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Abstract Coastal marine heatwaves (MHWs) modulate coastal climate through ocean‐land‐atmosphere interactions, but little is known about how coastal MHWs interact with coastal cities and modify urban thermal environment. In this study, a representative urban coastal environment under MHWs is simplified to a mixed convection problem. Fourteen large‐eddy simulations (LESs) are conducted to investigate how coastal cities interact with MHWs. We consider the simulations by simple urban roughness setup (Set A) as well as explicit urban roughness representation (Set B). Besides, different MHW intensities, synoptic wind speeds, surface fluxes of urban and sea patches are considered. Results suggest that increasing MHW intensity alters streamwise potential temperature gradient and vertical velocity direction. The magnitude of vertical velocity and urban heat island (UHI) intensity decrease with increasing synoptic wind speed. Changing urban or sea surface heat flux also leads to important differences in flow and temperature fields. Comparison between Set A and B reveals a significant increase of vertical velocity magnitude and UHI intensity. To further understand this phenomenon, a canopy layer UHI model is proposed to show the relationship between UHI intensity and urban canopy, thermal heterogeneity and mean advection. The effect of urban canopy is considered in terms of an additional vertical velocity scale that facilitates heat transport from the heated surface and therefore increases UHI intensity. The model can well explain the trend of the simulated results and implies that overlooking the effect of urban canopy underestimates canopy UHI in urban coastal environment.
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The Impact of Vertical Wind Shear on the Outcome of Interactions Between Squall Lines and Cities
Abstract This study examines how organized lines of deep convective storms can be impacted by a large city with a prominent urban heat island and how low‐level environmental vertical wind shear may influence the outcomes of that interaction. Idealized simulations of squall lines are conducted in which a simplified urban area—defined by perturbations to skin temperature and surface roughness length—is placed in the center of an otherwise horizontally homogeneous domain. Simulations are conducted with three different magnitudes of low‐level vertical wind shear representing “weak,” “medium,” and “strong” shear environments. Results show that storms experience noticeable modification—including enhanced downwind precipitation—after interacting with a prominent urban heat island in all three shear configurations. However, the details of the modification are a function of the shear magnitude. In the medium and strong shear simulations, updrafts are enhanced via increased buoyancy after passing over a prominent urban heat island. In contrast, little updraft strengthening is evident in the weak‐shear simulations. Instead, near‐surface winds are enhanced downwind of the urban heat island due to a more prominent descending rear‐inflow jet.
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- Award ID(s):
- 1953791
- PAR ID:
- 10395642
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Atmospheres
- Volume:
- 128
- Issue:
- 3
- ISSN:
- 2169-897X
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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