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

Abstract

Previous analysis of Oklahoma City (OKC), Oklahoma, temperature data indicated that urban heat islands (UHIs) frequently formed at night and the observed UHI intensity was variable (1°–4°C). The current study focuses on identifying meteorological phenomena that contributed to the variability of nocturnal UHI intensity in OKC during July 2003. Two episodes, one with a strong UHI signature and one with a weak signature, were studied in detail using observations along with simulations with the Weather Research and Forecasting model. Mechanical mixing associated with low-level jets (LLJs) played a critical role in moderating the nocturnal UHI intensity. During nights with weak LLJs or in the absence of LLJs, vertical mixing weakened at night and strong temperature inversions developed in the rural surface layer as a result of radiative cooling. The shallow stable boundary layer (SBL < 200 m) observed under such conditions was strongly altered inside the city because rougher and warmer surface characteristics caused vertical mixing that eroded the near-surface inversion. Accordingly, temperatures measured within the urban canopy layer at night were consistently higher than at nearby rural sites of comparable height (by ~3°–4°C). During nights with strong LLJs, however, the jets facilitated enhanced turbulent mixing in the nocturnal boundary layer. As a consequence, atmospheric stability was much weaker and urban effects played a much less prominent role in altering the SBL structure; therefore, UHI intensities were smaller (<1°C) during strong LLJs. The finding that rural inversion strength can serve as an indicator for UHI intensity highlights that the structure of the nocturnal boundary layer is important for UHI assessments.

Theoretical Urban Heat Island Circulation in the Temperature Inversion Profile

“…The UHI circulation distributions for the other two STI cases presented by the dotted and dash-dotted lines in Figure 1a are also calculated (figure omitted). The results suggest that a stronger inversion intensity benefits to a stronger circulation cell, which could be confirmed by observational and numerical experiments (e.g., see [23,24]). …”

Measurements of Turbulent Flow and Ozone at Rooftop and Sidewalk Sites in a High-Rise Building Area

“…Weak easterly synoptic-scale surface winds were observed on 26 June, but no distinct synoptic-scale surface winds were observed on the other days. In contrast to the synoptic-scale surface winds, somewhat strong westerly sea breezes reported that a nighttime peak of ozone concentration appears when low-level jets enhance vertical mixing and hence transport ozone in the residual layer to the surface (Reitebuch et al 2000;Hu et al 2013). This possibility in the present study needs to be investigated using a coupled meteorology-air quality model.…”

Observing and Simulating the Summertime Low-Level Jet in Central Iowa

“…In their simulations, WRF produced insufficient wind shear below the jet, which they attributed to the overly diffusive nature of the chosen PBL schemes. They used WRF-ARW version 3.1.1, however, and many of the PBL parameterizations have since received significant modification, particularly the YSU scheme in version 3.4.1 [as examined in Hu et al (2013)]. …”

“…During the summer months, winds aloft accelerate after sunset with remarkable consistency in numerous regions throughout the central plains and Midwest states. These wind accelerations are known as low-level jets (LLJs), and their influence has been noted in many fields of study, such as clear-air turbulence (e.g., Banta et al 2002), convective storm formation (e.g., Means 1952Means , 1954Curtis and Panofsky 1958), forest fire propagation (e.g., Barad 1961), urban pollution transport (e.g., McNider et al 1988;Banta et al 1998;Hu et al 2013), and turbulent transport (e.g., Prabha et al 2007). …”

“…Despite the progress that has been made at understanding the theoretical basis behind the GPLLJ, most observational work has occurred within Oklahoma and Kansas, a region described as the spatial core of the jet (e.g., Lundquist and Mirocha 2008;Hu et al 2013). The availability of a wide suite of instruments at the Atmospheric Boundary Layer Experiments (ABLE) facility in Kansas has engendered extensive study into jet properties at that location (e.g., Whiteman et al 1997;Poulos et al 2002;Banta et al 2002;Song et al 2005;Werth et al 2011), though some recent work has focused on winds aloft north of the jet core (Walton et al 2014).…”