This study explores the conditions under which we can expect an early evening transition period (EET) to be established, and what measuring multiple terms of the surface layer energy budget can do to provide insight to the problem of the establishment and cessation of differential rates of cooling throughout a small valley. A network of temperature sensors, sonic anemometers, and net radiometers was deployed to the Shale Hills Critical Zone Observatory, a 0.08 km2 forested valley in central Pennsylvania with a 49 m elevation change from valley bottom to ridge top. These sensors collected data during March-June 2013. Measurements were taken above-canopy (30 m above ground) and below-canopy (.91 m and 1.83 m above ground).
Study nights were separated to identify the EET period. Nights with an EET differed in several ways from non-EET nights; nights with an EET tended to have more negative average net radiation, calmer above-canopy wind speeds, and higher stability than non-EET nights. Three-month composites of these variables across the nighttime period of 1930-0600 LST show differences in temporal evolution throughout the night for net radiation, above-canopy wind speed, and air temperature decrease on EET vs. non-EET nights.
Ranking histograms indicated that both EET nights and non-EET nights showed spatial temperature patterns. The more variable spatial temperature pattern was found in the cluster with an EET, suggesting radiative sheltering does not fully explain the spatial temperature pattern on these nights. A primitive energy budget computed for three different case studies throughout the measurement period confirmed that many other terms were contributing to the valley cooling rate besides the radiative cooling. Horizontal advection and horizontal and vertical turbulent flux divergence both were of a larger magnitude than the radiative flux divergence on the EET nights.
No clear transition in the ABL, such as the development of down-valley drainage flow, was revealed to cause the change in the rate of cooling across the valley.
Burkely Twiest (2014): SPATIAL AND TEMPORAL EVOLUTION OF THE NOCTURNAL SURFACE LAYER IN A SMALL, STEEP WATERSHED. Master of Science, Meteorology, The Pennsylvania State University, p. 85.
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