Abstract
Convective response under multiscale 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 0 and 0.5 days, corresponding to instantaneous and noninstantaneous responses, 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 (qm). In the first phase, as qm 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 sustaining M and qm through noninstantaneous convection–convergence 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 (instantaneous convection–convergence feedback) also contributes to an increase of qm. The comparison of the initiation and propagation stages of MJO indicates that τ is shorter in the propagation stage since M and qm are larger therein.
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