overview motivation description of the electrification numerical scheme results : (1) single cloud...
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Modeling Cloud Electrification With The RAMS Model
Orit Altaratz1, Tamir Reisin2 and Zev Levin1
1Department of Geophysics and Planetary Science, Tel Aviv University, Israel.
2Soreq Nuclear Research Center, Yavne, Israel.
Overview
•Motivation
•Description of the electrification numerical scheme
•Results : (1) single cloud simulations
(2) cloud field simulations
•Summary
•Conclusions
Focus on the contribution
of geographical factors: • Temperature differences between
land and sea• The coast shape• Topography
Motivation
Better understanding of differences between lightning activity over land and sea
The RAMS microphysical scheme
Bulk microphysical scheme
Water categories: vapor, cloud droplets, rain, pristine ice, snow, aggregates, graupel and hail.
A generalized gamma function is assumed for the size spectrum of the categories
)exp(1
)()(
1),( 1
nnn
ngam D
D
DD
DDDf
Processes: nucleation, condensation, evaporation and melting, collision and coalescence, drops breakup, secondary ice production, shedding, sedimentation.
Graupel
Ice particle
Supercooled water
The noninductive charging mechanism
T, LWC
Three parameterizations were implemented into the model:1) Saunders et al. (1991). 2) Takahashi (1978, 2002).3) Based on Saunders et al. (2003)
The electrification scheme stages
1. Calculation of the noninductive charging rate of the particles in the cloud. (RAMS)
• Interactions of graupel-pristine ice, graupel-snow, graupel-aggregates
3. Calculation of the electric potential from Poisson’s equation. (offline)
4. Calculation of the electric field from the potential by Gauss’ law. (offline)
2. Tracking the charge on the particles. (RAMS)
Spatial distribution of charge
t
Saunders’ scheme
Vg and Vi - terminal fall velocities of the graupel and ice
k - constant ( 3 m s-1 )
G(Di) - a polynomial fit to the experimental data of Keith and
Saunders (1989)
Charge per separation event
)(),(
3
iig
ig DGk
VVDDq
Charge (in fC) gained by graupel as a function of temperature and liquid
water content.
Takahashi’s scheme
Charge per collision
Takahashi (1978)
The polarity of charge gained by graupel
1) Based on the experimental studies of Saunders et al. (1991). (Black bold dashed lines).
2) Based on the experimental results of Takahashi (1978, 2002). (Black thin lines).
3) Based on a modified scheme suggested by Saunders et al. (2003). (Red bold dashed lines).
The noninductive charging rate
The rate of change of charge density on graupel particles:
giggiigiigig dDqdDDnDnEVVDDt
)()()(
42
Vg and Vi - terminal fall velocities of the graupel and ice
Dg and Di - diameters
Egi – collision-separation-charging efficiency
δq – charge per separation event
Using a standard numerical solver (NAG) for the electric potential at all grid points by Poisson equation:
The electric potential
2
The electric field
E
Solving for the electric field at all grid points:
Single Cloud Simulation - Setup
Warm-humid bubble initialization
Vertical wind shear
Bet Dagan – January 5, 2000
1 grid 105 X 105 X 27 cells
32 X 32 X 12 Km
single cloudCloud base (m) 1200
Cloud base (°C) 4°
Cloud top (°C) -28°
Max updraft (m/s) 14
Max LWC (g/Kg) 2.5
Max Snow Content (g/Kg) 0.3
Max Graupel Content (g/Kg) 2.7
Max Aggregates Content (g/Kg) 1.6
Single Cloud Simulation: Results
@ 25 min of simulation
Mass content (g/Kg) at 11 min
Cloud drops Pristine ice
Snow Graupel
Graupel
Cloud drops
Snow Aggregates
Pristine ice
Mass content (g/Kg) at 21 min
Pristine ice Snow
Graupel Total
Charge density (fC/l) at 11 min with Takahashi’s scheme
Pristine ice Snow
GraupelAggregates
Charge density (fC/l) at 21 min with Takahashi’s scheme
Total
Total Charge density (fC/l) at 21 min with Takahashi’s scheme
Total
+1111
-2515 +115
Pristine ice Snow
Graupel Total
Charge density (fC/l) at 11 min with Saunders’ scheme
Pristine ice Snow
GraupelAggregates
Total
Charge density (fC/l) at 21 min with Saunders’ scheme
Total
Total Charge density (fC/l) at 21 min with Saunders’ scheme
+10
-155
+72+77
Takahashi’s scheme
Saunders’ scheme
According to original charging zones
According to modified charging zones
Total Charge density (fC/l)
at 21 min with
Saunders’ schemes
Maximal electric field in the cloud
29 min
Cloud water content at 2616 m
Clouds over the land
19:22 UTC
Clouds over the sea
Cloud Field Simulation
Clouds over the sea
Clouds over the land
Charge
density (fC/l)
with
Takahashi’s
scheme
before first
flash.
ag
gr
gr
ag
ag
ag
gr
gr
Sea 2Sea 1
Land 1 Land 3
The maximal electric field in the clouds in the Haifa simulation (Takahashi’s scheme)
Summary
•A new electrification scheme was implemented into the mesoscale RAMS model
•Simulations of the electrification of a single cloud and a cloud field thunderstorm were performed.
•Three parameterizations of the charge separation mechanism were implemented.
Conclusions
* Takahashi’s scheme predicts charge distribution (tripole/
dipole) and charging rate that compares well with
measurements.
* Saunders’ original scheme predicts an inverted dipole (in
contrast to observations) until close to first flash. Then, a
small upper charge center appears.
* Assuming our modification to Saunders’ charging zones,
the model predicts a tripole that develops at an earlier stage
but with main charge centers in disagreement with
observations.
Conclusions
* Takahashi’s scheme predicts charge distribution (tripole/
dipole) and charging rate that compares well with
measurements.
* Saunders’ original scheme predicts an inverted dipole (in
contrast to observations) until close to first flash. Then, a
small upper charge center appears.
* Assuming our modification to Saunders’ charging zones,
the model predicts a tripole that develops at an earlier stage
but with main charge centers in disagreement with
observations.
Conclusions
* Takahashi’s scheme predicts charge distribution (tripole/
dipole) and charging rate that compares well with
measurements.
* Saunders’ original scheme predicts an inverted dipole (in
contrast to observations) until close to first flash. Then, a
small upper charge center appears.
* Assuming our modification to Saunders’ charging zones,
the model predicts a tripole that develops at an earlier stage
but with main charge centers in disagreement with
observations.
Conclusions (cont.)
* The stronger dependence of the charging rate on the size of
the particles in Saunders’ scheme leads to a lower charging
rate than in Takahashi’s.
* In clouds that develop over the sea, charging begins later
but with a higher rate in comparison to clouds over the land.
* The time to the first lightning flash is shorter for clouds that
develop over the sea. This could explain the higher frequency
of flashes over the Mediterranean Sea.
Conclusions (cont.)
* The stronger dependence of the charging rate on the size of
the particles in Saunders’ scheme leads to a lower charging
rate than in Takahashi’s.
* In clouds that develop over the sea, charging begins later
but with a higher rate in comparison to clouds over the land.
* The time to the first lightning flash is shorter for clouds that
develop over the sea. This could explain the higher frequency
of flashes over the Mediterranean Sea.
Conclusions (cont.)
* The stronger dependence of the charging rate on the size of
the particles in Saunders’ scheme leads to a lower charging
rate than in Takahashi’s.
* In clouds that develop over the sea, charging begins later
but with a higher rate in comparison to clouds over the land.
* The time to the first lightning flash is shorter for clouds that
develop over the sea. This could explain the higher frequency
of flashes over the Mediterranean Sea.