Characterizing CCN Spectra to Characterizing CCN Spectra to Investigate the Warm Rain ProcessInvestigate the Warm Rain Process

bybySubhashree MishraSubhashree Mishra

OutlineOutline• An introduction to the warm rain processAn introduction to the warm rain process

• Theory of droplet nucleationTheory of droplet nucleation

• Warm Rain HypothesesWarm Rain Hypotheses

• Role of CCN in initiation of warm rainRole of CCN in initiation of warm rain

• Measurement of CCN in the RICO projectMeasurement of CCN in the RICO project

• DataData

• Results and ConclusionsResults and Conclusions

Warm RainWarm Rain

• Characterized by precipitation due to cumulus Characterized by precipitation due to cumulus convection at temperatures > 0 convection at temperatures > 0 ˚̊CC

• Occurs in clouds with sufficient liquid water and Occurs in clouds with sufficient liquid water and updraft lifetime to sustain collision–coalescence updraft lifetime to sustain collision–coalescence growthgrowth

• Mostly produced in shallow convections over the Mostly produced in shallow convections over the tropics and the midlatitudes tropics and the midlatitudes

• Cumulus convection is limited by the inversion cap Cumulus convection is limited by the inversion cap and hence extends no more than 4kmand hence extends no more than 4km

Why Warm Rain?Why Warm Rain?

• Cumulus convection resulting in warm rain is one Cumulus convection resulting in warm rain is one of the most prevalent cloud types in the planetof the most prevalent cloud types in the planet

• These clouds are ubiquitous over the tropical These clouds are ubiquitous over the tropical oceansoceans

• They play a vital role in estimating the global They play a vital role in estimating the global climate change and energy balanceclimate change and energy balance

• The processes controlling their evolution and The processes controlling their evolution and coverage are not yet well understoodcoverage are not yet well understood

Comparative sizes, concentrations and terminal velocities of typical precipitation particles (from McDonald, 1958)

Nucleation of dropletsNucleation of droplets

• Homogeneous nucleationHomogeneous nucleation

-- -- highlyhighly improbable in natural clouds improbable in natural clouds since it requires supersaturations≥400%since it requires supersaturations≥400%

• Heterogeneous nucleationHeterogeneous nucleation

-- On Insoluble particles-- On Insoluble particles (less likely (less likely because of large contact angles)because of large contact angles)

-- On Soluble particles -- On Soluble particles (most (most favorable for droplet nucleation)favorable for droplet nucleation)

Heterogeneous Nucleation on Heterogeneous Nucleation on Soluble ParticlesSoluble Particles

• Governed by Governed by

-- Kelvin (curvature) effect -- Kelvin (curvature) effect

-- Raoult (solution) effect-- Raoult (solution) effect

Combining Kelvin and Raoult Effect gives the equation for obtaining Kohler curve as

S = e's (r)/ es (∞) = 1+a/r- b/r3

Curvature term

Solution term

Kohler curve for Ammonium Sulfate (Rogers, R. R., and M.K. Kohler curve for Ammonium Sulfate (Rogers, R. R., and M.K. Yau, 1989, A Short Course in Cloud Physics)Yau, 1989, A Short Course in Cloud Physics)

a ≈ 3.3 * 10-5 / T (cm)

b ≈ 4.3 * i * M/ms (cm3)

i = coefficient of ionic activity of

the salt

Warm Rain HypothesesWarm Rain Hypotheses

• The Giant Nuclei HypothesisThe Giant Nuclei Hypothesis

• Turbulence Induced MechanismsTurbulence Induced Mechanisms

--Entrainment Entrainment

-mixing (homogeneous and inhomogeneous)-mixing (homogeneous and inhomogeneous)

-Stochastic condensation-Stochastic condensation

-Entity mixing-Entity mixing

Measurement of CCN in RICOMeasurement of CCN in RICO

• CCN or Cloud Condensation Nuclei are essentially a subset CCN or Cloud Condensation Nuclei are essentially a subset of CN (condensation nuclei) of CN (condensation nuclei)

• activated at low supersaturations, typical in natural clouds activated at low supersaturations, typical in natural clouds and are mostly water solubleand are mostly water soluble

• CCN measurements in RICO were done using the two DRI CCN measurements in RICO were done using the two DRI CCN spectrometers mounted on the C 130CCN spectrometers mounted on the C 130

• Use of two instruments ensured Use of two instruments ensured -- redundancy in data -- redundancy in data -- expanded the supersaturation (S) range -- expanded the supersaturation (S) range -- enabled in flight calibrations and -- enabled in flight calibrations and -- optimized measurements by providing operation -- optimized measurements by providing operation over different S rangesover different S ranges

Instrumentation for CCN Instrumentation for CCN Measurement On the C-130Measurement On the C-130

Adjustment of the DRI CCN spectrometer inlet tip Adjustment of the DRI CCN spectrometer inlet tip (from David C. Rogers, NCAR)(from David C. Rogers, NCAR)

RICO Air MassRICO Air Mass (generated by using the NOAA HYSPLIT model (generated by using the NOAA HYSPLIT model

trajectories)trajectories)

Mean RICO soundingMean RICO sounding

Source: Rauber et al, 2006, In the Driver’s Seat – RICO and Education. (In preparation for Bull. Amer. Soc. Meteor. )

Comparison of CCN Measurements made by the two DRI CCN

Spectrometers

Total particle (CN) and cumulative CCN concentrations during a sounding

Vertical soundings of CCN at various S versus CN Vertical soundings of CCN at various S versus CN concentration during the AIRS2 project on November concentration during the AIRS2 project on November 24th, 200324th, 2003

Figure 3: Sizes of CCN measured in RICO. Also plotted is the theoreticalversus critical supersaturation (Sc) relationship

Cloud adiabaticity versus droplet Cloud adiabaticity versus droplet concentrationconcentration

r2 = 0.88r2 = 0.97

Table 2: Effective Supersaturations determined from Table 2: Effective Supersaturations determined from adiabatic droplet concentrations (Nadiabatic droplet concentrations (Naa))

Date CN Na(min) Seff(min) Na(cm-3) Seff (%) r2

December 16 (RF 06) 385±193 100 ~0.3 195 ~ 0.8 0.677

December 17 (RF 07) 163±94 35 ~0.09 102 ~ .8 0.662

December 19 (RF08) 202±37 35 ~0.06 106 ~ 1.0 0.501

January 5 RF(10) 297±134 80 ~0.1 160 ~ 1.5 0.679

January 7 RF(11) 272±269 110 ~1.0 282 ~ 2.2 0.516

January 11 RF(12) 194±73 80 ~0.8 164 ~ 1.6 0.880

Table 3: Effective supersaturations estimated by taking Table 3: Effective supersaturations estimated by taking values corresponding to the mean, maximum and minimum values corresponding to the mean, maximum and minimum droplet concentrations obtained for Lc/La in the range of 0.9 droplet concentrations obtained for Lc/La in the range of 0.9 to 1.1.to 1.1.

Results and ConclusionsResults and Conclusions

• High standard deviations in particle concentrations implied significant day-to-day variability in concentrations

• Variability in particle concentrations at higher altitudes (~600mb) suggested long-range transport of continental aerosol

• Shift in vertical CCN distribution at approximately the cloud layer suggested cloud processing of aerosols

• Decrease in number of small CCN below clouds may be due to Decrease in number of small CCN below clouds may be due to coagulation that results in increased concentration of large coagulation that results in increased concentration of large CCNCCN

• High concentrations of large CCN at low altitudes may also be a High concentrations of large CCN at low altitudes may also be a result of sea sprayresult of sea spray

• Volatility measurements suggested that most particles in Volatility measurements suggested that most particles in RICO were highly soluble like NaCl or Ammonium SulphateRICO were highly soluble like NaCl or Ammonium Sulphate

• Size supersaturation measurements combined with Size supersaturation measurements combined with volatility measurements suggested that most CCN in the volatility measurements suggested that most CCN in the boundary layer were more consistent with sulfate than NaClboundary layer were more consistent with sulfate than NaCl

• Effective supersaturations in these clouds were as high as Effective supersaturations in these clouds were as high as 1.6 %1.6 %

• Inhomogeneous mixing seemed to be more common in Inhomogeneous mixing seemed to be more common in these clouds although the presence of homogeneous these clouds although the presence of homogeneous mixing cannot be ruled out mixing cannot be ruled out

• All the above factors combined together support the All the above factors combined together support the formation of large droplets that can enhance gravitational formation of large droplets that can enhance gravitational collection process and hence explain warm rain initiationcollection process and hence explain warm rain initiation

Thank you !!Thank you !!

Questions ?Questions ?