Eddy Dissipation Rate Boundary Layer Climatologies Emphasizing Very Low Eddy Dissipation Rates at Memphis

Times of low Eddy Dissipation Rates (EDR) are of interest to the Federal Aviation Administration because aircraft wakes existing in these environments can have longer lifespans and may contain increased safety risk to following aircraft. Because these risks are greatest near the ground, understanding the prevalence of low EDR periods near the altitude of one wingspan are of importance. Understanding the variability in likelihood of these periods can have implications for correctly assessing the true risk as opposed to the averaged or expected likelihood based on limited sets of observations. This study provides an analysis of these periods as recorded in a dataset collected in the vicinity of Memphis International Airport between May 2013 and March 2015 to provide insight into this issue. The sensor suite [1] included a tower, profiler, lidar and data from the on-airport Automated Surface Observing System (ASOS) sensor.

Distributions of EDR were constructed by season and local hour to seek times of increased likelihood of low EDR. These distributions showed that the variability of the Memphis boundary-layer EDR distributions can exhibit as much or more variability on an interannual (i.e., seasonal) basis as on a diurnal basis. This variability characteristic is also seen for the low-EDR tail of the distributions. In an effort to move towards expansion of this effort to other sites, a companion analysis of stability was undertaken. The relative contributions of buoyant and shear instabilities to Turbulent Kinetic Energy (TKE), which is more widely available, is often computed using the Obukhov length; however, this computation did not have ideal correlations to EDR. Instead, buoyant stability was assessed using layer potential temperature gradients (both dry and moist). Stabilities over various layers were compared to low level EDR observations to establish the best correlation. Overall correlations across the entire distribution were quite poor, however there was the expected strong relationship between low EDR values and high stability, especially in the lowest 50m layer, which yields potential to leverage stability as a predictor for low EDR likelihood.

[1] Pruis, Matthew J., et al. "Summary of NASA Wake and Weather Data Collection at Memphis International Airport: 2013-2015." 8th AIAA Atmospheric and Space Environments Conference. 2016.