A field project will be conducted from 20 May - 20 June 2011 near the Point Conception, California. The fundamental goal of this field experiment is to directly measure the dynamic forcing responsible for the wind field within the marine atmospheric boundary layer (MABL). The study will focus on dynamics associated with the northerly low-level coastal jet that is commonplace in the California coasts during summer. Because there are many potentially non-linear interactions occurring at the interface of the strong northerly (thus potentially difficult to simulate), along-shore wind regime with the cyclonic circulation within the California Bight, direct measurements of meteorological variables in this region are necessary to pars out the important components and their interactions at MABL. If a Catalina eddy and/or coastally-trapped wind reversal should occur, dedicated flights will be conducted to explore the dynamics and thermodynamics associated with those events.
Intellectual merit. The University of Wyoming King Air aircraft (UWKA) will be deployed in the California coasts, which has been used in previous research projects. A total of 50 flight hours are planned. This project will build on recently developed airborne technology that enables detection of small horizontal pressure signals using differential correction of GPS heights of an airborne platform. Such measurements will allow fine-scale assessment of the horizontal pressure gradient at levels within the MABL, and thus permit analysis of the forcing terms in the horizontal equations of motion. Determination of ageostrophic winds and accelerations of the MABL flows will be possible from the airborne sampling. In addition, a request has been made for inclusion of the University of Wyoming Cloud Radar and Cloud Lidar as part of the instrumentation onboard the UWKA. Such instrumentation will enable measurements of attendant cloud properties, such as cloud height and microphysical parameters. The standard cloud physics probes on the UWKA will also allow a host of other cloud and microphysical measurements.
A critical measurement for dynamics of the MABL under conditions of a Catalina eddy or coastally trapped wind reversal is the height of the MABL stratus. Detailed information regarding the vertical structure of humidity and temperature within the MABL from the airborne missions will enable fine-scale determination of microwave refractivity profiles in this strategically sensitive region. The airborne observational dataset will be supplemented with the considerable body of surface-based instrumentation including a host of buoys surrounding Point Conception. Data collected will provide the basis for comparison with high-resolution (~ 1 km) numerical simulations with the Weather Research and Forecasting Model (WRF).
Broader impacts. The Point Conception area is of strategic importance and national security interests. Meteorological phenomena in the coastal environment have prompted several major research efforts during the past two decades. Data collected as part of this field experiment will provide unique and unprecedented measurements that will be useful for validation of previous conceptual models and efforts to numerically simulate detailed MABL dynamics. This project will offer an opportunity for graduate student training in the observational aspects of atmospheric science, a focus from that was identified as a key element for the June 2009 Advanced Study Program Colloquium. As with previous field experiments, graduate students will have an opportunity to plan a research mission and serve as flight scientist on the UWKA. A new academic program, Earth System Science, has been initiated at the University of Wyoming and data from this project will be used in a research project as part of an atmospheric dynamics project.
was a three-year study of the atmospheric marine boundary layer and wind off the coast of Southern California from June 2011-May 2014. The focus of PreAMBLE was a field study that was conducted from 17 May to 17 June 2012 based in Point Mugu, CA. The University of Wyoming King Air research aircraft was the primary measurement platform with the majority of the flights in the lowest 500 meters of the atmosphere. The coastal mountains modulate the marine boundary layer and wind along the West Coast of the United States. The central theme of research was a detailed examination of the atmospheric dynamics associated with the wind and temperature fields within the summertime marine layer near Point Conception and the California Bight area using an airborne measurement platform. PreAMBLE had three major research goals. First, assess the influence of the Point Conception topography on the dynamics of the adjacent marine environment that often produces strong wind. Second, examine the forcing of the circulation within the California Bight, commonly known as a Catalina Eddy. Third, investigate the initiation of the anomalous northward propagating surges of stratus along the coastal terrain known as coastally-trapped wind reversals. All major goals specified in the original proposal were completed during PreAMBLE. The weather was favorable throughout the field study. There were 15 research flights totaling 50 hours during PreAMBLE addressing the three primary PreAMBLE research goals. Individual flights can be found on the PreAMBLE website (http://flights.uwyo.edu/projects/preamble12/). Ten flights were designed to examine aspects of flow around Point Conception, three flights to sample the Catalina eddy environment, and one flight was conducted to investigate the pre- coastally-trapped wind reversal environment about the Point Conception area. Recent advancements in measurement technology were employed in PreAMBLE. In particular, the ability of the University of Wyoming King Air to map the horizontal pressure field is exploited. This is critical since knowledge of the horizontal pressure gradient force is prerequisite to understanding the dynamics of the flow regime. A second airborne measurement is the marine boundary layer height obtained from the Wyoming Cloud Lidar, which was designed for retrieval of cloud and aerosol properties. The Wyoming Cloud Lidar provided an unexpectedly rich data set that allowed interpretation of the environment that the University of Wyoming King Air was sampling, especially during clear sky conditions. Subtle differences in lidar depolarization returns enabled aerosol origin to be evaluated and thus boundary layer structure could be evaluated and understood. An example of a lidar image is shown in Figure 1. Fine-scale mapping of the horizontal pressure field was conducted about the Point Conception headland. Airborne measurements during cases of strong northerly winds show an abrupt adjustment of the marine layer near Point Conception, including a strong gradient in the horizontal pressure gradient force and a near-collapse of the marine boundary layer. Dynamics of the flow are modified significantly by offshore wind at Point Conception. Measurements were made of the cloud structure in the marine layer and the horizontal pressure field associated with a Catalina eddy using the University of Wyoming King Air research aircraft. Airborne measurements show that the coastal mountains south of Los Angeles block the flow, resulting in enhanced marine stratus heights and a local pressure maximum near the coast. The horizontal pressure field also supports a south-to-north movement of marine stratus. Little evidence of lee-side troughing south of Santa Barbara was observed for this case implying that the horizontal pressure field is forced primarily through topographic blocking by the coastal terrain south of Los Angeles and the ambient large-scale circulation associated with the mean flow. A coastally-trapped wind reversal formed during the last few days of PreAMBLE. A case study flight on June 16 captured its initiation phase. Results from the aircraft mission and a previous aircraft study conducted back in 2006 as well as the numerical simulations have prompted a slightly different interpretation of the dynamics of these wind reversals. We are now advocating that such features owe their northward propagation to a favorable horizontal pressure gradient force in the coastal environment. We view the role of rotational trapping by the coastal mountains as secondary to the northward progression of such events. As a result, we have advocated that the term "coastally-trapped" is inappropriate since the influence of such rotational trapping is secondary to the primary forcing owing to the ambient environment.
