EAPS

Special Weather & Climate Lecture Series - Fuqing Zhang (Penn State)
Date Time Location
December 1st, 2015 2:30pm-3:30pm 54-915
"Predictability and Dynamics of Weather and Climate at the Regional Scales | Impacts of air-sea and boundary layer fluxes on the intensity and structure of TCs"

Speaker: Fuqing Zhang, Professor of Meteorology, Director, Center for Advanced Data Assimilation and Predictability Techniques, Penn State, University

Title: Impacts and parameterizations of air-sea and boundary layer fluxes on the intensity and structure of tropical cyclones

Abstract: The intensity and structure of tropical cyclones can be strongly influenced by large fluxes of heat and momentum from the air-sea interface, and in the boundary layer. These fluxes cannot be explicitly resolved by NWP models and therefore must be parameterized. Unfortunately, there is a great deal of uncertainty as to the behavior of air-sea surface and boundary layer turbulence fluxes, especially under strong (hurricane-force) winds. The first part of the lecture presents the sensitivity of WRF-simulated TCs to parameterizations of the surface exchange coefficients for drag (Cd) and moist enthalpy (Ck). In agreement with theory, increased Ck yields a stronger TC both in terms of minimum central pressure and maximum 10-m wind speed. The impacts of Cd are not as straightforward: increased drag does reduce the maximum 10-m wind speed (in agreement with theory), but also deepens the minimum central pressure (opposite of what is predicted by theory) – in other words, Cd changes the pressure-wind relationship of simulated TCs. Cd also profoundly impacts TC structure, such that increased drag yields a more compact primary circulation. Our study also points to future directions in further reducing uncertainties in these coefficients.

The second part of the lecture presents the impact of boundary layer parameterizations on tropical cyclones through a series of “Large Eddy Permitting (LEP)” simulations (with Δx as small as 111 m) of Hurricane Katrina (2005). These LEP runs, which use a nonlinear backscatter with anisotropy (NBA) subgrid-scale parameterization scheme, exhibit features resembling small-scale turbulence for Δx as large as 333 m. The size of these turbulent structures depends on Δx, indicating convergence to true LES has not been reached. That said, however, the azimuthally averaged low-level secondary circulation and the total momentum fluxes in the boundary layer are remarkably similar between these LEP runs, all of which have clear advantage over simulations with parameterized PBL at Δx = 333 m.