Nocturnal Boundary-Layer Structure Inside a Valley Observed During the Perdigão Field Experiment

The University of Oklahoma (OU) Collaborative Lower Atmospheric Profiling System (CLAMPS) was deployed near Vale Do Cobrão from May 1, 2017 through June 15, 2017 for the Intensive Observation Period (IOP) of the Perdigão Field Experiment. CLAMPS includes a Halo Photonics Streamline Doppler lidar (DL), Atmospheric Emitted Radiance Interferometer (AERI,) and a multi-channel microwave radiometer (MWR). Additionally, near-surface temperature, humidity and wind speed were measured with a Vaisala WXT-530 weather transmitter that is part of the MWR instrumentation. The CLAMPS DL performed a cross-valley Range Height Indicator (RHI) scan, along-valley RHI scan, and a 70 degree Velocity Azimuth Display (VAD) scan every 15 minutes. The remainder of the time it was operated in zenith stare mode to capture vertical velocities with a sampling frequency of 1 Hz, which provide information about turbulent mixing. The AERI passively measures downwelling spectra over the spectral range of 550 cm-1 to 3500 cm-1 (3.3 μm to 18.2 μm) with a temporal resolution of 30 s, which are then processed with the AERI Optimal Estimation retrieval algorithm to obtain temperature and water vapor profiles as well as cloud properties.

Vale Do Cobrão is a roughly 1.4 km wide valley bounded by two nearly parallel, 5-km long ridges that are about 200 m high. Climatologically, the two wind directions perpendicular to the ridges, SW and NE, are most prevalent, but during the IOP the wind direction at ridge height typically had a pronounced diurnal variation turning in around sunset from westerly to northerly and north-easterly winds. The CLAMPS observations provide detailed information about atmospheric stability and flow, both inside the valley and above the ridges. Excluding days with precipitation and malfunctioning instruments, data for 25 days have been analyzed. The flow patterns inside the valley and at ridge height are classified based on stability and wind direction above the ridges.