Tyler Bell-June 18

ANALYSIS OF FLOW AND THERMODYNAMIC CHARACTERISTICS AT A SITE IN COMPLEX TERRAIN

The Perdigão Field Experiment was designed to study atmospheric flows in complex terrain and to collect a high-quality dataset for the validation of meso- and micro-scale models. An Intensive Observation Period (IOP) was conducted from May 1, 2017 through June 15, 2017 where a multitude of instruments were deployed in and around two nearly parallel, 5 km long ridges separated by a 1.4 km wide valley perpendicular to the prevalent wind directions in the region. During this IOP, the Collaborative Lower Atmospheric Mobile Profiling System (CLAMPS) was deployed and operated in the valley between the ridges. The CLAMPS facility, which was developed as a joint effort between the School of Meteorology at OU and NOAA’s National Severe Storms Laboratory (NSSL), takes advantage of a microwave radiometer (MWR), an atmospheric emitted radiance interferometer (AERI), and a scanning Doppler lidar (DL) to profile the boundary layer with a high temporal and spatial resolution. Optimized DL scanning strategies and joint retrievals for the MWR and AERI data provide detailed information about the wind, turbulence and thermodynamic structure from the surface up to 1000 m AGL on most nights; sometimes the maximum height range is even higher. Over the course of the IOP, CLAMPS observed many different phenomena. During some nights, with stronger prevailing winds that were directed perpendicular to the valley, waves were observed at the ridges and in the valley. At the same time, radiational cooling led to drainage flows in the valley, particularly during nights when the mesoscale forcing was weak. 

In addition to CLAMPS, the Technical University of Denmark (DTU) operated eight Leosphere Windcube 200S scanning DLs and the German Aerospace Center (DLR) contributed three DLs of the same kind to capture flow features above the ridges and in the valley. The arrangement of DLs presented an opportunity to retrieve four virtual towers every 15 minutes where Range Height Indicator (RHI) scans of individual instruments intersected. The virtual towers typically cover heights from 50m to 600m above the valley floor, extending the range of traditional in-situ observations located throughout the valley. Additionally, they fill in low altitude areas where other DL processing techniques (such as VADs or DBS scans) may have trouble retrieving accurate wind speeds due to the high spatial flow variability and prevalence of significant vertical motions in complex terrain. Along with the wind speed and direction, uncertainties associated with the DLs were propagated through the retrieval. A few case studies will be presented to highlight the usefulness of these virtual towers.