Boundary Layer, Urban Meteorology and Land-Surface Processes

Urban heat island of Dallas Fort-Worth during July 2011 North-Central Texas drought: WRF-SLUCM model verification and sensitivity study of UHI’s dependence on land-use category.

Jinxin Wang
OU School of Meteorology

10 April 2015, 3:00 PM

National Weather Center, Room 5600
120 David L. Boren Blvd.
University of Oklahoma
Norman, OK

World-wide rapid growth of urban areas has been shown to be a reason for growing environmental issues. The urban heat island (UHI) is one of the most concerning issues related to urban meteorology. UHI is defined by the temperature difference between urban and surrounding rural areas, and is especially obvious at nighttime within the Planetary Boundary-Layer (PBL). Daytime insolation stored in urban structures released at nighttime is believed to be the main reason of UHI formation. Many studies have already shown that the variability of UHI has close relationships with wind speed and cloud cover. The sky-view factor, material of urban buildings, urban size, land-use conditions, and synoptic conditions of each city, also affect UHI. Therefore, each city deserves to be investigated independently. Many research papers concerning various cities, like New York, Houston, Beijing, Tokyo, have come out in recent decades. The UHI intensity among these cities varies and seasonal UHI trends exist for each city. One of the interesting questions about UHI is how we can reasonably reveal the magnitude of the UHI.
Under high-pressure, hot-weather synoptic conditions with clear skies, high insolation, low wind speeds, and small latent heat fluxes, the UHI is usually obvious. In this study, we focus on the UHI of Dallas Fort-Worth (DFW) during the north Texas drought in July 2011. Synoptic analysis shows this drought occurred under high-pressure, hot-weather synoptic conditions. The Meteorological Assimilation Data Ingest System (MADIS) observed 2m temperature indicates UHI is very obvious during this drought period and the magnitude of UHI is very sensitive to the land-use category of the urban/rural stations we use. When we select fixed urban station(s), the UHI increased with choosing rural station(s) located in savanna, grassland, and cropland; when we select fixed rural station(s), the UHI increased with choosing urban station(s) located in low-residential, high-residential, and commercial/industrial area. Even though our Weather Research and Forecast model coupled with the Single-Layer Urban Canopy Model (WRF-SLUCM) cannot guarantee to reproduce atmospheric conditions well for each model grid-point, the mean UHI is still acceptable, especially the UHIs between urban and grassland. Spatial and temporal analysis of UHI from model output shows that the vertical extension of UHI is no larger than 300m above the ground but shows a signature of slowly growing from day to day. A cold anomaly keeps appearing above the UHI. Urban Cool Island (UCI) at daytime is obvious in observation but does not appear in control experiment output. With replacing the urban with cropland, grassland and savanna, we conduct three sensitivity experiments. The difference between the control experiment and sensitivity experiments prove the variation of UHI based on different land-use categories shown in observations. A weak signature of UCI appears in the model output difference between control and sensitivity experiments.

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