Wednesday, 5 August 2015: 11:00 AM
Republic Ballroom AB (Sheraton Boston )
The failure of global models to explain the decadal trend of stratospheric water vapor may stem from a lack of representation of rapid water transfers from troposphere to stratosphere. This motivates our modeling study of Hector the Convector on 30 November 2005. This was a particular case of very deep convection over the Tiwi Islands, Australia, for which plumes of ice particles reaching 19 km altitude were observed. We performed a Giga Large-Eddy Simulation (100 m horizontal resolution, more than 1 billion grid points) using cutting-edge computing resources, together with a series of simulations with coarser resolutions, from 200 m to 1600 m. Whatever the resolution, two updrafts overshot the tropopause, carrying ice crystals into the stratosphere and leading to moistening. Grid spacing of the order of 100 m may be necessary for a reliable estimation of hydration (Dauhut et al. ASL 2015, doi: 10.1002/asl2.534). The mechanisms leading to very deep convection were investigated. The low-level convergence lines determined the locations of the tallest updrafts. Created by the sea breeze in the morning, they appeared along the coasts over which mid-level convection popped up at noon. The associated congestus then produced cold pools that supplied the convergence lines with surface buoyant air. Deep convection became visible shortly after, due to the enhanced buoyancy both at the base of the updrafts and above the freezing level with the generation of icy hydrometeors. By continuing to move inland, the convergence lines drew together and reinforced each other. Above them, very deep convection developed as two updrafts expanding over an 8-km width and keeping their core undiluted up to the tropopause. Large cold pools were then generated and drove convective cells at their edges. The convective cells merged into each other above the island center, leading to several mesoscale convective complexes. Although they did not reach the stratosphere, the convective complexes still transported a large amount of water into the upper troposphere. Finally, the resulting heavy precipitations fed a massive cold pool that cut Hector the Convector off from the ground and expelled its remnants outside the Tiwi Islands. A comprehensive analysis of individual updrafts and their properties shows that the overshooting updrafts present higher buoyancy, stronger vertical velocities and larger hydrometeor contents than ordinary deep convective updrafts and mesoscale convective complexes. In the tallest updrafts, the equivalent potential temperature exceeds the maximum value at the surface by 10 K, the upward wind speed reaches 50 m/s, and the loading of condensed water exceeds 10 g/kg. Both the low-level convergence lines intensified by cold pools and the reduced mixing in the troposphere are found to be determinant for the transition from deep to very deep convection. This study was supported by the StratoClim project.
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