The role of low-level winds on the morning transition in Dallas-Fort Worth

Abstract

In highly urbanized areas, anthropogenic changes to the landscape lead to the well-known urban heat island (UHI) where the temperature over urban areas is warmer than the surrounding rural regions. This urbanization modifies the temperature profile above the city and its connection to the mixed-layer depth is related to health risks including heat stress and respiratory illness that are linked with air pollution. This work focuses on the morning transition, which is defined as starting at dawn and ending at the onset of the rapid growth of the mixed layer. Mean temperature and wind profiles are compiled every half-hour from aircraft landing and departing Dallas-Fort Worth International (DFW) and Dallas Love Field (DAL) from 2010 to 2019 using data provided by the Aircraft Meteorological Data Relay (AMDAR) program. The Dallas-Fort Worth area is selected as the study region due to its large urban area that is located in the central Great Plains with no major topographic features or coastal influences. Results using a Richardson number formulation with a threshold of 0.25 offers the best characterization of boundary layer (BL) height. Observations with cloud oktas less than five are selected to eliminate transient weather events and top-down forcings caused by cloudy conditions that impact the BL more than local effects. Strong northerly winds can be associated with clear, post- frontal conditions, so southerly winds are selected. The results show greater BL heights at dawn in the summer than in the winter. A breakpoint algorithm using piecewise linear regression on the diurnal BL height from four hours before dawn to ten hours after dawn offers the most reasonable approach to determine the end of the morning transition. Changing the start and end of the hours used in the breakpoint calculation or the Richardson number threshold from 0.20 to 0.30 does not significantly affect the morning transition durations. In the summer, stronger winds are associated with a more well-mixed layer at dawn when the morning transition starts. Stronger low-level winds at dawn lead to a weaker nocturnal temperature inversion and a shorter morning transition once insolation generates thermally-driven turbulence. This suggests that the Great Plains low-level jet is the primary factor for shorter morning transitions, earlier rapid mixed layer growth, and a quicker mixing of pollutants from the morning commute using a much larger dataset than previous studies.