An official website of the United States government
This study investigates how urban environments influence boundary layer processes during the passage of a Great Salt Lake breeze using a multi-scale modeling system, NCAR’s WRF-Coupled GPU-accelerated FastEddy® (FE) model. Motivated by the need for sub-10 m scale decision support tools for uncrewed aerial systems (UAS), the FE model was used to simulate turbulent flows around urban structures at 5 m horizontal resolution with a 9 km × 9 km domain centered on the Salt Lake City International Airport. FE was one-way nested within a 1 km resolution Weather Research and Forecasting (WRF) domain spanning 400 × 400 km. Focused on the late morning lake breeze on 3 June 2022, an FE simulation was compared with WRF outputs and validated using surface and radar observations. The FE simulation revealed low sensible heat flux and cool near-surface temperatures, attributed to a relatively low specification of thermal roughness suitable for previously tested FE applications. Lake breeze characteristics were minimally affected, as FE effectively resolved interactions between the lake breeze and urban-induced turbulent eddies, providing insights into fine-scale boundary layer processes. FE’s GPU acceleration ensured efficient simulations, underscoring its potential for aiding decision support in UAS operations in complex urban environments.
more »
« less
The boundary layer characteristics of coastal urban environments
The atmospheric boundary layer along the coastal-urban transect differs from that of urban or rural regions due to the distinctive interaction between the sea breeze and the urban heat island effect. In this manuscript, we present the observations of the atmospheric boundary layer in the Houston, Texas, area during the Coastal Urban Boundary Layer Experiment (CUBE) from June through September 2022. In order to understand the unique characteristics of the coastal urban boundary layer, we collected mean and turbulence data from micrometeorological towers and ground-based remote sensing instruments installed in the urban, coastal, bay, and rural sections within the greater Houston region. Furthermore, an urbanized weather research and forecast (WRF) model incorporating the Building Effect Parameterization and Building Energy Model (BEP-BEM) scheme is used to recognize the spatial variability of the meteorological conditions in the Houston Metro area. Compared to non-urban sites, the urban site exhibits a higher near-surface temperature throughout the day, with the highest temperature difference occurring at night due to the redistribution of the stored heat as sensible heat. During the dry period in June, we observed comparatively higher sensible heat flux in the urban site, demonstrating the heat island effect and lower latent heat flux due to lack of vegetation. The urban site had higher TKE values throughout the day than other sites because of the uneven roughness of the landscape. One of the unique findings of this study is the shift in spectral characteristics along the coastal-rural-urban transect. The power and co-spectra of zonal and vertical velocities and the vertical heat flux during the convective periods varied significantly across all the sites. The coastal site was influenced mainly by the local bay breeze shifting the peak to higher frequencies. The boundary layer height in the urban site was generally greater than in bay and rural sites due to increased convection in urban areas resulting from anthropogenic modification of land cover and waste heat from air conditioning use. The balance between the urban thermal and mechanical roughness effects was seen during the sea breeze front (SBF) event on the highest heat index day as SBF was triggered and accelerated by UHI.
more »
« less
- Award ID(s):
- 2114198
- PAR ID:
- 10529136
- Publisher / Repository:
- Springer Journals
- Date Published:
- Journal Name:
- Theoretical and Applied Climatology
- Edition / Version:
- 1
- ISSN:
- 0177-798X
- Format(s):
- Medium: X Size: 16.2MB
- Size(s):
- 16.2MB
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Significant knowledge gaps exist in our understanding of urban boundary layer processes, particularly the hygrothermal state. The earth system community has successfully used microwave radiometers for several decades. However, the applicability in complex urban environments has never been adequately tested. Here, observations from a microwave radiometer are compared to radiosonde readings in a densely urbanized site in Houston, Texas. The site was influenced by both an urban heat island and the sea breeze phenomenon. The analysis showed significant disagreement between the virtual potential temperature predicted by the microwave radiometer and the radiosonde for all periods within the boundary layer. However, the values were reasonably comparable above the boundary layer. The microwave radiometer incorrectly predicted an inversion layer instead of a mixed layer during convective periods. The microwave radiometer measurements deviated from the radiosonde measurements throughout the lower troposphere for the relative humidity.more » « less
-
Abstract The two-resistance mechanism (TRM) attribution method, which was designed to analyze the urban–rural contrast of temperature, is improved to study the urban–rural contrast of heat stress. The improved method can be applied to diagnosing any heat stress index that is a function of temperature and humidity. As an example, in this study we use it to analyze the summertime urban–rural contrast of simplified wet bulb globe temperature (SWBGT) simulated by the Geophysical Fluid Dynamics Laboratory land model coupled with an urban canopy model. We find that the urban–rural contrast of SWBGT is primarily caused by the lack of evapotranspiration in urban areas during the daytime and the release of heat storage during the nighttime, with the urban–rural differences in aerodynamic features playing either positive or negative roles depending on the background climate. Compared to the magnitude of the urban–rural contrast of temperature, the magnitude of the urban–rural contrast of SWBGT is damped due to the moisture deficits in urban areas. We further find that the urban–rural contrast of 2-m air temperature/SWBGT is fundamentally different from that of canopy air temperature/SWBGT. Turbulent mixing in the surface layer leads to much smaller urban–rural contrasts of 2-m air temperature/SWBGT than their canopy air counterparts. Significance Statement Heat leads to serious public health concerns, but urban and rural areas have different levels of heat stress. Our study explains the magnitude and pattern of the simulated urban–rural contrast in heat stress at the global scale and improves an attribution method to quantify which biophysical processes are mostly responsible for the simulated urban–rural contrast in heat stress. We highlight two well-known causes of higher heat stress in cities: the lack of evapotranspiration and the stronger release of heat storage. Meanwhile, we draw attention to the vegetation types in rural areas, which determine the urban–rural difference in surface roughness and significantly affect the urban–rural difference in heat stress. Last, we find the urban–rural contrasts of 2-m air temperature/SWBGT are largely reduced relative to their canopy air counterparts due to the turbulent mixing effect.more » « less
-
High spatial/temporal resolution mobile transects were used to examine the thermal and moisture structure of the sea-breeze front (SBF) along the Mississippi coast during August 2014 and 2015. Compared to most similar studies, conditions were much warmer and more humid. Results show a 1-2 g/kg increase in mixing ratio across the mature SBF zone, and up to a 2.5°C temperature decrease. When SBF radar fine lines are identifiable, their position agrees very well with surface thermodynamic changes. Although temperatures were cooler at the coast, microscale offsets in location of thermal, moisture, and radiative features are noted in the vicinity of the SBF, particularly when the sea-breeze system is relatively weak or immature. At times, it seems that strong solar insolation causes the temperature to rise temporarily within the transition zone behind the kinematic SBF. These results are at variance with most other diagnostic studies. Some thermodynamic variations are noted within the marine air mass in connection to minor water bodies such as Biloxi Bay. The potential for passage of the SBF to at least temporarily increase human heat stress as described by heat index is also noted.more » « less
-
Abstract The influence of large bodies of water on convection initiation (CI) and the environment in which convection evolves is highly complex due to the wide range of parameters that control relevant processes. A substantial focus of sea-breeze front (SBF) CI research has focused on the role of mesoscale boundary or organized boundary layer circulation interactions with SBFs; however, less research has focused on heterogeneities in convective parameters and how those may affect CI coverage, timing, and location. In this two-part series, the authors present a parameter study of SBF CI through Cloud Model 1 (CM1) large-eddy simulations across a parameter space varying the cross-shore wind component and radiative forcing and water surface temperatures consistent with June, July, and August in the Great Lakes region. In Part I of this series, simulations under conditions of calm, weak, and moderate offshore flow (0, −2.5, and −5.0 m s−1) are presented. CI occurred in all calm simulations, with decreased coverage and frequency of CI in weak offshore flow. CI was least frequent during moderate offshore flow, despite stronger convergence, due to a shear profile that favored undercutting by the SBF under conditions of moderate offshore flow. Surface-based convective available potential energy (SBCAPE) maxima developed on the cool side of the SBF, with convection occurring on the cool side of the SBF in some cases. Analyses are presented with a focus on the nature of the SBF, distribution of convective parameters, and their implications on CI.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1007/s00704-024-05036-z
