92nd American Meteorological Society Annual Meeting (January 22-26, 2012)

Thursday, 26 January 2012
On the High Resolution Simulation of Tropical Storm Ivan (2004) in the Southern Appalachians: Sensitivity to Planetary Boundary Layer Schemes and the Impact of Cumulus Parameterization
Hall E (New Orleans Convention Center )
Xiaoming Sun, Duke University, Durham, NC; and A. P. Barros

The sensitivity of the planetary boundary layer (PBL) schemes and the impact of cumulus parameterization on the high resolution simulation (down to 1 km grid spacing) of Tropical Storm Ivan (2004) in the Southern Appalachians were investigated using the Weather Research and Forecasting (WRF-ARW3.1) model. The two PBLs examined are the Yonsei University (YSU) and the Mellor-Yamada-Janjić (MYJ) schemes. The cumulus physics involved is the Kain-Fritsch parameterization.

Sensitivity experiments on boundary layer dynamics show that by transporting moisture away from the surface to higher levels more deeply and strongly, the nonlocal YSU favors development of deep convection. In contrast, the MYJ underestimates vertical mixing, resulting in underestimated entrainment which produces a colder and moister PBL. This effect can lead to premature and unrealistic rainfall, impairing the simulation of principal rainbands during the later stage of the model integration. The inferior performance of the MYJ is attributed to its insufficiency in resolving the counter gradient fluxes due to its local closure nature.

Although it is well known that the cumulus parameterization should be deactivated in high resolution simulations, detailed documentation on the effect of cumulus parameterization on convective allowing simulations is relatively lacking. Our results show that when the cumulus scheme is activated at high resolution, the updrafts penetrate farther aloft than in explicit numerical experiments. The detrainment at high altitude of the tropical cyclone disperses the hydrometeors and under the humid conditions of a tropical storm, more widespread light rainfall is predicted. On the other hand, the more localized intense rainfall in the explicit experiment is associated with the increased availability of spatially concentrated condensates in the atmosphere. Further, when the cumulus parameterization is activated, the energy balance components change considerably, affecting the vertical wind velocity and the horizontal wind patterns, showing one reason for timing/location variation among different model configurations.

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