Role of large‐scale moisture advection for simulation of the MJO with increased entrainment

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Abstract
The intraseasonal moisture budget is analyzed in simulations with the UK Met Office climate model, which produces a reasonable representation of the Madden–Julian Oscillation (MJO) when the entrainment rate in the convective parametrization is increased by 50%. Analysis of the moisture budget shows that parametrized convection tends to dry the troposphere and that large‐scale vertical advection moistens the troposphere. These two tendencies mostly balance each other. However, the total moisture tendency is asymmetric relative to the maximum precipitation, corresponding to the recharge and discharge process of organized convection in the Tropics. This moistening before and drying after the maximum precipitation is largely due to large‐scale horizontal advection of moisture. By comparing to the control run that does not simulate a realistic MJO, we find that increasing the entrainment acts to reduce deep convection in the relatively dry environment by increasing the mixing of the dry air. As a result, large‐scale convection and large‐scale advective processes play a stronger role during the development stage, which implies that convection is necessarily better organized.

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