Large-Scale Vertical Motions and MJO Convective Onset

Description

Reanalysis (ERA-Interim), radar, and rawinsonde data collected during the 2011–2012 DYNAMO/AMIE field experiment reveal the impact of circumnavigating large-scale vertical motions on Madden-Julian Oscillation (MJO) convective onset over the central, equatorial Indian Ocean. The large-scale vertical motions, derived from reanalysis, are associated with large negative anomalies of 150 hPa velocity potential that are dominated by circumnavigating anomalies of upper tropospheric zonal wind with zonal wavenumber one structure. The corresponding low-frequency structure of the vertical motion indicates anomalous upward motion throughout the troposphere, with a maximum near 500 hPa, below where large-scale divergence near 150 hPa takes place. Using large-scale forcing data derived from a sounding array in conjunction with ground-based radar, typical profiles of Q1, Q2, vertical motion, and moisture advection are computed for periods prior to those during which deep convection is prevalent and those during which moderately deep cumulonimbi do not form into deep clouds. In both environmental regimes, convection with tops between 3 and 7 km are present, meaning that the mere presence of congestus clouds does not precondition the environment for deep convection to develop. However, the relative humidity below 600 hPa is as much as 20 percent larger prior to formation of deep convection than prior to periods when deep convection does not form. It is shown that the large-scale reduction in subsidence that occurs over the Indian Ocean as the wavenumber one structure moves eastward promotes shallow to moderately deep moist convection by causing a reduction in adiabatic warming that exceeds the reduction in radiative cooling caused by an upward shift in the profile of water vapor content. Deep convection becomes more prominent as the lower troposphere is moistened in response to the changes in large-scale vertical motion.Upper-tropospheric moistening on the large-scale could be accomplished by deep convection alone, but it occurs more rapidly because of anomalous upward motion in the upper-troposphere associated with the large-scale divergence. We thus provide evidence for how anomalies of zonal wind in the upper-troposphere are connected to MJO convective onset.