Download - 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)