Warm cloud microstructures

Warm cloud microstructures

• Liquid water content (LWC): amount of water per unit volume of air

• Droplet concentration: # droplets per unit volume of air

• Droplet size distribution/spectrum: droplet concentration vs. size interval

Liquid water content & entrainment

• Liquid water content (LWC) correlated with updraft speed; large intra-cloud variability

• Actual LWC << adiabatic (skew-T-predicted) LWC due to entrainment of unsaturated ambient air

Liquid water content & entrainment

• Cloud water evaporates into (subsaturated) entrained air cools, sinks

• Parcels can descend several km, even within updrafts (penetrative downdrafts)

• Causes patchy LWC distributions and broadens DSDs

Marine vs. continental warm clouds

• CCNs more concentrated over land (soil particles, forest fires, pollution) LWC distributed over more droplets

• Thus, smaller mean droplet sizes and narrower drop size distributions (DSDs) in continental clouds

• Marine clouds can be shallower and still precipitate due to larger mean droplet size

Cold cloud microphysics

Ice nucleation

• Useful analogies between warm/cold microphysics

• For supercooled (i.e., T < 0) droplet to freeze, ice embryo must be large enough that growth decreases system energy

• Both homogeneous and heterogeneous nucleation mechanisms (latter requires less extreme environment)

Ice nucleation (cont.)

• Homogeneous nucleation – chance aggregation of water molecules to form ice embryo exceeding critical size (T < -40)

• Heterogeneous nucleation – water molecules collect on freezing nucleus within droplet (can occur at much warmer T)

• Contact nucleation – external particle contacts droplet (may occur at still higher T)

• Deposition – vapor changes directly to ice on suitable particles

Ice nucleation (cont.)

• Particles with ice-like molecular structure and that are water-insoluble tend to be more effective ice nuclei (e.g., certain clays, organic materials)

• Occurs at higher T if air supersatured relative to water rather than to ice only (since this allows condensation-freezing)

• Ice nuclei concentration increases exponentially as T decreases

Ice multiplication• Observed ice particle concentration often exceeds

predicted ice nuclei concentration• Ice crystal breakup• Supercooled droplets freezing in isolation• Freezing of droplets onto ice particle (riming) –

numerous ice splinters shed by droplets encountered by falling particle

• Last mechanism probably most important, but still doesn’t explain explosive growth in ice particle concentration observed in some clouds (more research needed)

Growth by deposition

• Analogous to droplet growth by condensation, except nonspherical shape must be accounted for (elecrostatic analogy)

• Supersaturation w.r.t. ice much greater than w.r.t. water (10-20 % vs. 0-1 %)

• Thus, ice particles grow much faster from vapor than do droplets

• Growth maximized ~-14 C - difference between saturation vapor pressures of water vs. ice maximized

Ice crystal habits• Basic habits determined by T during

vapor deposition (plates columns plates columns as T decreases)

• All essentially hexagonal, but axis ratio varies greatly

• Basic shapes embellished when air nearly saturated (or supersaturated) relative to water

Growth by riming (accretion)

• Ice particles collide with supercooled droplets• Graupel –original habit indiscernible• If hailstone collects supercooled water rapidly,

latent heat release can prevent some of collected water from freezing – “wet growth” (light, bubble-free layers in stone)

• Hailstone lobes – enhanced collection efficiencies for droplets

Growth by aggregation

• Ice particle collisions much more likely when terminal fall speeds different

• Collision frequency enhanced by riming since fall speeds of rimed particles more sensitive to dimensions, amount of riming

• Adhesion frequency determined by habit (e.g., higher for dendrites than plates) and T

Growth to precipitation size

• Growth by deposition alone too slow to produce large raindrops

• Depositional growth proceeded by riming and aggregational growth, which both increase with size

• Bright band – melting ice particles have higher radar reflectivity; upon melting completely, terminal fall speeds increase, reducing concentrations below

Related Topics

Cloud modification• Warm cloud seeding with hygroscopic nuclei

– Fog mitigation: seeded droplets grow at expense of fog droplets and fall out– Rain initiation: inject water droplets or nuclei into cloud base;

condensational growth occurs within updraft, then collision-coalescence as droplets descend

• Cold cloud modification– Likely more efficient since ice particles can grow very rapidly in presence of

supercooled droplets– Precip initiation: dry ice induces homogeneous nucleation, raising ice nuclei

concentration toward optimal level– Dissipation of supercooled clouds/fog: overseed with dry ice or silver

idodide, glaciating the cloud ice crystals become small and supersaturation relative to ice low crystals evaporate

Cloud modification (cont.)

• Hail suppression– Artificial nuclei should decrease average size of ice

particles by increasing competition for supercooled water

– Overseeding could cause nucleation of most supercooled droplets, reducing growth by riming

• Cloud modification has had mixed success

Thunderstorm electrification

• Graupel or hailstones (rimers) become negatively charged by, and positively charge, cloud particles (precise mechanism unknown)

• Positive charge carried aloft by updrafts

• Electric field intensifies until dielectric strength of air exceeded lightning

Cloud-to-Ground Lightning• 90 % of ground flashes negatively charged• Stepped leader – discharge originating between main

negatively charged region and positively charged cloud base

• Travels groundward in discrete steps• Induces (+) charge on ground (repels electrons) ,

triggering discharge that moves upward• Once two discharges connect, electrons flow to ground

and visible lightning stroke propagates upward to cloud• See book for subsequent details

Cloud-to-Ground Lightning (cont.)

• Understand what’s going on in these figures!

Thunder

• Return stroke heats air to > 30,000 K• Pressure in channel increases to 10-100 atm• Induces supersonic shock wave in addition to

sound wave (thunder)• At distances > 25 km, thunder generally

refracted above earth’s surface (inaudible)