ON THE RESPONSE OF HAILSTORMS
TO ENHANCED CCN CONCENTRATIONS
William R. Cotton
Department of Atmospheric Science, Colorado State University
Approaches to hailstorm
simulation
• 2 moment bulk microphysics
• Hybrid approach: Bulk 1-M or 2-M
microphysics with bin representation of
hail
• Full bin storm model
• 3 moment bulk model
Bin approach
The full bin approach is the most realistic
approach but is very computationally
expensive and has only been done for
hailstorms in two dimensions
Previous studies of CCN
impacts on hail
To date, only two studies have examined the impacts of increasing
CCN on hail (for same storm) with contrasting results
• Noppel et al. (2010):used 2M microphysics in 3D storms and found
more numerous but smaller hailstones, reduction in hail amounts
reaching surface for greater CCN concentrations
• Khain et al. (2011): used a full bin microphysics but in 2D and found an
increase in total hail mass and sizes of hailstones, larger hail reaches
surface for greater CCN amounts
• Both these studies were for the same storm
Adrian Loftus developed a 3moment hail model in RAMS
Most bulk models are either 1-moment(predict
on mass), or 2-moment(predict on mass and
concentration) or a few are 3-moment(predict
on mass, concentration, and the 6th moment
which is proportional to radar reflectivity)
The 3-moment scheme is still computationally
less demanding than a bin model but allows
representation of the tail of the distribution
where hail stones reside
Brief model description
RAMS v.6 with bin-emulating two-moment
microphysics (Saleeby & Cotton, 2004, 2008).

processes are weighted by dist’s function of Nx and Qx
For this study we added a triple-moment hail microphysics
scheme (Loftus, 2012) that predicts of 6th and 0th moments
so all three parameters of the distribution are variable:

Nh, Qh, and
n
h
3MHAIL scheme development
3MHAIL scheme predicts 6th moment [hail reflectivity
factor (Zh)] in addition to Nth and rh in order to obtain nh
and thereby allow for fully prognostic size distribution
•Spectral width parameter (nh)
controls relative amounts of small
versus large particles
•For 3MHAIL scheme, nh ranges
from 1.0 to 10.0
•Hail mean mass diameter (Dmh)
ranges from 0.8 to 40 mm
(maximum Dmh is typically 10 mm
for 2M scheme in RAMS)
•Actual hail diameters (Dh) range
from 0.2 to 150 mm
Marshall-Palmer distribution
Dm = 20 mm
3M vs 2M
• Predicting 6th, 3rd, and 0th moments of
hail size spectrum allows all three gamma
distribution parameters to vary freelyremoves need to ‘tune’ parameters
• Variable nh improves realism in evolution of hail
distribution during sedimentation and melting
broadening/narrowing aloft, narrowing at low levels
mitigates artificial shifts towards large hail sizes
Adrian performed simulations for
an actual case:
29 June 2000 STEPS supercell
Goodland, KS WSR-88D loop of
lowest level radar reflectivity
2025 to 0410 UTC
Hail reports
< 1 in
1 in
1.75 in
F1 tornado
Triple-moment hail
microphysics
Largest hail at
surface following
right turn
Largest hail
collocated with
largest Ze values
Left: Swath of column maximum reflectivity from KGLD radar. Surface fallout locations for
hail ≥ 3 cm computed using particle growth model for 29 June 2000 supercell are overlaid
(black dots) [from Tessendorf et al. 2005]
Right: Swath of column maximum equivalent reflectivity factor (Ze) [dBZ] with contours of
surface hail concentrations [1x10-4, 0.01 m-3] having diameters ≥ 2 (blue), 3 (black) and 4 cm
(purple) for simulation using 3MHAIL scheme. Times (min) of maximum Ze also shown.
Sensitivity to CCN
experiments
•
•
•
•
•
Same model setup as in
verification simulations
(simulation time of 180 min)
Model initialized with 5
different horizontally
homogeneous CCN profiles
Cloud droplet nucleation is
CCN sink, no sources of CCN
CCN assumed to be
ammonium sulfate particles
CCN < 300 cm -3 not typical of
High Plains region (Hobbs et
al. 1985)
Initial CCN profiles
Approximate cloud
base height is 2 km
CCN sensitivity results
ccn100
ccn600
z=982m
ccn3000
Similar storm
structure and
evolution in all cases
z=4947m
Increase in
maximum Ze
as CCN h
Ze
[dBZ]
Sensitivity of hail size to CCN
ccn100
Time-height plots of maximum hail mixing ratio [g m-3]
associated with hail having diameters ≥ 2 cm reveal a
general trend of increases in large hail mass aloft as
CCN increases, in agreement with Khain et al. (2011:
bin microphysics) but in contrast with Noppel et al.
(2009: two-moment bulk microphysics).
ccn600
CCN numbers do not have appreciable impact on
maximum vertical velocities in simulations, similar
to results by Tao et al. (2007), Khain et al. (2011)
and Lim et al. (2011) for strong convection.
ccn3000
Sensitivity of hail size to CCN
D ≥ 2 cm
D ≥ 4 cm
Time series of surface maximum hail mixing ratio
[g kg-1 ] associated with hail having diameters ≥ 2
and 4 cm exhibit a clear trend of increases in large
hail mass at surface as CCN increases. A similar
trend is seen in time series of maximum hail
number concentrations associated with hail having
diameters ≥ 2 and 4 cm (not shown) in general
agreement with Khain et al. (2011).
Maximum accumulated amounts of hail increase
with increasing CCN, in contrast to Noppel et al.
(2009), whereas maximum accumulated amounts
of rain generally decrease for CCN values greater
than 300/cc.
Experiment
Rain [kg m-2]
Hail [kg m-2]
ccn100
17.3
27.6
ccn300
18.8
30.4
ccn600
18.5
38.6
ccn1500
16.4
44.4
ccn3000
14.4
46.0
Cold pool sensitivity to CCN
Surface cold pool area [km2]
Appears to be trend of increase in cold pool size and
strength (greater negative qe’) as CCN increases
from 100 to 600/cc, but then the opposite trend
occurs as CCN increases from 600 to 3000/cc.
These trends generally follow the trends in maximum
accumulated rain and hail, with less rain and more
(and larger) hail leading to decreased cooling at low
levels, similar to results from van den Heever and
Cotton (2004).
Minimum surface qe’ [K]
Mean surface qe’ [K] for
grid points having qe’ ≤ -1 K
Gustavo Carrio and I then exercised
Adrian’s model
A rather large number of
hailstorm simulations were
carried out varying both
[CCN]
and
cloud
base
temperature.
RAMS was set up as follows:

3-D domain: 100 X 100 X18 Km (Nx=Ny=200)

Dx = Dy = 500m , Dt = 1s

1st level 40m, Dz stretching up to 500m
Hot Bubble (2.5K), 30km upwind of the
center

Benchmark case
Marginally severe single
cell ordinary thunderstorm

Occurred over central
Tennessee on 15 May 2009


2.5 cm diameter hail stones

CAPE = ~ 1500 J kg-1
Sensitivity experiments
[CCN]: 16 values between 150 and
2400 cm-3 constant at low levels,
exponentially decreasing above 3km.
Cloud base: The sounding of the
case
study
was
systematically
perturbed by varying low-level vapor
contents from -12 to 12% (10 levels,
~ 500m).
Sensitivity experiments
Green arrow denotes the
profiles of May 15 2009

Blue arrow represents the
simulations with the lowest
[CCN] (150cm-3, “clean”).

In the following figures
each point is a quantity that
characterizes a run.

To isolate the dependence
on [CCN] many figures
show values normalized by
each corresponding “clean”
case (along the blue arrow).

Preliminary results


Precipitation
For storms with higher cloud bases, significant CCN impact on:
both total precipitation and hail fraction
For lower cloud bases → little impact
Hail integral mass and rates
Preliminary results
Large positive changes in both the integral mass of hail
precipitation and maximum rates for runs with higher cloud bases.

Hail concen and predominant size
Preliminary results

Large positive changes for N’s for runs with higher cloud bases.

Positive although more modest impact on predominant diameters.
Peak updrafts and SCLW
Wmax
Peak SCLW

Positive relative changes for storms with higher cloud bases.

Linked to the availability of supercooled liquid water.
Change in storm “size”
Increasing [CCN] tends to concentrate hail in smaller
areas for the dries cases.


Larger accumulations concentrate in smaller areas
precipitation (as in Carrió & Cotton, 2011).
of
Downdrafts

CCN → downdraft area ↑ for runs with lower cloud bases.
Downdrafts


CCN → downdraft area ↑ for runs with lower cloud bases.
Larger impact occurs for run with higher rain fraction of precipitation
Summary:
We find significant sensitivity to CCN
concentrations for storms with colder cloud
base temperatures or higher cloud bases
and with smaller distances between cloud
base and freezing level(D)
For this regime we find larger predominant
hail sizes and concentrations, greater
integral masses of hail, and greater hail
precipitation
For warmer cloud base
temperatures and greater D
The response to varying CCN amounts is
weak
Warm rain processes are much more active
Thank you!