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Which assumption does quasi-equilibrium convective closure make?

  • A.Convection occurs only where the large scale ascent exceeds a fixed threshold value at the level concerned
  • B.Convection is always absent
  • C.Convection is triggered by the local vertical velocity at the grid scale rather than by the buoyancy present
  • D.Convection consumes instability as fast as the large scale generates it

Answer: D

Arakawa and Schubert proposed that the convective mass flux adjusts so that the available energy remains near zero, which determines the intensity of the convection from the large scale forcing. It works well in the tropics where the timescale separation is clear. Over land with a strong diurnal cycle the assumption is less secure, which is a recognised source of model error. The quantity that varies and the quantity that does not are worth naming before any substitution is attempted.

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