This study investigated development processes of maritime meso-scale convective systems (MCS) and continental MCS in an inactive phase of a large-scale convective disturbance associated with MaddenJulian Oscillation (MJO), which is dominant in tropical region. Development processes of MCSs in an inactive phase of MJO have not well been revealed. Analysis of Outgoing Longwave Radiation (OLR) filtered within the period of 25 - 80 days including the spectral peak of the MJO indicated that maritime MCSs developed in active phases and an early inactive phase of MJO, and the continental MCSs developed in an inactive phase and an early active phase of MJO during Australia summer monsoon from 20 December 1998 to 28 February 1999, when an observation was hold over the tropical northern Australia (NAU). Moisture variations in the middle level over Darwin were positively related to precipitation averaged over the NAU region, and had a period of 5 - 10 days indicating the dominant spectral of Equatorial Rossby Wave (ERW).
This study focused on MCSs that developed on the days when daily rainfall averaged over NAU was above the seasonal average and the phase of ERW was active during an inactive phase of the MJO. From the days, a maritime MCS on 15 January 1999 during monsoon active regime (the low-level westerly), which was mainly defined as wind at 850 hPa averaged over the NAU region, and continental MCS on 21 January 1999 during break regime (the low-level easterly) were selected and investigated with the analysis of observational data mainly using Doppler radars and a cloud resolving model.
The analysis of observational data showed that the maritime MCS formed in a west several convective lines extensive stratiform region. The dual-Doppler radar analysis revealed that shallow convections formed on a convergence region between the low-level weak outflow with a weak downdraft within the stratiform precipitation and the moist westerly in the low level during the early developing stage of a convective line. A cyclonic circulation (mesovortex) with convergence in the middle-level was found in the stratiform region during the mature stage of the MCS. The development of several convective lines and the mesovortex resulted in the maintenance of the maritime MCS. The analysis of heating profiles revealed that a convective region had the maximum heating at the middle level and a stratiform region had the small cooling in the low level in the developing stage of the MCS or later, and a stratiform region had relatively large heating at the middle-upper level during the mature stage. The development processes of the maritime MCS that have not well been known was elucidated.
The continental MCS formed in a strong mid-level east-southeasterly flow of dry air that was influenced by tropical depressions associated with an ERW. The dual-Doppler radar analysis showed that the MCS formed a number of convective lines oriented parallel to low- and mid-level shears with relatively strong negative vortices and convergence (in the scale of convective cell) in the low-mid level. Positive (anticyclonic) vorticity was found with a descending rear inflow in the rear of the lines and the stratiform region of the MCS below the mid-level. The descending rear-to-front flow to the convective region strengthened the convergence and vertical wind-shear in the low-level and resulted in the development and maintenance of convective lines with intensive negative vortices. The analysis of heating profiles represented a convective heating maximum at the middle-level and a moderate stratiform cooling below the melting level in the developing stage of the MCS or later.
A comparison of the heating profiles in the stratiform parts of two MCSs indicated a larger low-level cooling in the continental MCS and a larger heating at the mid-upper level in the maritime MCS. It was suggested that the different stratiform heating of two MCSs in the inactive phase of the MJO played significant roles in their development processes.
To understand the processes of development and maintenance of the maritime and continental MCS, the processes of momentum transport were examined and the budgets of heating and drying profiles were analyzed using a cloud resolving storm simulator (CReSS). Analysis of momentum transport for the maritime MCS indicated the enhancement of high-θe southwesterly in the mid-level perpendicular to convective bands and the upward transport of the momentum within the stratiform region. This acceleration was accompanied with an increase of horizontal convergence and negative vorticity (mesovortex) in the middle level via stretching by the increase of horizontal convergence of moist air and upward vertical advection of vorticity in response to larger heating by deposition at mid-upper level than that of continental MCS. The maximum domain-mean vertical advection of heat and moisture for the maritime MCS was found in the mid- to upper levels. These results suggest that the maritime MCS had a measurable impact on the synoptic-scale circulation due to the extensive region of stratiform precipitation and the development of stratiform precipitation acted as a positive feedback to strengthen the southwesterly momentum in the meso-to-synoptic scale at the mid levels.