Impact of Low - Level Jets on the Nocturnal Urban Heat Island Intensity in Oklahoma City 1 2

Manuscript text (non-LaTeX) Click here to download Manuscript (non-LaTeX): Hu2013JAMC_text_revision_final_clean.pdf 2 Abstract 23 Previous analysis of Oklahoma City (OKC) temperature data indicated that urban heat 24 islands (UHI) frequently formed at night, but the observed UHI intensity was quite variable (1-25 4 o C). The current study focuses on identifying meteorological phenomena that contributed to the 26 variability of nocturnal UHI intensity in OKC during July 2003. Two episodes, one with a strong 27 and one with a weak UHI signature were studied in detail using observations along with 28 simulations with the Weather Research and Forecasting (WRF) model. Mechanical mixing 29 associated with low-level jets (LLJs) played a critical role in moderating the nocturnal UHI 30 intensity. During nights with weak LLJs or in the absence of LLJs, vertical mixing weakened at 31 night and strong temperature inversions developed in the rural surface layer due to radiative 32 cooling. The shallow stable boundary layer (SBL<200 m) observed under such conditions was 33 strongly altered inside the city because rougher and warmer surface characteristics caused 34 vertical mixing that eroded the near surface inversion. Accordingly, temperatures measured 35 within the urban canopy layer at night were consistently higher than at nearby rural sites of 36 comparable height (by ~3-4 o C). However, during nights with strong LLJs, the jets facilitated 37 enhanced turbulent mixing in the nocturnal boundary layer. As a consequence, atmospheric 38 stability was much weaker, urban effects played a much less prominent role in altering the SBL 39 structure, and thus UHI intensities were smaller (<1 o C) during strong LLJs. The finding that 40 rural inversion strength can serve as an indicator for UHI intensity highlights that the structure of 41 the nocturnal boundary layer is important for UHI assessments. 42 43 3