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In this study, thefirst of two parts, the planetary boundarylayer (PBL) depicted in high-resolution Weather Research and Forecast Model (WRF) simulations of Hurricane Isabel (2003) is studied and evaluated by direct comparisons with in situ data obtained during the Coupled Boundary Layer and Air‐Sea Transfer Experiment (CBLAST). In particular, two boundary layer schemes are evaluated: the Yonsei University (YSU) parameterization and the Mellor‐Yamada‐Janjic´ (MYJ) parameterization. Investigation of these schemes is useful since they are available for use with WRF, are both widely used, and are based on entirely different methods for simulating the PBL. In this first part, the model domains and initialization are described. For additional realism of the low-level thermodynamic environment, a simple mixed layer ocean model is used to simulate ocean cooling. The YSU and MYJ schemes are discussed, along with some modifications. Standard measures of the accuracy of the hurricane simulations, such as track, maximum surface wind speed, and minimum surface pressure are describedforavarietyofparameterchoicesandforthetwoparameterizations.Theeffectsontrackandintensity of increased horizontal and vertical resolutions are also shown. A modification of the original YSU and MYJ schemes to have ocean roughness lengths more in agreement with recent studies considerably improves the results of both schemes.Instantaneous wind maximaon the innermost grid with 1.33-kmresolutionare shown to be an accurate representation of the simulated 1-min sustained winds. Thesimulatedboundarylayersareevaluatedby directcomparison ofthePBLassimulatedandasobserved by in situ data from the CBLAST experiment in the ‘‘outer core’’ region of the storm. The two PBL schemes and their modifiedcounterparts reproduce the observed PBL remarkably well. Comparisonsare also made to the observed vertical fluxes of momentum, heat, and moisture. In Part II, the detailed comparisons of the intensities and structures of the simulated and observed innercore boundary layers are presented, and the reasons for the differences are discussed.

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Evaluation of Planetary Boundary Layer Parameterizations in Tropical Cyclones by Comparison of In Situ Observations and High-Resolution Simulations of Hurricane Isabel (2003). Part I: Initialization, Maximum Winds, and the Outer-Core Boundary Layer