Tyler Bell-April 27

Analysis of Flow and Thermodynamic Characteristics at a Complex Site



April 27, 2018 - 2:00 pm


April 27, 2018 - 3:00 pm


120 David L. Boren Blvd., Room 5600, Norman, OK 73072   View map

Analysis of Flow and Thermodynamic Characteristics at a Complex Site

The Perdigão Field Experiment set out 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 to profile the boundary layer with a high temporal and spatial resolution. Optimized Lidar 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, when stronger background prevailed and was 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.

An essential part of the instrumentation were scanning Doppler lidars (DL) strategically placed to capture flow features above the ridges and in the valley. The arrangement of DLs presented an opportunity to create virtual towers where Range Height Indicator (RHI) scans of individual instruments intersected. CLAMPS performed both cross- and along-valley RHI scans every 15 minutes. The Technical University of Denmark (DTU) operated eight Leosphere Windcube 200S scanning DLs upgraded with DTU’s WindScanner software and the German Aerospace Center (DLR) contributed three DLs of the same kind. Many of these DLs performed cross-valley RHI scans that intersected with the along-valley RHI scan from OU. Four virtual towers distributed along the valley could be retrieved every 15 minutes where the RHIs intersect. 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 case study will be presented to highlight the usefulness of these virtual towers.