Effect of dry large‐scale vertical motions on initial MJO convective onset

Abstract

Anomalies of eastward propagating large‐scale vertical motion with ~30 day variability at Addu City, Maldives, move into the Indian Ocean from the west and are implicated in Madden‐Julian Oscillation (MJO) convective onset. Using ground‐based radar and large‐scale forcing data derived from a sounding array, typical profiles of environmental heating, moisture sink, vertical motion, moisture advection, and Eulerian moisture tendency 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. Convection with 3–7 km tops is ubiquitous but present in greater numbers when tropospheric moistening occurs below 600 hPa. Vertical eddy convergence of moisture in shallow to moderately deep clouds is likely responsible for moistening during a 3–7 day long transition period between suppressed and active MJO conditions, although moistening via evaporation of cloud condensate detrained into the environment of such clouds may also be important. Reduction in large‐scale subsidence, associated with a vertical velocity structure that travels with a dry eastward propagating zonal wavenumbers 1–1.5 structure in zonal wind, drives a steepening of the lapse rate below 700 hPa, which supports an increase in moderately deep moist convection. As the moderately deep cumulonimbi moisten the lower troposphere, more deep convection develops, which itself moistens the upper troposphere. Reduction in large‐scale subsidence associated with the eastward propagating feature reinforces the upper tropospheric moistening, helping to then rapidly make the environment conducive to formation of large stratiform precipitation regions, whose heating is critical for MJO maintenance.

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

“…However, the domain-averaged echo top height increases prior to 15 December. This may have occurred because positive humidity and upward motion anomalies propagated into the Indian Ocean from the Maritime Continent after the November MJO event [Johnson and Ciesielski, 2013;Powell and Houze, 2015]. In fact, the real-time multivariate MJO index [Wheeler and Hendon, 2004] remained in Phase 4 or 5, indicative of widespread support for convection over the Maritime Continent, for nearly a month after active MJO conditions subsided over the central Indian Ocean during late November.…”

“…A key component to the hypothesis is the time scale of moisture buildup. Other hypotheses for MJO convective onset [Matthews, 2008;Powell and Houze, 2015] invoke temperature or divergence fields to explain how widespread deep convection may develop. Therefore, showing that time series of wind, temperature, and humidity near single rawinsonde locations are consistent with those of the mean large-scale profiles of each is important.…”

Diurnal timescale feedbacks in the tropical cumulus regime

“…Onset is manifest in simultaneous large-scale increases in tropospheric humidity, rainfall, and diabatic heating as subsidence relaxes [Lin and Johnson, 1996;DeMott and Rutledge, 1998;Johnson et al, 1999;Kiladis et al, 2005;Haertel et al, 2008;Johnson and Ciesielski, 2013;Powell and Houze, 2015;RJ15]. The coupling between these quantities is expected from WTG theory [Sobel et al, 2001], which describes the prevailing thermodynamic balance in the tropics between the diabatic warming and cooling by clouds and radiation and the adiabatic cooling and warming by largescale-averaged vertical motion [Yano and Bonazzola, 2009].…”

“…The impact of this rectified heating on large-scale subsidence-and hence on the onset process-is captured through the WTG framework. In nature, equatorial waves can also influence the onset process [Maloney, 2009;Powell and Houze, 2015;Maloney and Wolding, 2015]; this study's focus is limited to the influences of local diabatic processes, however. The experimental methodology is described next, with results in section 3, and summary and conclusions in section 4. lowest 2 km), smoothly stretched to Dz 5 350 m for z > 15 km.…”

Effect of Overturning Circulation on Long Equatorial Waves: A Low-Frequency Cutoff

“…Incompressible meridionalvertical background flow is here shown to produce no growing long wave disturbances. However, Powell and Houze (2015) observe anomalous large-scale vertical motion leading the MJO events recorded by the DYNAMO field campaign. Therefore, some assumptions in this model will be relaxed to allow for background zonal variation in concert with destabilizing upscale fluxes, and a source of instability, particularly one leading to MJO initiation, sought therein.…”