4.5 Microphysics of clouds

Cloud microphysics explores the intricate processes of cloud formation and evolution. It delves into nucleation, droplet growth, and ice crystal formation, providing crucial insights into precipitation patterns and atmospheric energy balance.

Understanding cloud particle size distributions and microphysical processes is key to predicting weather and climate. From collision-coalescence to riming and evaporation, these mechanisms shape cloud structures and precipitation formation, influencing global climate dynamics.

Cloud formation processes

• Atmospheric Physics explores the intricate processes of cloud formation, a fundamental aspect of weather and climate systems
• Understanding cloud formation mechanisms provides insights into precipitation patterns, atmospheric energy balance, and global climate dynamics

Nucleation and condensation

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• Nucleation initiates cloud droplet formation when water vapor condenses on tiny particles (aerosols)
• Heterogeneous nucleation occurs on aerosol particles (cloud condensation nuclei)
• Homogeneous nucleation happens in pure water vapor without a surface, requiring higher supersaturation
• Condensation continues as water vapor adheres to newly formed droplets, causing growth

Droplet growth mechanisms

• Diffusion growth involves water vapor molecules adhering to droplet surfaces
• Collision-coalescence occurs when larger droplets collide and merge with smaller ones
• Bergeron process facilitates droplet growth in mixed-phase clouds through vapor pressure differences
• Droplet size increases until reaching a critical radius, after which growth accelerates

Ice crystal formation

• Homogeneous freezing of pure water droplets occurs at temperatures below -38°C
• Heterogeneous ice nucleation involves ice-nucleating particles (INPs) at higher temperatures
• Ice crystal habits (shapes) depend on temperature and supersaturation conditions
• Secondary ice production mechanisms include rime splintering and collisional fragmentation

Cloud particle size distributions

Droplet size spectra

• Cloud droplet sizes typically range from 1 to 100 micrometers in diameter
• Size distributions often follow a gamma or lognormal function
• Droplet concentration varies widely, from tens per cubic centimeter in maritime clouds to thousands in continental clouds
• Drizzle drops (>50 micrometers) mark the transition to precipitation-sized particles

Ice crystal size spectra

• Ice crystal sizes span from a few micrometers to several millimeters
• Size distributions depend on temperature, supersaturation, and available ice nuclei
• Pristine ice crystals exhibit various habits (plates, columns, dendrites) based on growth conditions
• Aggregates and rimed particles contribute to larger sizes in the spectrum

Measurement techniques

• In-situ measurements utilize airborne instruments like cloud droplet probes and ice crystal imagers
• Ground-based instruments include fog monitors and precipitation disdrometers
• Remote sensing techniques employ radar and lidar to infer particle size distributions
• Satellite observations provide global coverage of cloud properties and particle size information

Cloud microphysical processes

Collision and coalescence

• Gravitational settling causes larger droplets to fall faster and collide with smaller ones
• Collision efficiency depends on droplet sizes, with larger size differences increasing efficiency
• Coalescence occurs when colliding droplets merge to form a single larger droplet
• This process is crucial for warm rain formation in clouds with temperatures above freezing

Riming and aggregation

• Riming involves supercooled water droplets freezing upon contact with ice particles
• Aggregation occurs when ice crystals collide and stick together, forming larger snowflakes
• Both processes contribute to the growth of precipitation-sized particles in mixed-phase clouds
• The degree of riming affects particle density and fall speed, influencing precipitation intensity

Evaporation and sublimation

• Evaporation reduces droplet size when relative humidity is below 100%
• Sublimation converts ice directly to water vapor, often occurring at cloud edges or in descending air
• These processes affect cloud lifetime, precipitation efficiency, and atmospheric moisture distribution
• Evaporative cooling can influence local temperature profiles and atmospheric stability

Cloud types and structures

Warm vs cold clouds

• Warm clouds consist entirely of liquid water droplets, with cloud tops below the freezing level
• Cold clouds contain ice crystals or a mixture of ice and supercooled water droplets
• Warm clouds typically produce precipitation through collision-coalescence processes
• Cold clouds often involve more complex microphysical processes, including the Bergeron process

Convective vs stratiform clouds

• Convective clouds form through strong vertical air motions, often associated with instability
• Stratiform clouds develop in stable atmospheric conditions with gentle lifting over large areas
• Convective clouds (cumulonimbus) can produce intense, localized precipitation
• Stratiform clouds (altostratus, nimbostratus) generate widespread, longer-duration precipitation

Mixed-phase clouds

• Contain both liquid water droplets and ice crystals, typically between 0°C and -40°C
• The Wegener-Bergeron-Findeisen process facilitates rapid ice crystal growth at the expense of droplets
• Complex interactions between liquid and ice phases influence precipitation formation and cloud electrification
• Mixed-phase clouds play a crucial role in global precipitation and radiative balance

Precipitation formation

Warm rain processes

• Occurs in clouds with temperatures above 0°C throughout their vertical extent
• Collision-coalescence dominates droplet growth, leading to raindrop formation
• Requires sufficient cloud depth and liquid water content for efficient droplet growth
• Common in tropical and subtropical regions, producing gentle to moderate rainfall

Cold rain processes

• Involves ice-phase particles in clouds extending above the freezing level
• The Bergeron process initiates ice crystal growth through vapor deposition
• Riming and aggregation contribute to the formation of larger precipitation particles
• Melting of ice particles below the freezing level produces cold rain

Graupel and hail formation

• Graupel forms when supercooled droplets freeze onto falling ice crystals or snow
• Hail develops through multiple cycles of riming and wet growth in strong updrafts
• Size and density of graupel and hail depend on available supercooled water and updraft strength
• These particles can cause significant damage and are associated with severe thunderstorms

Cloud electrification

Charge separation mechanisms

• Non-inductive charging involves collisions between ice particles in the presence of supercooled water
• Inductive charging occurs when polarized particles collide in an existing electric field
• Convective charging results from the vertical transport of charged particles in updrafts and downdrafts
• The magnitude and polarity of charge transfer depend on temperature, liquid water content, and particle sizes

Lightning formation

• Charge accumulation within clouds creates strong electric fields
• When the electric field exceeds the breakdown threshold, an initial lightning leader forms
• Stepped leaders propagate in a branching pattern, seeking opposite charges in the cloud or ground
• Return strokes produce the visible flash and thunder, neutralizing the charge difference

Cloud-aerosol interactions

Aerosol effects on cloud droplets

• Aerosols serve as cloud condensation nuclei (CCN), influencing droplet number and size
• Higher aerosol concentrations generally lead to more numerous, smaller cloud droplets
• This can affect cloud albedo, lifetime, and precipitation efficiency (first and second indirect effects)
• Absorbing aerosols (black carbon) can alter atmospheric stability and cloud formation processes

Cloud condensation nuclei

• CCN are particles on which water vapor can condense to form cloud droplets
• Common CCN include sea salt, sulfates, nitrates, and organic compounds
• CCN activation depends on particle size, chemical composition, and ambient supersaturation
• The CCN spectrum describes the relationship between supersaturation and activated particle concentration

Ice nuclei

• Ice nuclei (IN) are particles that facilitate ice crystal formation in clouds
• Effective IN include mineral dust, biological particles, and some anthropogenic aerosols
• IN activity varies with temperature, supersaturation, and particle properties
• The scarcity of effective IN at warmer subzero temperatures influences mixed-phase cloud processes

Microphysical parameterizations

Bulk vs bin schemes

• Bulk schemes represent cloud particles using moments of the size distribution (mass, number concentration)
• Bin schemes explicitly resolve the particle size distribution into discrete size categories
• Bulk schemes are computationally efficient but may oversimplify microphysical processes
• Bin schemes provide detailed microphysical information but are computationally expensive

Autoconversion processes

• Represent the transition of cloud droplets to rain drops in numerical models
• Parameterizations typically depend on cloud water content and droplet number concentration
• Threshold-based schemes initiate autoconversion when cloud water exceeds a critical value
• Continuous schemes allow gradual conversion based on collision-coalescence efficiency

Sedimentation and fallout

• Describe the gravitational settling of cloud and precipitation particles
• Fall speeds depend on particle size, shape, and density
• Parameterizations often use power-law relationships between particle size and fall speed
• Accurate representation of sedimentation is crucial for predicting precipitation timing and intensity

Observational techniques

In-situ measurements

• Aircraft-mounted instruments directly sample cloud particles and atmospheric conditions
• Cloud particle imagers capture high-resolution images of ice crystals and droplets
• Hot-wire probes measure liquid water content in clouds
• Counterflow virtual impactors separate cloud particles for chemical analysis

Remote sensing methods

• Passive remote sensing uses naturally emitted or reflected radiation to infer cloud properties
• Active remote sensing (radar, lidar) emits signals and analyzes returns to characterize clouds
• Multispectral satellite observations provide global coverage of cloud properties
• Ground-based remote sensing networks offer continuous monitoring of local cloud conditions

Radar and lidar applications

• Weather radars detect precipitation particles and provide information on storm structure
• Cloud radars use shorter wavelengths to detect smaller cloud particles
• Doppler capabilities measure particle velocities, providing insights into cloud dynamics
• Lidars offer high-resolution vertical profiles of cloud boundaries and aerosol distributions

Modeling cloud microphysics

Numerical representation

• Eulerian approaches simulate cloud processes on a fixed grid
• Lagrangian methods track individual particles or air parcels
• Spectral bin models resolve detailed size distributions but are computationally intensive
• Bulk microphysics schemes use simplified representations for computational efficiency

Microphysics in climate models

• Parameterizations represent sub-grid scale cloud processes in global climate models
• Cloud fraction schemes determine the spatial extent of clouds within model grid cells
• Aerosol-cloud interactions are increasingly incorporated to improve climate projections
• Convection parameterizations represent the effects of unresolved convective clouds

Uncertainty and sensitivity analysis

• Ensemble simulations explore the range of possible outcomes given parameter uncertainties
• Perturbed physics experiments assess the sensitivity of model results to specific microphysical processes
• Observational constraints help refine parameterizations and reduce model uncertainties
• Intercomparison projects evaluate the performance of different microphysics schemes across models