An Overview of Air-Sea Interaction Research in CASPER-West

One of the objectives of the Coupled Air-Sea Processes and EM Ducting Research (CASPER) is to correctly retrieve the vertical profiles of mean temperature and humidity in the marine atmospheric boundary layer (MABL) from diagnostic surface layer models or from mesoscale forecast models in order to characterize the atmospheric refractive properties affecting electromagnetic wave propagation in the marine and coastal environment. CASPER-West field campaign made extensive effort to address this objective. This presentation intends to provide an overview of the air-sea interaction component of CASPER-West.

CASPER-West field campaign occurred between 27 September and 26 October 2017 offshore of Point Mugu, California. It was designed to capture the MABL mean and turbulence characteristics and their horizontal and vertical variations using a combination of ships, at-sea moored platform, research aircraft, autonomous vehicles, and buoys and land stations. Two major at-sea platforms, the R/V Sally Ride (RVSR) and the stabilized platform R/P FLIP, were deployed during CASPER-West. FLIP was moored at about 25 km north of Santa Barbara Island. RVSR transected along the 47 km path between the CASPER shore site at Pt. Mugu and FLIP, stopping at multiple locations to make approximately an hour long extensive air-sea interaction measurements, a period referred to as the air-sea interaction (ASI) sampling module. Each ASI module included having the bow facing the mean wind direction for good quality turbulence measurements, deploying a small boat near Sally Ride to get turbulence closer to the surface than the bow mast flux package of RVSR and obtaining near-surface temperature and humidity profiles using a small tethered balloon. During the Santa Ana event days, RVSR continued along the path beyond FLIP location to about 78 km from Pt. Mugu. Coordinated with the ship-based measurements, the Twin Otter research aircraft operated by the Naval Postgraduate School Center for Interdisciplinary Remote-Piloted Aircraft (CIRPAS) flew a total of 50 flight hours on ten different days. On the shore site, the flux towers and coordinated three-lidar measurement configuration continuously monitored the near-shore atmospheric boundary layer. In addition, upper ocean and surface wave measurements were made from both RVSR, FLIP, three autonomous underwater vehicles, and a wave buoy deployed near FLIP.

R/P FLIP is the major platform in CASPER-West for multi-level concurrent and collocated measurements of the marine atmospheric surface layer (MASL). There were seven levels of high-rate sampling of turbulence and scalar perturbations and another ten levels of mean wind, temperature, and humidity measurements on the 13 m long vertical mast on the port side boom of FLIP with an additional tethered surface following instrumented float with mean scalar measurements down to less than a meter above the surface. The starboard boom of R/P FLIP was also equipped to sample fine-scale turbulence structure at the ocean surface or the atmosphere up to several meters from the surface using state-of-the-art laser-Doppler and infrared technology.

In this presentation, we will describe CASPER-West general experiment design for air-sea interaction study and show example results from various platforms. We will also introduce the CASPER-West large eddy simulation and mesoscale modeling efforts.