The effect of the size of CCN on drizzle and rain formation in convective clouds Roelof T. Bruintjes Research Applications Program, National Center for Atmospheric Research, P.O. Box 3000, Boulder, Colorado, June 2004 WMO INTERNATIONAL CLOUD MODELING WORKSHOP

Hygroscopic Seeding: History

Some Comments on Hygroscopic Seeding Two methods: add giant/ultra-giant, or add large particles. Most experiments have used UGA, but recent experiments use flares that produce both. UGA produce embryos that grow to raindrops, but continued influence on the cloud will depend on breakup or some other mechanism to overcome the problem that each embryo otherwise produces one raindrop. Large CCN instead rely on acceleration of the production of drizzle which can occur in higher concentrations because the CCN are much smaller.

Measured Size Distribution of Smoke from S.A. Flares: Two distributions used, one including 2D measurements (from another flight) PCASP Histogram: Measured Distribution, PCASP Smooth Lines: Sum of Fitted Log- Normal Distributions (with two or three components) Measurements made by flying behind seeding aircraft. Concentrations diluted (typically by a factor of 100) to allow for dilution as smoke enters cloud.

(a) assumed N=1000(SS/1%) 0.5 (b) use transition to Junge distri- bution for intermediate sizes (c) match (limited) observations at largest sizes (Alofs and Liu) (d) assumed ammonium sulfate for natural CCN, KCl for seeded. Note that assumed size distribution at large size is critical. Distributions of the form N=C(SS) k have infinite mass unless k>2 and give unrealistic concentrations of giant CCN. Cloud Condensation Nuclei

Comparison of “special” natural and seeded cases: Significant acceleration of the precipitation process results. Condensate after 30 min: 0.02 % in raindrops vs 82% or 85% in raindrops

If a size distribution having 1 um geometric-mean diameter (instead of 0.5 um) is used, without the large and giant components of the particle size distribution, the process is still faster and the effect of early broadening of the droplet size distribution is more evident. Drizzle concentrations are enhanced substantially.

Transmission Electron Microscope (TEM) and Scanning Electron Microscope (SEM) particle analyses KCl crystals in young smoke from two different flaming fires.

TEM images from more aged smoke (20 to 30 min). KCL has transformed into potassium sulfate and nitrate and form inclusions within organic particles.

Similar chemical transformation have been observed in polluted marine environments (McInnes et al. 1994) The reaction between sea salt and acific nitrate and sulfate is expected to liberate HCl gas leaving the the particles enriched in nitrate and sulfate. KNO 3 (g) + NaCl(p)  HCl(g) + NaNO 3 (p) H2SO 4 (g) + 2NaCl(p)  2HCl(g) + Na2SO 4 (p)

Li et al. 2002: Pristine and partly reacted sea salt particles over the North Atlantic

Sizes and concentrations 1.Biomass burning particles are mostly sub-micron with KCl dominant initially in flaming fires. 2.Initial particles through chemical processes transform from potassium chloride to potassium sulfate and nitrate and are usually smaller. 3.Ultimately in regional haze ammonium sulfate particles dominate by far and are submicron. 4.The larger than 1 µm diameter particles primarily are comprised of NaCl and mineral dust. Closer to the oceans NaCl dominates and further inland mineral dust dominates. 5.Near ocean NaCl also transforms to sodium sulfate and nitrate in polluted environments.

Fresh smoke seem to contain the most effective CCN while the effectiveness diminishes with age especially at supersaturations less than 1% with aerosol particles experiencing chemical transformations. Aerosol and CCN spectra were found to be fairly homogeneous in regional haze and the CCN number concentration N as a function of supersaturation S can be described by the relation N = 692S Peak cloud droplet concentrations (>800 cm -3 ) in clouds during SAFARI were typical for clouds growing in highly polluted environments. SUMMARY AND CONCLUSIONS

Aerosol size distributions CCN activation spectra

Effect of accumulation mode aerosol concentration

drizzle raindrops Pristine continental Rural continental Industrial continental PRODUCTION OF DRIZZLE AND RAINDROPS WITH INCREASING ACCUMULATION MODE AEROSOL CONCENTRATION

Pristine continental Industrial continental Polluted maritime No large aerosolsWith large aerosols drizzleraindrops EFFECT OF LARGE AEROSOLS ON DRIZZLE AND PRECIPITATION PRODUCTION

POLLUTED COASTAL Aerosols <1  m diameter Aerosols <10  m diameter Aerosols <50  m diameter

INDUSTRIAL CONTINENTAL Aerosols <1  m diameter Aerosols <10  m diameter Aerosols <50  m diameter

PRISTINE CONTINENTAL Aerosols <1  m diameter Aerosols <10  m diameter Aerosols <50  m diameter

CONCENTRATION OF DROPLETS WITH DIAMETER >40  m Continental conditions Coastal conditions

The cloud droplet size distribution is dependent on the chemistry, size and concentrations of the aerosol population and aerosols. Course mode aerosols (CCN) between 0.8 and 5 µm diameter governs the drizzle production in convective updrafts. Transfer of water via coalescence process to rain water proceeds more rapidly with higher drizzle concentrations. Drizzle also mixes through larger parts of the cloud resulting in larger parts of the cloud developing an effective coalescence process. These studies support the rainfall enhancement studies using hygroscopic flares. SUMMARY AND CONCLUSIONS

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