Tropical Weather in a World without Land or Seasons

Tropical weather systems are important components of Earth’s climate, yet our understanding about their properties, evolution, and impacts is limited by the lack of formal theories or detailed observations of the tropical regions (20°S–20°N). Unlike weather systems in other regions, tropical systems span multiple scales—from individual thunderstorms to tropical cyclones to cloud clusters that circumnavigate the globe. Moreover, tropical systems are poorly represented in weather and climate models, which further limits our ability to predict their day-to-day impacts and their potential changes in light of a warming climate.

With the goal of both increasing our understanding and identifying pathways for improved prediction, this study employs a simplified framework—called an aquaplanet—that is able to capture the multi-scale nature of tropical weather systems. An aquaplanet is an idealized representation of Earth, without complexities introduced by continents, topography, sea-ice, or seasons. The aquaplanet framework was employed in the Model for Prediction Across Scales (MPAS)—a global, non-hydrostatic model with grid refinement capability. This capability is used to produce simulations with system-resolving resolution (3-km cell spacing) in the tropics. A snapshot from this simulation (see Figure) shows that the aquaplanet framework in MPAS captures the desired features: an active region of clouds and precipitation in the tropics, including high-impact weather systems such as tropical cyclones.

This and other simulations with different horizontal resolution are being used to investigate four important issues regarding tropical weather systems:

  • tropical cyclogenesis and its relationship with planetary-scale waves,
  • factors conducive to heavy precipitation for prolonged periods of time,
  • expected changes of tropical weather systems in a warmer climate,
  • deficiencies of weather models without resolved convection.
Snapshot from an aquaplanet simulation produced with the Model for Prediction Across Scales. Gray shading shows outgoing longwave radiation, where light colors indicate the presence of clouds and dark colors indicate clear skies. Color shading shows precipitation rate.
Figure A: Snapshot from an aquaplanet simulation produced with the Model for Prediction Across Scales. Gray shading shows outgoing longwave radiation, where light colors indicate the presence of clouds and dark colors indicate clear skies. Color shading shows precipitation rate.