Boundary Layer Influences on Hurricane Structure and Forecast Skill: Results from Idealized and Retrospective Simulations

We investigate the role of planetary boundary layer (PBL) mixing in influencing storm size, track, and intensity, utilizing both idealized and retrospective simulations with the Hurricane WRF model. Bu et al. (2017) has shown that significant variations in storm size can result from different PBL schemes, owing in large part to disparities in the vertical mixing of water vapor with respect to magnitude and depth. Enhanced mixing elevates the relative humidity in the boundary layer, which can lead to enhanced convective activity in the hurricane outer core. Diabatic forcing by convection results in a radially expanded wind field, and can affect intensity and storm motion as well.

Historically, HWRF has been using a version of the GFS PBL parameterization, a descendant from Troen and Mahrt (1986) that generates relatively vigorous and deep mixing in tropical cyclones. Recognizing that the mixing was likely excessive, Gopalakrishnan et al. (2013) revised the GFS scheme to suppress mixing magnitude, leaving PBL depth largely unaffected. Another Troen-Mahrt descendant is the YSU parameterization, which owing to differences with respect to the determination of PBL depth, results in substantially shallower and weaker mixing than the GFS scheme. We test how versions of YSU and other PBL schemes fare in retrospective tests of the HWRF system with respect to storm size, track, intensity, and other important hurricane characteristics.

References:

Bu, Y. P., R. G. Fovell, and K. L. Corbosiero, 2017: The influences of boundary layer mixing and cloud-radiative forcing on tropical cyclone size. J. Atmos. Sci., 74, 1273-1292.

Gopalakrishnan, S. G., F. Marks, J. A. Zhang, X. Zhang, J.-W. Bao, and V. Tallapragada, 2013: A study of the impacts of vertical diffusion on the structure and intensity of the tropical cyclones using the high-resolution HWRF system. J. Atmos. Sci., 70, 524–541.

Troen, I., and L. Mahrt, 1986: A simple model of the atmospheric boundary layer; sensitivity to surface evaporation. Bound.- Layer Meteor., 37, 129–148.