Objectively Determined Fair-Weather CBL Depths in the ARW-WRF Model and Their Comparison to CASES-97 Observations

Abstract

igh-resolution 24-h runs of the Advanced Research version of the Weather Research and Forecasting Model are used to test eight objective methods for estimating convective boundary layer (CBL) depth h, using four planetary boundary layer schemes: Yonsei University (YSU), Mellor?Yamada?Janjic (MYJ), Bougeault?LaCarrere (BouLac), and quasi-normal scale elimination (QNSE). The methods use thresholds of virtual potential temperature Θ?, turbulence kinetic energy (TKE), Θ?,z, or Richardson number. Those that identify h consistent with values found subjectively from modeled Θ? profiles are used for comparisons to fair-weather observations from the 1997 Cooperative Atmosphere?Surface Exchange Study (CASES-97).The best method defines h as the lowest level at which Θ?,z = 2 K km?1, working for all four schemes, with little sensitivity to horizontal grid spacing. For BouLac, MYJ, and QNSE, TKE thresholds did poorly for runs with 1- and 3-km grid spacing, producing irregular h growth not consistent with Θ?-profile evolution. This resulted from the vertical velocity W associated with resolved CBL eddies: for W > 0, TKE profiles were deeper and Θ? profiles more unstable than for W < 0. For the 1-km runs, 25-point spatial averaging was needed for reliable TKE-based h estimates, but thresholds greater than free-atmosphere values were sensitive to horizontal grid spacing. Matching Θ?(h) to Θ?(0.05h) or Θ? at the first model level were often successful, but the absence of eddies for 9-km grids led to more unstable Θ? profiles and often deeper h.Values of h for BouLac, MYJ, and QNSE, are mostly smaller than observed, with YSU values close to slightly high, consistent with earlier results.