Parameterizating the Radiative Effect on Microphysics for MJO and ENSO

Weather and climate models usually have a bias of “excessive water vapor” and “too dense clouds” that distort the energy and water cycles of the atmosphere via wrong cloud radiation. To mitigate the bias, we propose to introduce a new cloud process, the radiative effect on microphysics (REM) that leads to two common phenomena observed at the surface-dew and frost-and works as a precipitation production process in clouds. Since precipitating particles fall to the ground surface quickly, REM is a candidate to remove the modeling bias.

Plentiful observations from different platforms (e.g., aircrafts, field campaigns, and satellites) on the footprints of REM showed REM exists from the Arctic to the tropics, in different clouds such as diamond dust (or clear sky precipitation) in the Arctic, subvisual cirrus clouds in the tropical tropopause layer, and other cirrus clouds from the low to high latitudes.

To reveal the role of REM in weather and climate phenomena, we parameterized REM into very low order systems (e.g., a two-column two-layer model) and found that the instability of tropical clouds and radiation leads to the atmospheric oscillations with distinct timescales observed. That is, the instability of the boundary-layer quasi-equilibrium leads to the quasi-two-day oscillation, the instability of the radiative-convective equilibrium leads to the Madden-Julian oscillation (MJO), and the instability of radiative-convective-flux equilibrium leads to the El Niño-southern oscillation (ENSO).

In the preceding low order models the parameterization of REM is a key to replicate the oscillations with distinct timescales observed, which provides a novel clue for the weather and climate models to replicate MJO and ENSO. The instability criteria derived further suggest the weather and climate models can successfully predict the oscillations if properly representing cirrus clouds via REM and convective downdrafts in the tropics.