Diurnally Modulated Cumulus Moistening in the Preonset Stage of the Madden–Julian Oscillation during DYNAMO*

Abstract

Atmospheric soundings, radar, and air-sea flux measurements collected during Dynamics of the Madden-Julian Oscillation (DYNAMO) are employed to study MJO convective onset (i.e., the transition from shallow to deep convection) in the tropical Indian Ocean. The findings indicate that moistening of the low-midtroposphere during the preonset stage of the MJO is achieved by simultaneous changes in the convective cloud population and large-scale circulation. Namely, cumuliform clouds deepen and grow in areal coverage as the drying by large-scale subsidence and horizontal (westerly) advection wane. The reduction of large-scale subsidence is tied to the reduction of column radiative cooling during the preonset stage, which ultimately links back to the evolving cloud population. While net column moistening in the preonset stage is tied to large-scale circulation changes, a new finding of this study is the high degree to which the locally driven diurnal cycle invigorates convective clouds and cumulus moistening each day. This diurnal cycle is manifest in a daytime growth of cumulus clouds (in both depth and areal coverage) in response to oceanic diurnal warm layers, which drive a daytime increase of the air-sea fluxes of heat and moisture. This diurnally modulated convective cloud field exhibits prominent mesoscale organization in the form of open cells and horizontal convective rolls. It is hypothesized that the diurnal cycle and mesoscale cloud organization characteristic of the preonset stage of the MJO represent two manners in which local processes promote more vigorous daily-mean column moistening than would otherwise occur.

Evolution of precipitation and convective echo top heights observed by TRMM radar over the Indian Ocean during DYNAMO

“…That time scale is consistent with DYNAMO sounding network data analyzed by Johnson and Ciesielski [2013] and Ruppert and Johnson [2015]. A similar length of time was observed during a 1993 Tropical Ocean-Global Atmosphere Coupled Ocean-Atmosphere Response Experiment (TOGA COARE) MJO event for the cloud population to evolve from a suppressed stage with smaller clouds to a mature stage consisting of clouds with extensive anvils [Kikuchi and Takayabu, 2004].…”

“…The red boxes along the time axis represent periods of convectively active conditions. They represent dates during which the derived latent heating profile from a sounding network Johnson et al [2015] near and north of the equator between about 70°E and 80°E does not indicate suppressed conditions as shown by Ruppert and Johnson [2015]. The vertically oriented, black, dashed lines represent dates (16 October, 18 November, and 15 December) on which a mesoscale convective system was first observed by the ground-based S-PolKa precipitation detecting radar in association with three MJO events at Addu City.…”

Multiple and spin off initiation of atmospheric convectively coupled Kelvin waves

“…These case studies suggest that both the upper ocean temperature diurnal variability and ocean surface flux variability are important factors for formation of convection and precipitation, especially when they are organized in multiple CCKWs. Such link between atmospheric convection and upper ocean diurnal variability was noted previously (Chen et al 2015; Ruppert and Johnson 2015) and it will be further elaborated using composites of multiple CCKW initiation cases and a modeling study.…”

“…In the variable source simulations, the geopotential height anomaly, the wind anomaly and the wind convergence are stronger than in control simulation with the constant source. This link between the diurnally varying heat source and Kelvin waves and a relationship between the diurnal SST variability and development of atmospheric convection during the MJO suppressed phase, noticed by Chen et al (2015) and Ruppert and Johnson (2015), indicates that our simplified simulations are qualitatively agreeing with observations with the caveat that some of the incoming diurnal shortwave signal can be partitioned to directly heat the atmosphere and influence convection development (Ruppert 2016;Ruppert and Johnson 2016). Furthermore, shallow water model solutions suggest that the link between short-term variability of a source and a structure of Kelvin wave response is truly nonlinear and exemplifies interactions across many temporal and spatial scales.…”

“…A recent study by Ruppert and Johnson (2015) shows that diurnal SST variability is reflected in the diurnal cycle of the heating profiles in the atmosphere. Those authors show that the low to midlevel moistening during the suppressed phase of the MJO is accomplished by a population of shallow cumulus and congestus clouds that exhibits a pronounced diurnal cycle in response to oceanic diurnal warm layers.…”