The Role of Shallow Cloud Moistening in MJO and Non-MJO Convective Events over the ARM Manus Site

Abstract

Observations from the Atmospheric Radiation Measurement Program (ARM) site at Manus Island in the western Pacific and (re)analysis products are used to investigate moistening by shallow cumulus clouds and by the circulation in large-scale convective events. Large-scale convective events are defined as rainfall anomalies larger than one standard deviation for a minimum of three consecutive days over a 10° × 10° domain centered at Manus. These events are categorized into two groups: Madden–Julian oscillation (MJO) events, with eastward propagation, and non-MJO events, without propagation. Shallow cumulus clouds are identified as continuous time–height echoes from 1-min cloud radar observations with their tops below the freezing level and their bases within the boundary layer. Daily moistening tendencies of shallow clouds, estimated from differences between their mean liquid water content and precipitation over their presumed life spans, and those of physical processes and advection from (re)analysis products are compared with local moistening tendencies from soundings. Increases in low-level moisture before rainfall peaks of MJO and non-MJO events are evident in both observations and reanalyses. Before and after the rainfall peaks of these events, precipitating and nonprecipitating shallow clouds exist all the time, but their occurrence fluctuates randomly. Their contributions to moisture tendencies through evaporation of condensed water are evident. These clouds provide perpetual background moistening to the lower troposphere but do not cause the observed increase in low-level moisture leading to rainfall peaks. Such moisture increase is mainly caused by anomalous nonlinear zonal advection.

Long‐Lived Mesoscale Convective Systems of Superparameterized CAM and the Response of CAM

“…There are two moisture sink peaks in SPCAM at approximately 3 and 7 km in Figures c, which is also observed in nature (Zermeño‐Díaz et al, ) during active phases of large MCSs. Whereas in CAM, there is no moisture sink but a moisture source at 3 km throughout the MCS life cycle.…”

Global climate impacts of stochastic deep convection parameterization in the NCARCAM5

“…The increase in shallow convection is porbably due to its interaction with deep convection: the reduced deep concection favors the buildup of CAPE and more frequent shallow convection [ Zhang and Mu , ], which serves to precondition the lower troposhere for MJO [ Zhang and Song , ; Cai et al ., ; Zermeo‐Daz et al . ]. Wang et al .…”

Possible Role of the Diurnal Cycle in Land Convection in the Barrier Effect on the MJO by the Maritime Continent

“…The vanguard of precipitation appears to be associated with the low‐level easterly flow and premoistening. Triggered by diurnal forcing, land convection produces the anomalously high vanguard of precipitation in place of shallow convection commonly observed to the east of MJO convection centers over the open ocean (Lau & Wu, ; Kikuchi & Takayabu, ; Del Genio et al, ; Barnes & Houze, ; Bellenger et al, ; Rowe & Houze, ; Ruppert & Johnson, ; Xu et al, ; Zermeno‐Diaz et al, ). In contrast, MJO‐B events do not establish such a well‐defined zonal overturning circulation and westward title structure until their convection centers enter the MC (Figures e–h).…”