Convective Response in a Cloud-permitting Simulation of the MJO: Time Scales and Processes

Convective response under multi-scale forcing is investigated in this study using a month-long cloud-permitting simulation of the MJO. Convective response time scale (τ) is defined as the time lag between moisture convergence and convective heating. Results imply that τ is dependent on spatial and temporal scales of convective systems. Particularly, estimated τ for slowly varying signals (periods above 2.0 days) on the microscale and synoptic scale is about zero and 0.5 day, corresponding to instantaneous and non-instantaneous response respectively.

There are two main phases related to the processes of convective response: shallow convection development and shallow-to-deep convection transition. They are controlled by synoptic-scale boundary layer moisture convergence (M) and lower-tropospheric specific humidity (q_m). In the first phase, as q_m is small and lags the development of shallow convection, shallow convection occurrence is solely dominated by M (given suitable thermodynamic conditions in the boundary layer). In the second phase, shallow convection further preconditions the atmosphere for shallow-to-deep convection transition by increasing M and q_m through convection-convergence non-instantaneous feedback, i.e., shallow convection drives large-scale circulation that enhances moisture convergence and upward moisture transport. Additionally, eddy moisture upward transport by shallow convection itself (convection-convergence instantaneous feedback) also contributes to increase of q_m.

Comparison of the initiation and propagation stages of MJO indicates that τ is shorter in the propagation stage since M and q_m are larger therein.