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Colloquium

Department of Atmospheric Science

Tues., Feb. 13, 3:10 pm, EN6085

Modification of Marine Boundary Layer Flow by Topography along the Western United States Coastline

Tim Juliano

University of Wyoming

Abstract

Coastal marine environments are regions of significant meteorological interest as a result of the direct impact of atmospheric processes on, for example, commerce and trade, naval operations, and civilian activities. Because the coastal topography of California acts as a rigid, lateral boundary for the marine layer, it plays a critical role in the life cycle of various atmospheric processes. The work presented here will examine two examples of marine phenomena and illustrate the influence of large-scale meteorological conditions on mesoscale and microscale processes.

 

In the first part of the talk, observations from the University of Wyoming King Air are compared with simulations using the Weather Research and Forecasting (WRF) model for a case study of an atmospheric hydraulic jump offshore California on 24 May 2012. Strong, northwesterly flow rounded the Point Arguello–Point Conception complex and encountered the remnants of an eddy circulation in the Santa Barbara Channel. The aircraft flew an east–west vertical sawtooth pattern that captured a sharp thinning of the marine boundary layer (MBL) and the downstream development of a hydraulic jump. The WRF model produced results in agreement with the observations. A strongly divergent wind field, consistent with expansion fan dynamics, is present upwind of the hydraulic jump. The model accurately resolves details of the MBL collapse into the jump. Results from large-eddy simulations show a large increase in the turbulent kinetic energy field coincident with the hydraulic jump.

 

The second portion of the talk focuses on a set of 23 coastally trapped disturbances (CTDs) – spanning the years from 2004 to 2016 – from a climatological perspective using several data products, including model reanalyses, buoys, and satellites. While it is well known that broad stratiform cloud decks accompany CTDs, which surge from south to north, very little is known about their cloud microphysical and macrophysical properties because they are difficult to forecast and therefore in situ observations are few and far between. Previous works investigating MBL clouds off the California coast have focused on those that form under the typical northerly flow regime during the boreal warm season. Results shown here suggest that CTD cloud decks may play a unique role in the radiation budget due to a combination of aerosol sources that enhance cloud droplet number concentration and diminish cloud top effective radius. This particular type of cloud regime should therefore be treated differently than that which is more commonly found in the summertime months over the northeast Pacific Ocean. The influence of a coherent wind stress cycle on sea surface temperatures is also explored.

 


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