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Xiao-Ming Hu, Ming Xue, and Xiaolan Li

Abstract

Since the 1950s, a countergradient flux term has been added to some K-profile-based first-order PBL schemes, allowing them to simulate the slightly statically stable upper part of the convective boundary layer (CBL) observed in a limited number of aircraft soundings. There is, however, substantial uncertainty in inferring detailed CBL structure, particularly the level of neutral stability (z n), from such a limited number of soundings. In this study, composite profiles of potential temperature are derived from multiyear early afternoon radiosonde data over Beijing, China. The CBLs become slightly stable above z n ~ 0.31–0.33z i, where z i is the CBL depth. These composite profiles are used to evaluate two K-profile PBL schemes, the Yonsei University (YSU) and Shin–Hong (SH) schemes, and to optimize the latter through parameter calibration. In one-dimensional simulations using the WRF Model, YSU simulates a stable CBL above z n ~ 0.24z i, while default SH simulates a thick superadiabatic lower CBL with z n ~ 0.45z i. Experiments with the analytic solution of a K-profile PBL model show that adjusting the countergradient flux profile leads to significant changes in the thermal structure of CBL, informing the calibration of SH. The SH scheme replaces the countergradient heat flux term in its predecessor YSU scheme with a three-layer nonlocal heating profile, with f nl specifying the peak value and z*SL specifying the height of this peak value. Increasing f nl to 1.1 lowers z n, but to too low a value, while simultaneously increasing z*SL to 0.4 leads to a more appropriate z n ~ 0.36z i. The calibrated SH scheme performs better than YSU and default SH for real CBLs.

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