lecture 6 Ice growth in clouds Weather modification Format of this lecture We will continue our work on the growth of ice crystals in clouds We will discuss the principles of weather modification cloud seeding and holepunch clouds ID: 584006
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Slide1
Atmospheric Physicslecture 6
Ice growth in clouds
Weather modificationSlide2
Format of this lecture
We will continue our work on the growth of ice crystals in clouds
We will discuss the principles of weather modification (cloud seeding) and “hole-punch clouds”
mixture of board work & slidesSlide3
Reminder of last week
We showed that the warm rain process can occur, but the processes of diffusion and collision-coalescence are both inefficient for radii near 30
μ
m (bottleneck)
However majority of precipitation actually forms as ice (and then melts to form rain in mild climates like the UK)Slide4Slide5
Saturation vapour pressure for ice
BLUE SOLID LINE = SVP for liquid water
RED DASHED LINE = SVP for ice
Key point:
Saturation vapour pressure for ice is lower than for liquid waterSlide6Slide7Slide8
Cloud Seeding: is it effective?
Seeding a thin cloud layer of
supercooled
liquid droplets with dry ice
Liquid has disappeared (
hole) and has been replaced by ice crystal
fallstreaks
= Bergeron-
Findeison
process
Seeding is successful
But cloud is thin and effect is localised – not much precipitation
liquid cloud layer
What about this cloud?
-
can we tell if it’s been seeded?Slide9
Problem – we have no control experiment
The seeding of the layer cloud was easy to see, but it’s not very useful: only a small amount of water is available to precipitate from such a cloud
Really we want to seed cumulus clouds or deep orographic clouds
Much more complicated and unpredictable
How can we tell if seeding has worked, or if cloud was going to evolve that way anyway?
Need randomised trials, and a lot of them!
This is still an on-going problem. Trials have been performed, but there are many problems with the methodology
What data there is suggest that any enhancement due to cloud seeding effect is likely to be <15%.Slide10
https
://www.youtube.com/watch?v=2D5s2FlA_5k
this is a short film of Vincent Schafer’s early experiments on cloud seeding at GE
HOMEWORK
: watch this video
after the lecture
can you understand the physics of what’s going on?Slide11
Hole-punch clouds
i
ce crystal fall streaks (this part is
glaciated
)
Thin cloud layer of
supercooled
liquid droplets
Note this is just like the photo of dry ice seeding. But no cloud-seeding agents were used here.
Hole punch clouds are very common – what causes them?Slide12
23rd
October 2010Slide13
Aircraft!
Remember in lecture 2 we talked about aerodynamic contrails where air was expanded rapidly as it passed over
propellor
/ wing tips by 10s of °C
this led to homogeneous nucleation of droplets in a trail behind the aircraft
Now imagine we have this occurring in a cold cloud
Air will be chilled to below -35°C and the droplets in the trail will freeze homogeneously
In effect we have just injected millions of tiny ice particles into the
supercooled
cloud
Bergeron
Findeison process operates as before‘inadvertent’ cloud seedingSlide14Slide15Slide16Slide17
Crystal structure of
ice
Ih
The reason we see hexagonal symmetry in ice crystals is because of the way that the atoms are packed in the crystal
Oxygen
Hydrogen bond
Hydrogen
There is controversy over whether cubic ice (
Ic
) may be present in the very cold upper troposphere in the tropicsSlide18Slide19
19
Ukichiro Nakaya - 1930s
Came to Hokkaido university in 1930
Wanted to work on nuclear physics - but university had no facilities
But he did have a microscope. And an unlimited supply of snowflakes in the winter...
He made > 3000 photomicrographs, and established a classification system
He made the first systematic fall speed measurements of snowflakes
Then he did something completely new:
he started to make his own
https://www.youtube.com/watch?v=1mri_ZqaBac
‘snow crystals may be called letters sent from heaven’
from 1:05
Discovered that the growth of a crystal depends on its
temperature
Birth of modern
ice microphysicsSlide20
Long thin ‘needle’ crystals near -5C
Large thin plate / dendritic crystals near -15C
Examples of crystals grown at different temperatures
(
Takahashi et al 1991)Slide21
General behaviour
0 to -3
Plate
-3 to -8
Column
-8 to -25
Plate-like
-25 to -40
Plate-polycrystal
-40 and colder
Bullet-rosetteSlide22
Why is growth so much faster at -5 and -15C?
Mass
vs
Time
vs
T
emperatureSlide23
At -5C crystals are 10 x as long as they are wide
At -15C crystals are 100 x as wide as they are long
We will see that this is why growth at these temperatures is so much fasterSlide24