Table of Contents
Precipitation
Virga: Liquid water or ice that falls towards but does not reach the earthís surface.
Processes that control the growth of precipitation.
3. Collision: Two hydrometers striking each other and combining to form a larger hydrometer.
Nucleation
Number of CCNs of various sizes in the atmosphere.
The smaller the size of the CCN, the greater the typical number found in the air. The number per unit volume, for nuclei greater than 0.2 mm, can be related to the radius of the CCN by: where, n = number per volume, R = radius of particle, c = a constant dependant on total concentration of particles.
Curvature Effect
The smaller the droplet, the greater the supersaturation (with respect to a flat surface) is required to keep the droplet from evaporating.
Solute Effect
Assume an unsaturated volume of air contains cloud condensation nuclei of varying sizes.
As RH approaches 100%, the curvature effect becomes negligible for the larger nuclei but remains appreciable for smaller droplets.
Eventually, water vapor is removed at the same rate it is being produced.
Kohler Equation
Where, T = absolute temperature (oK), R = drop radius (mm), i = number of ions per molecule in solution, ms = mass of solute in the droplet, Ms = molecular weight of solute, c1 = 0.3335 oK mm, c2 = 4.3 X 1012 mm3 g-1
Critical Radius, pg. 161
Location of peak changes with solute mass changes.
Left of the peak on Kohler curve:
Right of the peak on Kohler curve:
The radius of the droplet where the critical point occurs (i.e., whether it will grow- be activated or reach equilibrium and remain small) is given by: where: i (number of ions per molecule in solution) Ms (molecular weight of solute) from table 8-1 T in oK
The supersaturation fraction at this critical peak is given by: where: i (number of ions per molecule in solution) Ms (molecular weight of solute) from table 8-1 T in oK ms = mass of solute in the droplet
Activated Nuclei
The number is small compared to the total number of particles in the air.
Nucleation of Ice Crystals
Condensation freezing: Occurs when the nuclei is more attractive as a condensation nuclei than as a deposition nuclei. Water condenses on the nucleus and immediately freezes.
Droplet Growth by Diffusion (condensation and deposition)
For Ice Crystal Growth by Diffusion
Wegener-Bergeron-Findeisen Process
Collision and Collection (Coalescence)
Fall rate of droplets
Precipitable Water
g = -9.8 m/s2
Rainfall estimates by Radar
For hydrometers, it is dependant on drop-size distribution, number of particles per unit volume, physical state of hydrometers (ice or water), shapes of the individual elements of the group, if asymmetrical, their aspect to the radar.
A more complex, but accurate expression is: where, N(D) = number concentration of size D droplets per volume interval: where:
A radar expression is given as: The radar reflectivity factor, Z, is usually expressed in terms of decibels where:
Z-RR (rainfall rate)
For drizzle: For thunderstorms: For snow: For hail: varies from:
Problems:
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Author: Alcorn
Email: alcorn@ariel.met.tamu.edu
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