Hurricane Isabel (2003) boundary layer characteristics and patterns using the Richardson number

The Richardson number, a ratio of buoyancy to shear as described in Troen and Mahrt (1986), used in boundary layer calculations is applied to a hurricane to determine the characteristics and patterns of the hurricane boundary layer thickness. The Troen and Mahrt method of diagnosing the boundary layer thickness using the Richardson number is a basis for the Hong and Pan or MRF boundary layer schemes available in many hurricane models including WRF and MM5.

Hurricane Isabel (2003) is analyzed from dropwindsondes and GFDL model output. The patterns in the in situ observations are compared to those seen from the model output. Patterns in boundary layer thickness decrease with increasing radius away from the radius of maximum wind indicating a maximum extent of boundary layer depths and a region of structured environmental boundary layer modification due to presence of the hurricane. Additionally, boundary layer structure of the GFDL forecasted output does not agree with the model initialization.