Aerosol effects on rain and hail formation and their representation using polarimetric radar signatures

Eyal Ilotovich, Nir Benmoshe and Alexander KhainThe Hebrew University of Jerusalem, Israel

IAAR annual meeting, TAU, May 2013

Layout• Introduction: New implementations in HUCM, goals of

simulations

• The meaning of “ZDR columns”

• Results of simulations: microphysics vs polarimetric signatures

• Conclusions: combining cloud microphysical model and polarimetric signatures is a power tool to investigate cloud microphysical processes and aerosol effects.

Hebrew University Cloud Model (HUCM)

• Detailed description of time dependent freezing and wet growth of hail.

• Implementation of liquid water fraction in snow, graupel and hail. Liquid water is advected, diffused and sediment together with size distributions of ice particles.

• Calculation of all polarimetric radar parameters

HUCM is a spectral bin microphysics mixed-phase cloud model. It includes 8 types of hydrometeors (water drops, 3 types of crystals, snow, freezing drops, graupel and hail).To describe cloud-aerosol interaction a CCN size distribution is included. All distributions are represented on a mass grid containing 43 mass bins.

New implementations:

Deep convective clouds (storms) were simulated under mid-latitude (Oklahoma) conditions and tropical environmental conditions

In each case two CCN concentrations were applied: 100 cm-3 (clean) and 3000 cm-3 (polluted)

Simulations:

GOALS:

1. To test new algorithms of time depending freezing

2. To investigate aerosol effects on formation and growth of rain and hail

3. To investigate mechanisms leading to formation of Zdr Columns

Differential reflectivity ZDR

[ ] 10log( )HDR

V

PZ dB

P

HP - Horizontal Power Returned

VP - Vertical Power Returned

Differential reflectivity ZDRSpherical

dB 0< ZDR PV < PH

dB 0> ZDR PV > PH

(droplets, hail, graupel)

(rain drops, melting hail)

dB 0= ZDR PV ~ PH

(some ice crystals)

Horizontally oriented

Vertically oriented

Beginning of rain formation: warm rain

Fields of mass contents. Raindrops produce freezing drops which freeze producing graupel and hail.

Time dependent freezing:Freezing drops freeze quite fast producing hail. Significant liquid

water faction may exist within a column of 500-800m

Radar reflectivity from different hydrometeors .Reflectivity spreads down because of precipitating fall, mainly raindrops

Fall of raindrops along cloud edges

Formation of Zdr column in warm rain:Total Zdr consists of raindrops and freezing drops.

Zdr columns arise at the edge of updraft, where falling raindrops collect ascending cloud droplets

Mature stage: hail growth

G H

Wet growth

Wet growth of hail takes place when hail falls through a deep zone of large CWC.

In case of tilt of vertical updraft wet growth is replaced by dry growth

Low Zdr column: melting of hail

MAXIMUM VALUES OF FREEZING DROP CONTENTS: different aerosols and different soundings: MID-Oklahoma; T-tropical clouds

Maximum masses of freezing drops: MID (latitude) at low CCN concentrationIn Tropics the mass of freezing drops is small

Mass contents of hail are maximum in polluted mid latitude storms,Minimum: in Tropics with low aerosol concentration.

Mass contents of hail are maximum in polluted mid latitude storms, Minimum: in Tropics with low aerosol concentration. Size of hail is maximum in polluted clouds.

Conclusions 1. Warm rain: ZDR columns develop from above, where

rain forms. It means that the thickness of Zdr columns increases with the increase in the aerosol concentration

2. During storm evolution dry growth and wet growth of hail alternate. Wet growth takes place at high W and coincide of hail and high cloud water content. These conditions lead to formation of big hail as well as of high Zdr columns. It means that formation of high Zdr columns is a predictor of big hail.

3. High CWC is the result of instability and high aerosol concentration. Thus aerosols favor the formation of big hail.

THANK YOU!

Zdr is maximum in Mid-latitude clouds, and minimum in clean tropical clouds.

Conclusions

• The ZDR column is closely associated with the area of intense updraft where new cloud cell forms.

• In the updraft, collisions of cloud drops and small raindrops with ice particles lead to formation of graupel and small hail, which fall down from the titled updraft.

In case of tilt of vertical updraft wet growth is replaced by dry growth

Low Zdr column: melting of hail