• Liquid precipitation
  • Rain: Liquid water droplets that fall from clouds when they become too heavy to remain suspended in the air (drizzle, shower, downpour)
• Frozen precipitation
  • Snow: Solid precipitation in the form of ice crystals that fall from clouds when the temperature is below freezing throughout the lower atmosphere (flurries, blizzard)
  • Sleet: Partially melted snow that refreezes into ice pellets before reaching the ground, often bouncing upon impact (ice pellets, freezing rain)
  • Freezing rain: Liquid rain that falls through a layer of subfreezing air near the surface, causing it to freeze upon contact with objects on the ground, creating a glaze of ice (black ice, icing)
  • Hail: Solid precipitation in the form of ice balls or lumps that form within strong thunderstorm updrafts, often with distinct layers of ice (pea-sized, golfball-sized)

Formation of precipitation

• Cold cloud processes
  • Bergeron-Findeisen process: The growth of ice crystals at the expense of supercooled liquid water droplets in mixed-phase clouds, due to the lower saturation vapor pressure over ice compared to liquid water
    1. Ice crystals grow by vapor deposition, while nearby water droplets evaporate
    2. Evaporated water vapor is deposited onto the ice crystals, causing them to grow larger
• Warm cloud processes
  • Collision-coalescence: The growth of liquid water droplets by colliding with and merging with other droplets in clouds with temperatures above freezing
    1. Larger droplets fall faster than smaller ones, leading to collisions
    2. Upon collision, droplets merge to form even larger droplets (coalescence)
• Melting
  • The process by which frozen precipitation, such as snow or sleet, melts into liquid water as it falls through a layer of above-freezing air (thawing, liquefaction)
  • The melting layer, or bright band, is often visible on weather radar as a zone of enhanced reflectivity due to the high dielectric constant of water compared to ice

Factors affecting precipitation

• Moisture availability
  • Atmospheric moisture content: The amount of water vapor present in the air, which directly influences the potential for precipitation formation (humidity, dewpoint)
  • Proximity to moisture sources, such as oceans, lakes, or regions with high evapotranspiration (maritime air masses, lake-effect snow)
• Vertical motion
  • Upward motion: Rising air cools adiabatically, leading to condensation and the formation of clouds and precipitation
    • Mechanisms for upward motion include convection (thunderstorms), frontal lifting (cold fronts), and orographic lifting (mountain ranges)
  • Downward motion: Sinking air warms adiabatically, leading to the evaporation of clouds and the suppression of precipitation (subsidence, high pressure systems)
• Topography
  • Orographic effects: The influence of mountains and other terrain features on precipitation patterns
    • Windward side: Enhanced precipitation due to forced ascent and cooling of air as it encounters a mountain barrier (orographic lifting, upslope flow)
    • Leeward side: Reduced precipitation due to the descent and warming of air after passing over a mountain range, creating a rain shadow (downslope flow, föhn winds)

Measurement of precipitation

• In-situ measurements
  • Rain gauges: Instruments that collect and measure the amount of liquid precipitation at a specific location
    • Types include standard rain gauges (cylinder with funnel), tipping bucket gauges (records each 0.01" of rain), and weighing gauges (measures weight of collected water)
  • Snow gauges: Specialized instruments designed to measure the liquid water equivalent of solid precipitation (snow pillow, snow depth sensor)
• Remote sensing techniques
  • Weather radar: A system that emits microwave pulses and detects the backscattered energy from precipitation particles in the atmosphere
    • Reflectivity ($Z$) is related to the size and number of precipitation particles in a given volume (dBZ)
    • Doppler radar can measure the velocity of precipitation particles, providing information on wind speed and direction (m/s)
  • Satellite observations: The use of Earth-orbiting satellites to monitor precipitation patterns and intensities over large areas
    • Visible and infrared imagery can be used to identify and track precipitation systems (cloud tops, convective cells)
    • Microwave sensors can provide estimates of precipitation rates and amounts based on the scattering and emission of microwave energy by precipitation particles (GPM, TRMM)