CASES-99

Project Abstract

Studies of the Stable Boundary Layer During CASES-99
Using the High-Resolution Doppler Lidar

The Atmospheric Lidar Division deployed its High-Resolution Doppler Lidar (HRDL) to the CASES site in fall 1999 as a part of a comprehensive instrumentation deployment to study the nocturnal stable boundary layer (SBL). HRDL is a scanning, active remote sensing system that maps out the Doppler velocity and aerosol backscatter fields in the vertical and horizontal in the atmosphere.

Despite the profound impact of processes in the atmospheric boundary layer (ABL) on a wide range of human activity, these processes are still not well understood and are difficult to predict on the distance and time scales required. This is especially true for the nocturnal SBL, as well as the morning and evening transition boundary layers, over land. When the ambient, synoptic flow is not strong, surface cooling at night produces a strong nocturnal inversion layer. The SBL under these conditions, called the very stable boundary layer (vSBL), is characterized by intermittent turbulent mixing. As a result of the nature of this intermittency, traditional analysis techniques, e.g., stationary time series analysis, do not work and are not available to help understand these complex processes. Flow phenomena on a larger scale than the turbulence events, such as gravity and Kelvin-Helmholtz waves, low-level jets, and drainage flows, most likely modulate and sometimes control turbulence, vertical transport, and therefore turbulent fluxes near the surface. In this case, advances in our understanding of SBL processes will depend on the ability to measure simultaneously the turbulence and fluxes near the surface and the characteristics of these larger-scale flow phenomena above the surface layer. It will also be necessary to determine the spatial and temporal characteristics and distributions of the turbulence events.

To address this difficult and complex problem, a field measurement study called CASES-99 was organized for October 1999. A goal of CASES-99 was to perform the simultaneous measurements needed to determine the relationship between processes in these two regions. Arrays of surface-flux towers over a small (5 x 5 km) area, plus profiling instrumentation and aircraft, were deployed to study processes in the vSBL. A key instrument in achieving this goal was HRDL. Because of its scanning capabilities, HRDL is a powerful instrument for revealing the vertical and horizontal structure of flow features and turbulence events, and will be critical during the analysis of the CASES-99 dataset both for probing the flow structure itself, and for providing the meteorological context of in-situ flux-tower and other measurements. Used in conjunction with the other instrumentation in this manner, these scans provide an overview of flow features affecting the measurements, so that one does not have to guess or infer that gravity waves or pulsating drainage winds, for example, are contributing to the behavior of the data.