Applications of Aerosol Remtoe Sensing Products in Climate Studies Zhanqing Li Dept of Atmos. & Oceanic Science University of Maryland
2000s: aerosol climatologies, aerosol-cloud interaction 2003: GLAS 2002: GLI 1999: MODIS, MISR 1990s: analysis of POLDER, ATSR, methods for MODIS, MISR, radiation budget 1996: POLDER 1991: ATSR 1980s: study of transport aerosol species - effect on climate; stratospheric, aerosol overland: Stowe, McCormicK 1984: Earth Radiation budget satellites 1981: AVHRR afternoon 1979: SAGE 1976: dust - Landsat, Fraser 1975: GOES-VISSR 1972: ATS III dust transport Carlson and Prospero, 1972: Landsat, 1967: Sekera, aerosol from satellites polarization meas. 1967: ATS III 1965: stratospheric aerosol profiles from Vostok 6 - Rosenberg and Tereshkova
The effects of aerosol pollution on clouds • Since the 1960s, measurements have provided numerous and consistent pieces of evidence that an increase in pollution leads to increases in Cloud Condensation Nuclei (CCN) [a sub-set of atmospheric aerosols].
Indirect Effect Haywood and Boucher Revs. Geophys. (accepted) 2000 1) Increased CCN - reduces reff 2) Drizzle suppression - increases LWC 3) Increased cloud height 4) Increased cloud lifetime ‘First’ indirect effect ‘Second’ indirect effect
Ice in clouds Ice nuclei (IN) are a much smaller sub-set of atmospheric aerosols than CCN. Their role in precipitation formation in certain clouds is critical. Finding correlation between the concentrations of ice nuclei and ice crystals in clouds is difficult because of the low concentrations of IN and the numerous mechanisms by which ice crystals can form including ice multiplication mechanisms.
How do clouds form? Clouds form in regions of the atmosphere where water vapor is supersaturated. We focus on liquid water clouds. Water vapor supersaturation is generated by cooling (primarily through expansion in updraft regions and radiative cooling) Cloud droplets form from pre-existing particles found in the atmosphere (aerosols). This process is known as activation. Aerosols that can become droplets are called cloud condensation nuclei (CCN). Cloud CCN that activates into a cloud drop Aerosol particle that does not activate
Activity Spectrum Let Nc be the number of particles per unit volume that are activated to become cloud droplets. Data from cloud chamber measurements are often parameterized as Nc = C (S-1)k where C and k are parameters that depend on air mass type. Rogers gives: Maritime air: 30 < C < 300 cm-3; 0.3 < k < 1 Continental air: 300 < C < 3000 cm-3; 0.2 < k < 2 Thus, for the same saturation ratio, one would expect to find small numbers of CCN per unit volume in maritime air and large numbers per unit volume in continental air.
How can humans affect clouds? • By changing CCN; cloud properties are a strong function of their concentration. • This phenomenon is known as aerosol indirect effect. • The aerosol indirect effect can lead to climatic cooling by: • Increasing cloud reflectivity (albedo) • Increasing cloud lifetime & coverage. Higher A l bedo Lower A l bedo CCN CCN Clean Environment Polluted Environment (few CCN) (more CCN)
Is the indirect effect globally important? Pollution is a global problem. CCN are emitted together with greenhouse gases. Asian pollution plumes. Biomass burning in the Amazon.
Why is the Indirect Effect Poorly Characterized? • Aerosol-cloud interactions take place at smaller spatial scales than global climate models can resolve, and must be parameterized. • Aerosol-cloud interactions are complex; many aspects are unknown or poorly understood. • Climate models provide important but limited information about clouds and aerosols.
? Quantification of the Indirect Effect Aerosol Size Distribution and Chemical Composition Cloud Radiative Properties Cloud Droplet Number and Size Well Defined This problem has historically been reduced to finding the relationship between aerosol number concentration and cloud droplet number concentration. Empirical relationships are often used.
Aerosol-Cloud Interaction Modules • Goal • Couple all aerosol-cloud-radiation interactions within a framework of parameterizations appropriate for global models. • “Input” variables (from GCM) • Cloud liquid water content. • Aerosol size distribution and chemistry. • Wind fields. • Static stability/turbulence. • “Output” variables (to GCM) • Droplet number, distribution characteristics • Cloud optical properties • Cloud coverage, subgrid statistics
Very large variability. • Why? • Meteorology • Cloud microphysics • Chemical composition • etc… Droplet Concentration (Boucher & Lohmann, 1995) Aerosol sulfate concentration Simplest aerosol-cloud interaction module: correlations Pro: Very simple relationship to implement. Fast computation. Con: Large predictive uncertainty, without chance of improving.
Maximum at AOD ~ 0.25 Giant CCN shift max to greater AOD Andreae, ACPD 2008
Unaccounted “chemical” effects on droplet activation Slightly soluble compounds (Shulman et al., 1996): They add solute to the drop as it grows; this facilitates their ability to activate. Examples: organics (succinic acid), CaSO4. Soluble gases (Kulmala et al., 1993): They add solute to the drop as it grows; this facilitates their ability to activate. Examples: HNO3, HCl, NH3. A(g) A(g) A(g) A(aq) A(aq) A(aq)
Unaccounted “chemical” effects on droplet activation Surface-active soluble compounds (Facchini et al., 1999): They decrease surface tension of droplets; this facilitates their ability to activate. Examples: organics (succinic acid, humic substances). Surface tension data from cloud and fog water samples. Pure water 75 The departure from pure water values can be very large! Surface tension change is different for each CCN. 70 65 Droplet concentration range at activation Surface tension (dyne/cm) 60 55 Charlson et al., Science, 2001 50 1e-4 1e-3 1e-2 1e-1 -1 C(mol l )
Film breaks water molecule water molecule Slow Rapid Unaccounted “chemical” effects on droplet activation Film-forming compounds (e.g., Feingold & Chuang, 2002): They can slow down droplet growth. Once the film breaks, rapid growth is resumed: Examples: hydrophobic organics. Such substances do not necessarily alter droplet thermodynamics; they affect the kinetics of droplet growth. If present, such substances can strongly affect droplet number.
Physically-based aerosol-cloud interaction modules Uncertainties can be decreased by using first principles. Cloud droplet balance: Activated droplets for updraft w Probability of updraft w • Activation is the direct aerosol-cloud microphysical link. Two types of information are necessary for its calculation: • Aerosol chemistry and size distribution (CCN) • Representation of subgrid dynamics in cloud-forming regions. • Embedding a numerical activation model is too slow; must parameterize.
drop growth activation aerosol Mechanistic parameterizations: underlying ideas • Approach: • Assume an aerosol size distribution and chemical composition below cloud. • Aerosols rise into cloud. • Expansion generates cooling and supersaturation. • Aerosols activate into droplets. • Köhler theory links aerosols to CCN properties. t Smax S • Major challenge: • Derive expression for the condensational growth of CCN; include within the supersaturation balance for the parcel, and solve for the maximum. • Solution: • Depends on the approach used in each parameterization. • (e.g. Nenes and Seinfeld, JGR, 2003)
N & S (2003) evaluation: compare with numerical model 0.70 Numerical Simulation (s.t. effects present) Parameterization (s.t. effects present) 0.60 Parameterization (s.t. effects absent) 0.50 0.40 Activation Fraction 0.30 0.20 0.10 Nenes and Seinfeld, in press Nenes and Seinfeld, JGR, 2003 0.00 0.1 1 10 Updraft Velocity (m/s)
pristine polluted Nenes and Seinfeld, JGR, 2003 Underprediction: common to many parameterizations Abdul-Razzak et al. parameterization “family” = 1.0
Observational evidence of indirect effect Satellite observation of aerosol indirect effect in the Black Sea. Red: clouds with large drops. White: clouds with small drops. Rosenfeld et al., Science
Observational evidence of indirect effect Satellite observation of aerosol indirect effect in the Black Sea. Power plant Lead smelter Port Oil refineries Red: clouds with large drops. White: clouds with small drops. Rosenfeld et al., Science
DER-AOD relationship Yuan et al. (2008, JGR)
AIE efficiency distribution Yuan et al. (2008)
Global Analysis Student-t test indicates except India the difference among different loading of aerosols are statistically significant at least at the 95% level
Can the slowing of auto-conversion result in increasingprecipitation?
Conceptual model: HAIL Graphics by Robert Simmon, NASA
Conceptual model: HAIL Graphics by Robert Simmon, NASA
Why is Continental - Maritime classification so fundamental? Annual average lightning density [flashes km-2] Lightning prevail mostly over land, whereas rainfall is similar over land and ocean, indicates fundamental differences between continental and maritime rainfall.
Annual average lightning density [flashes km-2] Why is Continental - Maritime classification so fundamental? TRMM annual average rainfall amount [mm / day] There is little relation between lightning and rainfall amount
Global N. Hemisphere S. Hemisphere Ocean Land Global effects of pollution on precipitation GCM-- estimates 0 to - 4.5% change in global mean precipitation over the last 100 years due to the direct and indirect aerosol effects. The differences among models over land range from -1.5% to -8.5%.
A lot have been done concerning aerosol’s impact on rainfall Rainfall Suppressed by Aerosols Rainfall Enhanced by Aerosols Rosenfeld, 1999; Rosenfeld, 2000; Andreae, 2004; etc Koren, 2005; Lin, 2006; Bell, 2007; etc Observational studies Khain, 2004，2005; Tao 2007, Fan, 2007; Van den Heever, 2007 etc Modeling studies
But little has been done for rain frequency! While rain amount and frequency change in harmony in general, the impact of aerosol on initiation of rain is likely to be more significant than rain amount, as the latter is dictated more by dynamics and abundance of available water
Datasets Used • Daily ARM SGP data 2003-2008 (~20000 data samples) • Most complete and highest quality measurements of aerosol, cloud, atmospheric state • Key variables used: • Aerosol CN number concentration on the ground • Tipping bucket rain gauge • LWP from microwave radiometer • Cloud bottom and top heights from cloud radar & lidar • NOAA/NCAR Reanalysis • MODIS cloud particle size
Rainfall Frequency for clouds with different liquid water path at SGP(All-Season Data)
T P WV Wind
Cloud Top Heights(for clouds of cloud base <1km) Summer Only All Seasons For clouds with CBH<1km, clouds are classified into two categories with cloud top heights greater (blue) and less than (red) 2km.
Cloud Top Heights Summer Only All Seasons
Competition of two opposite effects Meteorological effects Responses of Rainfall frequency to increasing CN Invigoration Effects Depend critically on cloudbase !! Increase rainfall Not always increase Suppress rainfall Depend critically on available water Microphysical Effect
The WMO/IUGGINTERNATIONAL AEROSOL PRECIPITATION SCIENCE ASSESSMENT GROUP(IAPSAG) Aerosol Pollution Impact on Precipitation: A Scientific Review Zev Levin, Chairman William Cotton, Vice Chairman Approved by the WMO - May. 2007
Recommendations • Implement a series of international projects targeted toward unraveling the complex interactions among aerosols, clouds, and precipitation. • WMO/IUGG should take the lead in such projects together with other UN and International Organizations. • Some of these could be sponsored and financially supported by the countries involved. For example: • Study the effects of an evolving industrial economy such as China on precipitation. • Study the effects of biomass burning and dust in some of the African regions.