Airborne Doppler Wind Lidar Measurements of a California Coastal Jet During 2022 SWEX

During April and May of 2022, NSF and NCAR funded the Sundowner Winds Experiment (SWEX), a combined ground-based and airborne campaign based out of Santa Barbara/Oxnard CA. SWEX was designed to investigate the downslope winds at the lee of the Santa Yanez Mountains, often called Sundowners, and the important role they play in the evolution and movement of wild fires in the region. A ground-based observation network and airborne instrumentation, along with operational and research numerical modeling (WRF), was put in place with a goal to improve the understanding and prediction of these winds and the PBL structure.

The ground based network included rawinsondes, surface flux towers, radar wind profilers, microwave radiometers, infrasound sensors, ceilometers and six ground-based lidars. These instruments were operated between April 1 and May 11 2022. In addition, a truck based mobile wind lidar operated by UVA (University of Virginia Wind Observatory on Wheels (UWOW) )was also able to take mobile measurements of wind and aerosol profiles. A Naval Postgraduate School/CIRPAS Twin Otter Aircraft was also instrumented and included the NCAR AVAPS dropsonde system which provided vertical profiles of temperature, humidity and winds and a compact rotational Raman lidar (CRL) which obtained 2D distributions of water vapor, aerosols, temperature in the boundary layer at a vertical resolution of about 50 m. More important to this current study, the Twin Otter was also equipped with the Twin Otter Doppler Wind Lidar (TODWL) to obtain vertical profiles of horizontal winds at a vertical resolution of 50m and a horizontal resolution < 10 km.

Missions were flown on 13 days targeting different synoptic conditions which influenced the strength and existence of the Sundowner winds. Most days (8) had two missions to capture the temporal evolution of the PBL structure and the low level wind field. As part of each of these missions, numerous N-S transects from the mountains to the coastline and across the waters of the Santa Barbara channel were flown as well as numerous generally E-W transects over the water and along the coast.

The work presented here utilized these transects to investigate the changes in the low level wind field as it goes from the complex terrain over land to the open water of the Santa Barbara channel and the high resolution variability in both time and space of the coastal jet which often forms in this area. In addition to the flights mentioned above, through the support of the Office of Naval Research (ONR), a stand-alone dedicated mission with longer E-W and N-S transects over the water was flown on May 3, 2022 to concentrate on the wind field off and along the coast.

During these missions, the TODWL was able to make wind profile measurements every 3-7 km along the flight track (up to several hundred profiles per mission), a much higher density then has ever been possible in this location. The evolution and intensity of the jet have been reported and discussed in the literature before but the measurements taken during SWEX show a much higher resolution variability/fluctuation of the coastal jet wind field as well as the flow from the mountains and across the Channel. The analysis of these data will be presented including comparisons with the coincident dropsondes (5-10 per mission) over the water

In addition to comparisons with other observations such as the dropsondes, we will also present comparisons with numerical simulations generated by the UCSB WRF model (v4.2.2) 1 km resolution that was run as part of the SWEX campaign. This analysis will show how well the model captured the evolution, structure and variability of the low level jet over the water during several missions.