Analysis of Flow in Complex Terrain Using Innovative Multi-Doppler Lidar Retrievals

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 meteorological instruments were deployed in a study area with the unique feature of two nearly parallel, 5 km long ridges separated by a 1.4 km wide valley perpendicular to the prevalent wind directions in the region. 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.

The University of Oklahoma (OU) deployed its Collaborative Lower Atmospheric Profiling System (CLAMPS) at the lower Orange site located inside the valley during the IOP. CLAMPS includes a Halo Photonics Scanning DL that performed both cross- and along-valley RHI scans every 15 minutes. The German Aerospace Center (DLR) contributed three Leosphere Windcube 200S scanning DLs upgraded with DTU’s WindScanner software. Two of these DLs performed continuous RHI scans in the cross-valley direction. By combining these DLs with the OU DL scans, it is possible to retrieve the three-dimensional wind field in the form of a virtual tower where the planes intersect. In addition to the DLR DLs, the Technical University of Denmark (DTU) operated eight DLs of the same kind on top of the ridges and performed more cross-valley scans that intersected with the along-valley scan from OU. Adding these DLs it is possible to get three additional two-dimensional retrievals. In total, four virtual towers distributed along the valley are retrieved every 15 minutes. The virtual towers typically cover heights from 50m to 600m above the valley floor. The Perdigão project also included a network of meteorological towers of different heights with wind measurements at an exceptionally high density (55 towers with 184 sonic anemometers) that are critical for studying turbulent exchange processes but provide only limited information about the complex interactions between the flow inside the valley and higher up across the ridges. The virtual towers extend the range of traditional in-situ observations and can fill in low altitude areas where other lidar 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 of a nighttime stable boundary-layer flow with wave features in the valley will be presented to illustrate the usefulness of the virtual towers in analyzing the spatially complex flow over the ridges during the Perdigão campaign.