Climate models 101 for air quality Anand Gnanadesikan Department of Earth and Planetary Sciences Johns Hopkins University GAIA Conference on Climate Change and Public Health

What are the components of an Climate Model? A. Dynamical cores (atmosphere, ocean, ice): 1. Compute the pressure field. 2. Compute the flows resulting from this pressure field. 3. Use these flows to transport the quantities (heat, mass, etc.) that determine the pressure field. Equations are standard… numerics are not.

Pressure is an example of a variable that can be predicted relatively well. High-to-low pressure swings are ~30mb. Errors are 3!

What are the components of a Climate Model? B. Parameterizations of subgridscale processes 1. Vertical mixing (especially stratified BL) 2. Lateral mixing 3. Breaking waves. Relationship between parameterizations and mean state differs substantially between models. 4. Clouds and their effects. 5. Some (generally simplified) chemistry

Low cloud amount is one of our poorer predictions. Correlations are good over Atlantic, East Pacific Sector, poor over high latitudes, Indian Ocean.

Lessons  Can’t use model output blindly- need to match simulation to the area/process.  Recognize the difference between parameterization-dependent results and results that are driven by the fundamental conservation equations of the system.

An example:  Develop idealized insoluble tracer of pollution with emissions like CO but a fixed lifetime.  Develop a version of this tracer which can be washed out by precipitation. Fang, Fiore, Horowitz, Gnanadesikan, Held, Chen, Vecchi and Levy, The impacts of changing transport and precipitation on pollutant distributions in a future climate, in rev. for J. Geophys. Res.-Atmos.

Results Concentrations of insoluble tracer go up near ground at top of atmosphere, reflecting changes in vertical mixing. Concentrations of insoluble tracer rise everywhere- despite increase in precipitation!

Insoluble changes  Warmer atmosphere-> more moisture.  More moisture-> more latent heating aloft.  More latent heating aloft pushes tropopause up, but stabilizes exchange with lower half of troposphere.

Reason for soluble changes Precipitation changes from large- scale (stratus) which is fairly efficient in removing pollution per unit of rain to convective, which is less effective. Change in deposition tracks change in large-scale precipitation.

Results in context  Less vertical exchange driven by thermodynamics- likely to be common across models. This causes an increase in pollution.  Less wet deposition driven by representation of physical processes. Result is counterintuitive and may be true, but should be evaluated within the context of individual regions. All models are wrong. Some models are useful!