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.