Session 16.4 Microphysical-dynamical interactions in an idealized tropical cyclone simulation

Thursday, 20 August 2009: 11:15 AM
The Canyons (Sheraton Salt Lake City Hotel)
Steve Herbener, Colorado State Univ., Ft. Collins, CO; and W. R. Cotton

Presentation PDF (632.6 kB)

The physical processes that take place in clouds and storms are complex and extremely difficult to measure. Because of this, computer models have been extensively utilized to help researchers gain insight into these processes. One such study, Zhang et al., (2006), Zhang (2008), Zhang et al., (2009), utilized the Regional Atmospheric Modeling System (RAMS) to study the effects of dust acting as CCN on Atlantic tropical cyclones (TC). Early studies by Zhang et al.,(2006) demonstrated a strong response to CCN with greater amounts of CCN decreasing storm intensity. Subsequent simulations(Zhang, 2008; Zhang et al., 2009) exhibited a general of decreasing storm intensity with increasing CCN but the results were not monotonic. This demonstrates the complexity of interactions of microphysics and dynamics within a TC.

The work to be described is a continuation of the Zhang (2008) study with the purpose of untangling some of the complex microphysical-dynamical interactions in the TC simulations. This study focuses on the investigation of a hypothesis that has been presented to explain the non-monotonic response to enhanced CCN concentration levels. It is proposed that an increase of CCN in the outer rainbands cause reduced collision and coalescence, which results in more supercooled liquid water to be transported aloft which then freezes and enhances convection via enhanced latent heat of freezing. The intensified convection condenses more water which ultimately enhances precipitation in the outer rainbands. Enhanced evaporative cooling from the increased precipitation in the outer rainbands produces stronger and more widespread areal cold pools which block the flow of energy into the storm core and ultimately inhibits the intensification of the TC. However, the amount of suppression of the strength of the TC depends on the timing between the transport of CCN to the outer rainbands and the intensity and lifecycle stage of the outer rainband convection. If the elevated CCN levels are introduced when the outer rainband convection is weak, the transport of significant amounts of supercooled liquid water aloft and the associated dynamic response will not occur.

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