Organized layer overturning in mesoscale convective systems over the western Pacific warm pool
This study employs a high resolution numerical cloud model to investigate overturning of the tropical environment by mesoscale convective systems (MCSs) occurring over the westerly onset and strong westerly regions of the large scale Kelvin-Rossby wave pattern during the Tropical Ocean Global Atmosphere Coupled Ocean Atmosphere Response Experiment (TOGA COARE). Two aspects of deep convective overturning are addressed: a tendency for some low-level inflows to be layer-like in structure and momentum transport processes by organized regions of stratiform precipitation.Airborne Doppler radar data show that organized deep convective systems over the western tropical Pacific warm pool are often accompanied by layers of ascending inflow ∼0.5--4 km in depth, deeper than that described by current theory in which only boundary layer air is thought to rise significantly in buoyant, parcel-like updrafts. Trajectory analysis of convection simulated over the strong westerly region of the large-scale Kelvin-Rossby pattern demonstrates the existence deep inflow layers. In the westerly onset simulation, the parcel model prevails, with only the air in the lower part of the inflow actually rising in the deep convective updrafts. Increasing the relative humidity of the low and midlevel environmental air brings about the layer inflow mode. Proposed moistening mechanisms are inflow layers passing through tropical stratiform precipitation and mixing in the vertical by cumulus of moderate depth.Extensive regions of organized mesoscale stratiform descent act to transport downward midlevel momentum at low levels, easterlies over the westerly onset region and westerlies over the strong westerly region. In the westerly onset simulation, the convective and stratiform regions compete, with the convective attempting to accelerate the westerly momentum via pressure gradient and the stratiform attempting to increase the easterly momentum through downward vertical transport. Over the strong westerly region, both convective and stratiform components act to increase the low-level westerlies. Results imply MCSs in each flow regime may exert an influence on the large scale wave structure, negative or neutral during the westerly onset phase and positive during the strong westerly phase.
- Atmospheric sciences