A quick look at clouds: what is a cloud, what is its origin and what can we predict and model about its destiny?

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Lillian Wheeler
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1 A quick look at clouds: what is a cloud, what is its origin and what can we predict and model about its destiny? Paul DeMott Colorado State University

2 A look at clouds: what is a cloud, what is its origin and what can we predict and model about its destiny? Attempt to answer a biologist s questions: What are CCN and do bacteria play this role? Why does it rain? Why doesn't it rain? What is hail and how does it involve ice nucleation? Etc Give my own perspective: We need to consider bio-aerosols in the same context as other atmospheric aerosols that impact clouds and potentially climate. Fundamental data is needed on the basic function and distribution of cloud-active bio-aerosols. Clouds are complicated. When precipitation is involved, aerosol interactions and feedbacks are probably especially non-linear. The fact that bio-aerosols may behave as CCN and IN cannot be considered in absence of a total consideration of all other aerosols, thermodynamics and cloud dynamics.

Etc Give my own perspective: We need to consider bio-aerosols in the same context as other atmospheric aerosols that impact clouds and potentially climate.

3 There are many types of clouds of different origin -40ºC All of cloud < -20ºC 0ºC 20ºC

4 What makes it rain?: warm clouds What determines the initial liquid particle composition of clouds? Cloud condensation nuclei [CCN] (depend on RH > 100% in a rising parcel of air via size and composition) Bacteria do not operate in the classic sense of CCN Köhler Theory Single soluble CCN 100 bacteria ε m = soluble fraction

Cloud condensation nuclei [CCN] (depend on RH > 100% in a rising parcel of air

5 What makes it rain?: warm clouds continued Cloud dynamics (vertical motion), which drives condensation CCN activate and grow from a population of particles Activated drops deplete water vapor supersaturation Strength of updraft and CCN spectra itself determine peak RH Measure CCN spectra Drive parcel calculation A perfect parcel with only bacterial CCN rising at 2.5 m s -1 updraft (Franc and DeMott 1998)

grow from a population of particles Activated drops deplete water vapor supersaturation Strength of

6 What makes it rain?: warm clouds continued What is necessary for precipitation? Persistent vertical forcing Cloud drops must grow sufficiently during the lifetime of the cloud to settle against the updraft Depositional growth rate (proportional to 1/(drop size)) of drops is insufficient for precipitation. Drops must also collide and coalesce Berry and Rhinehart (1974)

cloud to settle against the updraft Depositional growth rate (proportional to 1/(drop size))

7 What makes it rain?: warm clouds continued Maritime regions tend to have fewer and larger CCN than continental, so cloud drops may grow faster to large sizes (more colloidally unstable ) Other factors: variable or pulsing updrafts, entrainment of environmental air into clouds, drop breakup, presence of giant CCN (GCCN) Large Eddy simulations Courtesy of Graham Feingold Observations - Levin et al. (1997)

drops may grow faster to large sizes (more colloidally unstable ) Other factors: variable or pulsing

8 CCN and modification of clouds: What are aerosol indirect effects on climate? sletter/23/noone.php IPCC (2001) + semi-direct effects (radiatively absorbing CCN) + ice indirect effects (another role for INA bacteria) Albedo effect only warm clouds only

php IPCC (2001) + semi-direct effects (radiatively absorbing CCN) +

9 Evidence for these indirect effects? Higher aerosol concentrations lead to higher CCN, > cloud drop numbers, lower droplet effective radii If giant CCN aerosols are present with enhanced aerosols, they destablize clouds, changing simple picture Ramanathan et al, Science, 2001

numbers, lower droplet effective radii If giant CCN aerosols are

10 What makes it rain (or snow)?: Cold or mixed-phase clouds Cold clouds may be entirely below 0 degc or encompass a regime both warmer and colder Ice nuclei initiate ice crystal formation but are relatively specious Secondary processes (not shown) may generate additional ice crystals, sometimes dominating the generation process and accelerating the precipitation process Crystals grow at the expense of water droplets due to vapor pressure differences (Bergeron-Findeisen process). Crystals rime (collect water droplets) and aggregate with each other to form precipitation sized particles If part of the cloud is above freezing, particles melt into raindrops Warm and cold processes may also interact

relatively specious Secondary processes (not shown) may generate additional ice crystals, sometimes dominating the generation process and accelerating the precipitation

11 Cold clouds - continued Nucleation processes (Gabor Vali s talk) Ice crystal growth (Ottmar Möhler s talk) Ice multiplication: Hallett-Mossop process: nature of freezing process on riming ice crystals or frozen large drops create ice splinters in the -3 to -8ºC temperature regime. collision-induced or evaporative fracture of branching ice crystals. Some observations indicate IN directly related to initial presence of ice in clouds, but other observations suggest that high concentrations of ice may form ultimately in all clouds, without the temperature dependence expected from activation of IN.

$collision-induced or evaporative fracture of branching ice crystals.$

12 Inferred ice concentrations in clouds from several Canadian projects (Korolev( et al. 2003)

13 Gain insights into how real clouds work through observation and through numerical modeling History of cloud modeling

14 Increase IN in an ideal cloud parcel to reflect observations of high IN in Saharan dust episodes in Florida Pressure (mb) ice (no dust) drops (no dust) ice (dust) drops (dust) Drop Conc (cm -3 ), Ice Conc ( L -1 ) Updraft = 12 m s -1 Cloud base = 15ºC Maritime CCN

900 ice (no dust) drops (no dust) ice (dust) drops (dust) 1000 10 100 1000 10000

15 Aerosol interactions in 3-D 3 D nested-grid cloud resolving model simulations of Florida Cumuli (Van den Heever et al., J. Atmos. Sci., 2006) Increasing any cloud active aerosols from a very clean case redistributes the water phase and decreases precipitation Aerosol profiles Cloud particles and rain But all clouds different, and changes in clouds also impact cloud dynamics, radiation and cloud lifetime, etc

, 2006) Increasing any cloud active aerosols from a very clean case redistributes the water phase and

16 But what is the base state for CCN and IN? - IN aerosol impacting Arctic cloudiness Global IN function often assumed IN measured In this case, knowing IN concentrations was critical to predicting observed cloud location and phase, liquid water path, and surface net infrared radiation. Allowing the nuclei to be redistributed and used up was also very important. North Slope Alaska Prenni et al. (2006) Observed cloudiness Cloud modeled with global IN Cloud modeled with observed IN allowed to deplete

was critical to predicting observed cloud location and phase, liquid water path, and surface net infrared radiation.

17 A quote from Dr. William Cotton s Abstract to the 2006 AMS Cloud Physics Conference Simulations in our group of aerosol influences on stratocumulus clouds, thunderstorms over Florida, and thunderstorms over and downwind of St. Louis, MO have revealed complex dynamical responses to variations in CCN, GCCN, and/or IN concentrations. This is a result of changes in precipitation rates from these clouds systems. It is shown that once the precipitation cycle is augmented in clouds, they are no longer subject to simple linear thinking as in the Twomey hypothesis or Abrecht s extension of it to drizzling clouds. Once the precipitation process is modified clouds may become optically thicker or thinner, they may rain more or they may rain less, they may become more vigorous or less so depending on the nature of the nonlinear response of clouds to changes in precipitation. Thus the climate response to changing aerosol populations becomes less predictable once those changes alter the precipitation cycle of many cloud systems.

downwind of St. Louis, MO have revealed complex dynamical responses to variations in CCN, GCCN, and/or IN concentrations. This is a result of changes in precipitation rates from these clouds systems.

18 A few words on hailstorms These are real beasts Hailstones can be formed in different ways but require presence of massive updrafts and recycling within cloud to form Conceptual models exist for hail suppression Why would bacterial ice nuclei be of relevance? warm temperature activation

recycling within cloud to form Conceptual models exist for hail

19 Hail formation and suppression concepts Krauss (1999)

20 Unproven conceptual models for hail suppression Beneficial Competition: growth limiting competition among hail embryos. Many more embryos must be introduced into the cell than would occur naturally. The competition for the available water reduces their size. Trajectory Lowering: Seeding causes much larger particles formed lower in the updrafts in the embryo formation region that potentially reduce hailstone sizes for all conditions of size sorting. Lower trajectories experience reduced liquid water content and shorter residence times. Early rainout (from the zone of hail embryos). Seeding the flanking line of weaker updrafts causes particles to grow to mm size and fall out without participating in a hail formation process.

Trajectory Lowering: Seeding causes much larger particles formed lower in the updrafts in the embryo formation region that potentially reduce hailstone sizes for all conditions of

21 Some expectations: Biological aerosols and clouds role as CCN? Secondary aerosols formed from biogenic gas emissions: Enter atmospheric aerosol in large numbers at small sizes, but are poor CCN in our experience. Bacteria: Activate more effectively than a wettable particle for their size (Bauer et al. 2003) Larger CCN, so system may be sensitive to them Concentrations? Pollen: Some extend to GCCN sizes (important in areas of convection?)

22 Some expectations: Biological aerosols and clouds role as IN? Secondary aerosols formed from biogenic gas emissions: Not expected but possibility for chemical processing to lead to sources as IN Bacteria, fungi, spores, pollen: Major widespread sources of INA types or a local/regional concern? Distribution of INA bacteria in atmosphere?

23 THE END

24 Growth of ice crystals by vapor diffusion

25 Crystal Fragmentation Delicate crystals can become broken Broken pieces become fragments that can act as new ice nuclei This and next slide from Greg Tripoli course lecture

26 Hallett-Mossop Multiplication Near 4 to 6 Celsius, cloud droplets greater than 12 micros radius riming onto an ice crystal form a shell of ice before freezing into their interior. When the ice formation spreads to the interior, the interior of the droplet expands and explodes through the ice shell producing small crystal splinters, as many as 360 splinters per droplet. These splinters are perfect ice nuclei that dramatically increases the IN content. The splinters grow immediately to form numerous needle shaped crystals

27 Bacterial ice nucleation

Chapter 8, Part 1. How do droplets grow larger? Cloud Droplets in Equilibrium. Precipitation Processes

Chapter 8, Part 1. How do droplets grow larger? Cloud Droplets in Equilibrium. Precipitation Processes Chapter 8, Part 1 Precipitation Processes How do droplets grow larger? Cloud contain water droplets, but a cloudy sky does not always mean rain. Cloud Droplets in Equilibrium In equilibrium water molecules