Effects of aerosols on precipitation and hail formation in midlatitude storms

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• Effects of aerosols on precipitation and hail
  formation in midlatitude storms
• Khain
• Department of Atmospheric Science, The Hebrew
  University of Jerusalem, Israel

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AEROSOL EFFECTS Most simulations of aerosol on
precipitation were carried out for conditions
characterizing by high freezing level of 4 km
(tropical convection, Texas clouds), where warm
rain is substantial and the increase in aerosol
concentration can prevent warm rain. Number of
studies where aerosol effects on precipitation
from mid-latitude convective clouds are simulated
is quite limited. The problem is that in
mid-latitude clouds warm rain processes are not
efficient independently on aerosol
concentration. So, what is aerosol effect on
precipitation from mid-latitude convective clouds
(or storms)?
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Hebrew University cloud model (HUCM) with
spectral (bin) microphysics (Khain et al 2004,
2008)

• Model includes 8 types of particles
• a) water drops, b) plate crystals c)
  columnar crystals
• d) dendrites e) snow f) graupel g) hail
  h) aerosols
• 2) Each type of particles is described by a size
  distribution function using a mass grid,
  containing 43 mass bins. The size of hail up to 6
  cm in diameter.
• 3) It solves an equation system for the size
  distribution functions.

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HAIL FORMATION SIMULATION OF HAIL SIZE MECHANISM
OF LARGE HAIL FORMATION Large amount of small
supercooled droplets in the deep layer up to 10
km These droplets can be collected by falling
hail particles Favorable conditions large W
large
amount of small aerosols
The main goal of the ANTISTORM project was to
investigate conditions of large hail formation
under mid-latitude environmental conditions (over
Germany)
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THE CASE STUDY
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Max reflectivity 65 dBZ
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Observations
HUCM, 3000cm-3
HUCM, 100cm-3
65 60 55 50 45 40 35 30 25 20 15 10 5 0 -5 dBz
Distance, km
Distance, km
Figure.8. Left Maxcappis of radar reflectivity
measured at Albis (near Zurich, Switzerland) on
28/06/2006 at 1725 UTC. Middle The radar
reflectivity field simulated by HUCM at t9000 s,
CCN concentration 3000 cm-3 Right The radar
reflectivity field simulated by HUCM at t9000 s,
CCN concentration 100 cm-3.
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Vertical velocities Maximum values 25m/s
(Ts23C) and 30 m/s (T24C), peaks are
larger and more frequent in polluted cases
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ACCUMULATED RAIN

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2
2
1
1
1
1
3
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1) Surface rain begins first at low AP
concentrations. 2) At tgt 60 min precipitation in
polluted air is dominating. At t180 min
precipitation is maximum at CCN3000 cm-3 . 3)
Accumulated rain in very clean air is
significantly lower than in polluted air
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Accumulated graupel and hail precipitation
3000 cm-3
3000 cm-3
-
3
100 cm

• The dependence on AP concentration is non
  monotonic with maximum at CCN conc 3000 cm-3
• Precipitation is minimum at CCN conc100 cm-3

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Hail and graupel kinetic energy at the surface
-3
3000 cm
3000 cm
-
3
-
3
3000-6000 cm
-
3
100 cm
-
3
-
3
-3
100 cm
-3
100 cm

• Hail and graupel kinetic energy
• 1) The kinetic energy is maximum in polluted
  cases. Size of hail increases with increase in
  aerosol concentration.
• 2) Kinetic energies are very small at CCN
  conc100 cm-3

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• Microphysical cloud structure
• Cloud water mass is higher in polluted case and
  reaches higher levels.
• Riming of graupel and snow leads to larger hail
  mass in polluted clouds

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Why precipitation increases in polluted
clouds? In polluted clouds hail contributes
significantly to precipitation. The loss of hail
mass by sublimation is negligible hail falls
fast and very concentrated in space. In clean
clouds ice particles are smaller, more snow forms
(because of lack of supercooled droplets). Snow
spreads over larger area and loose its mass by
sublimation. As a result, precipitation in
polluted clouds turns out to be larger than that
in clean air
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Mass and Size distributions of drops at different
heights
Clean air
Polluted air
3000cm-3
100cm-3
Max1 cm
Max0.4 cm
3000cm-3
100cm-3

• Size distributions
• In clean air the maximum raindrop diameter is 0.2
  cm. In polluted air the maximum raindrop diameter
  is 0.8 cm.
• Large raindrops form due to melting of snow,
  graupel and hail. Collisions of melting ice with
  water drops increases size of rain drops

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Mass and Size distributions of hail at different
heights
150 min
150 min
150 min
3000cm-3
100cm-3
3000cm-3
100cm-3
Max hail diameter at 10-1 m-3 mm-1 concentration
level
Collisions in melting layer
Typical hail size 0.25 cm in clean air and
0.8 cm in polluted air, Max hail size in
polluted air is 4.8 cm
7 6 5 4 3 2 1
No collisions in melting layer
100 min
3000cm-3
6 7 8 9 10 11 12 13
mm
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CCN100 cm-3
CCN3000 cm-3
Huge hail forms in case of very high supercooled
LWC

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THANK YOU!
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snow
3000cm-3
100cm-3
Max2.4 gm-3
Max0.45 gm-3
Max2.4 gm-3
Max3.5 gm-3

• Microphysical cloud structure
• The lack of supercooled water leads to the
  formation of larger snow mass in clean air

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• CONCLUSIONS
• Simulation of a thunderstorm in the southern
  Germany shows that
• The model reproduces well the main dynamical and
  microphysical features radar reflectivity and
  the size of graupel and hail.
• b) Precipitation starts earlier in clean air. At
  the same time storms are stronger and produce
  more precipitation in polluted air.
• c) Small aerosols foster the hail formation. This
  process is closely related to the increase in
  super cooled water content at upper levels and to
  intensification of riming. Large hail stones form
  in highly polluted air.
• d) Polarimetric parametrs of the storm are
  calculated. The main feature of hail storm is
  high W, high radar reflectivity at upper levels
  and a significant value of LDR within above the
  melting layer up to 10 km.

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• Questions for Ryzhkov
• 1. Wet growth of hail covered by a film of water
  at Tlt0. We do not take this into account.
• 2. Meaning of parameters. KDP in clean case0, in
  polluted case KDP is significant in melting
  layer.
• 3.How to proceed now? How to tune the parameters
  in formulas? Case studies? Examples of fields of
  polarimetric parameters to compare the results
  with observations.

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The equation for size distribution function fik
of a k-th mass bin for cloud particles of type i
is
is the fall velocity of cloud
particles of type i belonging to the k-th mass
bin
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In HUCM stochastic collision equations are solved
for all types of hydometeors Results of
collisions of hydrometeors of different type
hail graupel snow ice crystals drops
hail graupel or hail snow or graupel graupel or ice crystals drops drops
snow snow graupel or ice crystals ice crystals
snow snow snow or graupel snow
graupel or hail graupel
hail hail
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rimedsnow
rimedsnow
rimedsnow
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IMPROVEMENTS (cont)
Improvements (cont.)

z
graupel
Collision kernels increase with height
hail
graupel
drop
Wet growth
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Examples
Rimed fraction of snow Snow
bulk density
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• (Hail graupel) kinetic energy
• is maximum in polluted cases at CCN
  conc3000-6000 cm-3