May 24 - July 30
Boundary Layer Depth Measurement Disparities During the Evening Transitional Period
Isaac J. Medina, Tyler Bell, Elizabeth Smith, and Jacob Carlin
What is already known:
- Weather radar can be used to detect Bragg scatter-associated turbulence at the top of the boundary layer, which has been known for decades, but the method could be undependable due to obfuscation of Bragg scattering by returns from biota (insects, birds, etc.)
- Dual-polarization radar and new methods of visualizing and analyzing radar data can overcome some of the biota issues, but new questions have arisen around how to interpret and contextualize the resulting signatures in differential reflectivity, especially in the day-to-night transition and overnight periods.
- The Collaborative Lower Atmospheric Mobile Profiling System (CLAMPS) is a research-grade boundary layer profiling system which can be used to monitor boundary layer processes and evolution on scales of minutes and meters, including boundary layer transitions and nocturnal layers.
- The dual-polarization radar indications of boundary layer depth during the day-to-night transition and overnight period trend near one kilometer above the surface while indications of boundary layer depth from CLAMPS trend within 100 meters of the surface.
What this study adds:
- High resolute profiles from a Weather-sensing Uncrewed Arial System (WxUAS) were able to verify CLAMPS indications of a low-level nocturnal boundary layer via the detection of and isothermal layer near the surface at the height of the indication.
- WxUAS was able to confirm the presence of a distinct atmospheric layer at the height of the dual-polarization radar boundary layer height indication, however, the WxUAS profile was unable to confirm that the layer present was caused by the residual boundary layer.
- The verification of distinct layers present at both dual-polarization radar and CLAMPS-indicated boundary layer heights helped contextualize the indicated heights from both boundary layer height estimation methods.
Abstract:
The planetary boundary layer (PBL) is often under sampled by current operational atmospheric observation methods, especially during the early evening transition (EET). Other methodology such as specialized equipment at the Collaborative Lower Atmospheric Mobile Profiling System (CLAMPS) or dual-polarization NEXRAD radar could provide more consistent PBL measurements. Often, CLAMPS and the radar do not agree on the height of the PBL during the EET. During the EET the radar indication is often much higher than that of CLAMPS’s estimates. CLAMPS is better suited to detect the low SBL than the radar as it can use not only kinetic data but also thermodynamic data. The radar searches for areas of high Bragg scatter, which show up as near-zero ZDR to indicate the PBL height. We hypothesize that during the EET, the residual layer’s decaying mixing likely dominates any potential ZDR depression signal from the newly forming stable boundary layer—if that stable boundary layer is deep enough for the radar beam to reach it. This study used weather-sensing uncrewed aerial systems (WxUAS) to investigate the character of the layers that these two methods were detecting. These WxUAS data were compared to PBL measurement data collected near Norman Oklahoma on 27 August 2020. With its high-resolution thermodynamic profile measurements, the WxUAS was able to verify that CLAMPS was detecting the SBL, while the radar was seeing an elevated distinctly different layer of the atmosphere. These data were unable to confirm if this layer was a residual layer due to complex structure and lack of continued observation. While this distinct layer may or may not be the residual CBL, it helped contextualize the radar’s indications during the EET providing more understanding of the radar-based method’s capabilities.