Quantifying the Effects of Surface Thermal Heterogeneity on Dispersive Fluxes in an Idealized Environment
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Authors
Baldassare, Daniel
Issue Date
2020
Type
Thesis
Language
Keywords
Boundary Layer Meteorology , Dispersive Flux , Heat Flux , Surface Energy Balance , Turbulence
Alternative Title
Abstract
In the planetary boundary layer, diurnal variations in solar heating spawnturbulent mixing that modifies the distribution of state variables. The turbulence
contribution to state variable redistribution can be quantified by including a turbulence
term in the governing equations. The complexity in airflow dynamics caused by
turbulence is further increased by the presence of surface heterogeneities. Spatial
heterogeneities of albedo, temperature, moisture and roughness contribute to variability
in surface fluxes. The combined effects of these heterogeneities further increase the
difficulty in determining the individual effects.
The Idealized Planar-Array for Quantifying Surface Heterogeneity (IPAQS)
project was conducted during summers 2018 and 2019 in the Great Salt Lake Desert
Playa at the Dugway Proving Ground’s Surface Layer Turbulence and Environmental
Test location. Complexity was reduced by choosing a site that was flat and nonvegetated, with limited moisture availability. Heterogeneity was expressed by small-scale
differences in albedo, which cause surface temperature differences following solar
heating. A 5-km distributed temperature sensing (DTS) system was used to measure
surface temperatures at a spatial resolution of 12.5 cm and a temporal resolution of
roughly 3 s. A spatial array of ultrasonic anemometers placed in a 200 × 200 m grid
pattern measured air temperature and wind speed that were used to calculate turbulent
fluxes.
The spatial mean (SMEAN) of surface temperature reached maximum values of
roughly 40 °C in the early afternoon. The air temperature SMEAN occurred roughly two
hours later with maximum values of roughly 35 °C. Surface temperature spatial variation
as measured by the surface temperature spatial standard deviation (SSTD) reached
maximum daily values in the early afternoon. Air temperature SSTD did not change
significantly during the day. The SMEAN of wind speed was generally below 4 m s-1
during the convective period on sunny days. A decorrelation length scale was calculated
by finding the minimum distance at which the autocorrelation value dropped below 0.1.
The decorrelation length scale increased in the late morning and peaked in the early
afternoon, with a value of roughly 35 m on sunny days. These variables were combined
to create a heterogeneity parameter which represents the impact of surface thermal
heterogeneity on surface layer fluxes. Dispersive fluxes were found to increase slightly
with increasing heterogeneity parameter values during convective periods. Both surface
temperature variation and the vertical temperature gradient decreased with increasing
wind speed during convective periods. These results quantitatively illustrate the complex
interaction between surface thermal heterogeneities and wind. Surface temperature
variations drive air flow in low wind conditions, while stronger winds decrease the
intensity of thermal heterogeneities.