Leaving a Balanced World: Probing the Nonlinear, Unbalanced Dynamics of the Tropical Boundary Layer

Wayne Schubert and his research group typically focus on finding “simple” analytic solutions to systems of balanced dynamics. Through those solutions, his group significantly added to our knowledge of both tropical and non-tropical processes in the atmosphere. This body of work usually ignores the mechanisms that control the placement of diabatic or adiabatic forcing by instead focusing on the balanced atmospheric response to the forcing. Intrigued by flight-level data in Hurricane Hugo (1989), he spurred a new direction for research within his group by encouraging his students to examine the nonlinear terms in the boundary layer.

In the boundary layers of both the tropical cyclones and the intertropical convergence zone, the magnitude of nonlinear terms within the equations of motion often becomes non negligible. The Schubert research group noted that within the equation for the divergent component of the wind (radial in tropical cyclones and meridional in the intertropical convergence zone) is an embedded viscous Burger’s equation—the simplest differential question that allows for the formation of shocks due to the advection of the divergent wind by the divergent wind. This work uses a combination of numerical and analytic models to highlight regions where the boundary layer departs the traditional linear Ekman balance. The models are forced by a time-dependent, pressure field, which induces a transient response to the boundary layer divergent and rotational components of the wind. This work will examine how the boundary layer evolves with different prescribed pressure fields based on observations in both tropical cyclones and the intertropical convergence zone. The resulting wind field provides insight into the nonlinear response to different pressure gradients, the role the boundary layer plays in the placement of deep convection, and the abrupt changes observed in the divergent component of the wind in Hurricanes Allen (1980) and Hugo (1989).