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How do we predict weather and climate?

How do we predict weather and climate?. Review of last lecture. Well-defined pattern of heating, temperature, pressure and winds around the globe. Three-cell model. Mechanism for each cell Two characteristics of zonal mean temperature structure

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How do we predict weather and climate?

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  1. How do we predict weather and climate?

  2. Review of last lecture • Well-defined pattern of heating, temperature, pressure and winds around the globe. • Three-cell model. Mechanism for each cell • Two characteristics of zonal mean temperature structure • Two characteristics of zonal mean wind structure. Why does westerly winds prevail in the extratropical troposphere? What cause the jet streams? • Semipermanent pressure cells. Low pressure is associated with clouds and precipitation. High pressure is associated with warm surface temperature, drought, and desert. • What drives the ocean surface currents? In the case of Ekman spiral, what is the direction of surface current relative to surface wind?

  3. Review of last lecture Tropical climate: • Mean state: The two basic regions of SST? Which region has stronger rainfall? What is the Walker circulation? • El Nino and La Nina: Which region has warm SST anomaly during El Nino? 4-year period. • Land-sea contrasts: seasonal monsoon, diurnal sea and land breeze Extratropical climate: • Mean state: westerly winds, polar vortex • What is the primary way El Nino affect extratropics? (PNA) • The oscillations associated with strengthening/weakening of polar vortex: AO, AAO

  4. Birth of global climate models • 1922 - Lewis Richardson’s “forecast factory”: filled a vast stadium with 64,000 people, each armed with a mechanical calculator. Failed! • 1940s - von Neumann assembled a group of theoretical meteorologists at Princeton to run the first computerized weather forecast on the ENIAC. The results were encouraging. • 1954, 1955 - Routine forecast: The Swedish Institute of Meteorology, the US JNWP. Barotropic model.

  5. World’s Major Climate Modeling Centers

  6. The Global Climate System - Atmosphere, ocean, biosphere, cryosphere, and geosphere

  7. Framework of Climate System Model Atmosphere Coupler . Land Sea Ice Ocean

  8. Atmosphere general circulation model • General circulation models are systems of differential equations based on the basic laws of physics, fluid motion, and chemistry. • Scientists divide the planet into a 3-dimensional grid (100-500 Km wide), apply the basic equations within each grid and evaluate interactions with neighboring points.

  9. Atmosphere General Circulation Model: Basic equations (Conservation of monmentum) • This set of equations is called the Navier-Stokes equations for fluid flow, which are at the heart of the GCMs. • There are other equations dealing with the conservation of H2O, CO2 and other chemical species. (Conservation of mass) (Conservation of energy)

  10. Regional model (Mesoscale model) • Mesoscale: 1 Km- 1000 Km, 1 min - 1 day • Grid size: 1 Km - 10 km • Three characteristics: • Non-hydrostatic processes • Nested grid • Topography effects

  11. Video: Constructing a climate model http://www.nas-sites.org/climatemodeling/page_3_1.php

  12. The current status of weather and climate predictions • Weather prediction: Generally good within one week, not good beyond 10 days. The skill depends on season and location. • Tropical cyclone (hurricane, typhoon) prediction: good in track, but bad in intensity • Climate prediction: Seasonal prediction generally good within 6 months, but not good beyond. Skill depends on season. • Climate projection: All models project global warming in the 21st century, but with a 3-fold difference in magnitude.

  13. Why is it so difficult to understand and predict weather and climate?

  14. Problem I: Different parts of the world are strongly connected to each other (The “Teleconnection Problem”) Global atmospheric flow

  15. Example 1: Global impacts of El Nino

  16. Example 2: Factors affecting U.S. weather and climate Arctic N. Atlantic Atlantic/ Sahel Madden-Julian Oscillation El Nino Amazon

  17. One location is affecting every locations, and in turn is affected by every locations

  18. Problem II: Different components of the earth system (atmosphere, land, ocean, ice, clouds, etc) are strongly interacting with each other (The “Feedback Problem”)

  19. Example: Generation of hurricanes Heat from the ocean

  20. Problem III: The earth system composes of both very big objects (such as the whole Pacific Ocean) and very small objects (such as the cloud droplets), making it very difficult to draw them on the same page (The “Subgrid-Scale Problem”)

  21. Summary • 4 components of the climate system model. 3 usages of global climate models. • Atmosphere general circulation models: Grid size. Name of the basic set of equations for atmosphere model. • Regional models: grid size. 3 characteristics. • The current status of weather and climate predictions: (1) weather prediction good to 10 days, (2) tropical cyclone prediction good in track but not in intensity, (3) climate prediction good to two seasons, (4) climate change projections have a 3-fold difference in magnitude. • The main reasons of the difficulties in weather and climate predictions: (1) Teleconnection problem, (2) Feedback problem, and (3) Subgrid-scale problem.

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