Post on 05-Jan-2016
transcript
Water in the atmosphere
Water content of air
Mass mixing ratio, airdry mass
vapour watermassx
%110~ 1gkg
Saturated vapour pressure, equilibrium over flat surface
Rate of evaporation = rate of condensation
svp is a function of T only
Saturated vapour pressure
Condensation
Condensation when R.H.≥100%
R.H. can increase by
(i) increasing x,
(ii) cooling air
%100T same at svp
pressure vapourR.H. Humidity, Relative
Cooling damp air
• Radiation loss• Ascent (of air mass) • Descent, through inversion
• Advection, over cool region
• Influx of cold dry air• Convection
radiation foghill fog/cloud, frontssteam fogadvection fog, harArctic sea smokecumulus clouds
Warm Rain Process
• Saturation• Nucleation• Growth by condensation• Growth by accretion• Rainfall
Nucleation
For typical molecule, r~0.6nm, would need RH=740% (SS=640%)
In atmosphere maximum SS observed is ~1%
Condensation must be on particles with r~100nm
91 2 1.2 10exp 1
w
svp r
svp r RT r
s.v.p. over curved surface > s.v.p. over flat surface
Radius Name Conc. (cm-3) SS
< 0.1 µm Aitken nuclei 10,000 1%
0.1 - 1 µm large nuclei 100 0.1%
> 1 µm giant nuclei 1 0.01%
Nucleation
Hygroscopic nuclei
Kohler curves
9
3
1.2 101
svp r Bm
svp r r
Fewer water molecules in surface so lower evaporation rate
Sizes of droplets
Growth by Condensation
rdr
To grow by dr require this much water
24w wdm dV r dr
2
1
4w
dr dm
dt r dt
Same flux through every shell, so integrate to find Δp
Mass flux of water vapour diffusing through shell of radius n
24dm dp
F n Ddt dn
2
1
4 4
rr F dn F
p dpD n D r
24w w
dr F Dp
dt r r
1
w
dr D svp SS
dt r
Vapour pressure at droplet = svp (T)Vapour pressure in cloud = svp * RH
1
cloud drop surfacep p p
svp RH svp SS
Growth by condensation
Terminal velocity of
drops
6mg rv
22
9wgv r
343
6 6wr gmg
vr r
For laminar flow, at terminal velocitydrag force=weight
Growth by Collision/AccretionContinuous collection model
v1
v2
r1
r2
2{r1+r2}
2
1 2 1 2 l
dmr Ev
dtwr v
relative volume swept out per unit time
collection efficiency
liquid water content of cloud
22 11 1 2 1 24w s air
drr r r v v x Edt
1 2 1 2
11
if and
4s air
w
r r v v
x Edrv
dt
11
drr
dt
For large drops (>40µm) 1 1v r
Arial view of convection cells
Cloud streets